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Low Power, 2-Channel 24-Bit Sigma-Delta ADC AD7787
FEATURES
Power Supply: 2.5 V to 5.25 V operation Normal mode: 75 A max Power-down mode: 1 A max RMS noise: 1.1 V at 9.5 Hz update rate 19.5-bit p-p resolution (22 bits effective resolution) Integral nonlinearity: 3.5 ppm typical Simultaneous 50 Hz and 60 Hz rejection Internal clock oscillator Rail-to-rail input buffer VDD monitor channel Temperature range: -40C to +105C 10-lead MSOP
APPLICATIONS
Smart transmitters Battery applications Portable instrumentation Sensor measurement Temperature measurement Pressure measurement Weigh scales 4 to 20 mA loops
GENERAL DESCRIPTION
The AD7787 is a low power, complete analog front end for low frequency measurement applications. It contains a low noise 24-bit - ADC with one differential input and one singleended input that can be buffered or unbuffered. The device operates from an internal clock. Therefore, the user does not have to supply a clock source to the device. The output data rate from the part is software programmable and can be varied from 9.5 Hz to 120 Hz, with the rms noise equal to 1.1 V at the lower update rate. The internal clock frequency can be divided by a factor of 2, 4, or 8, which leads to a reduction in the current consumption. The update rate, cutoff frequency, and settling time scales with the clock frequency. The part operates with a power supply from 2.5 V to 5.25 V. When operating from a 3 V supply, the power dissipation for the part is 225 W maximum. It is housed in a 10-lead MSOP.
INTERFACE
3-wire serial SPI(R), QSPITM, MICROWIRETM, and DSP compatible Schmitt trigger on SCLK
FUNCTIONAL BLOCK DIAGRAM
GND VDD VDD REFIN
AD7787
DOUT/RDY SERIAL INTERFACE AND LOGIC CONTROL DIN SCLK CS
04477-0-001
AIN1(+) AIN1(-) AIN2 MUX BUF
- ADC
CLOCK GND
Figure 1.
Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.326.8703 (c) 2004 Analog Devices, Inc. All rights reserved.
AD7787
TABLE OF CONTENTS
Specifications..................................................................................... 3 Timing Characteristics..................................................................... 5 Absolute Maximum Ratings............................................................ 7 ESD Caution.................................................................................. 7 Pin Configuration and Function Descriptions............................. 8 Typical Performance Characteristics ............................................. 9 On-Chip Registers .......................................................................... 10 Communications Register (RS1, RS0 = 0, 0)........................... 10 Status Register (RS1, RS0 = 0, 0; Power-On/Reset = 0x8C)........................................................................................ 11 Mode Register (RS1, RS0 = 0, 1; Power-On/Reset = 0x02)......................................................................................... 12 Filter Register (RS1, RS0 = 1, 0; Power-On/Reset = 0x04)......................................................................................... 13 Data Register (RS1, RS0 = 1, 1; Power-On/Reset = 0x000000) ................................................................................ 13 ADC Circuit Information.............................................................. 14 Overview ..................................................................................... 14 Noise Performance ..................................................................... 14 Reduced Current Modes ........................................................... 14 Digital Interface.......................................................................... 15 Circuit Description......................................................................... 18 Analog Input Channel ............................................................... 18 Bipolar/Unipolar Configuration .............................................. 18 Data Output Coding .................................................................. 18 Reference Input........................................................................... 18 VDD Monitor ................................................................................ 19 Grounding and Layout .............................................................. 19 Applications ................................................................................ 19 Outline Dimensions ....................................................................... 20 Ordering Guide .......................................................................... 20
REVISION HISTORY
4/04--Revision 0: Initial Version
Rev. 0 | Page 2 of 20
AD7787 SPECIFICATIONS
Temperature range is -40C to +105C. VDD = 2.5 V to 5.25 V; REFIN = 2.5 V; GND = 0 V; CDIV1 = CDIV0 = 0; all specifications TMIN to TMAX, unless otherwise noted. Table 1.
Parameter ADC CHANNEL SPECIFICATION Output Update Rate ADC CHANNEL No Missing Codes1 Resolution Output Noise Integral Nonlinearity Offset Error Offset Error Drift vs. Temperature Full-Scale Error2 Gain Drift vs. Temperature Power Supply Rejection ANALOG INPUTS Bipolar Input Voltage Range Unipolar Voltage Range Absolute AIN Voltage Limits1 Analog Input Current Average Input Current1 Average Input Current Drift Absolute AIN Voltage Limits1, 3 Analog Input Current Average Input Current Average Input Current Drift Normal Mode Rejection1 @ 50 Hz, 60 Hz @ 50 Hz @ 60 Hz Common-Mode Rejection (AIN1) @ DC @ 50 Hz, 60 Hz1 REFERENCE INPUT REFIN Voltage Reference Voltage Range1 Average Reference Input Current Average Reference Input Current Drift Normal Mode Rejection1 @ 50 Hz, 60 Hz @ 50 Hz @ 60 Hz AD7787B 9.5 120 24 19.5 1.1 15 3 10 10 0.5 90 REFIN 0 to REFIN GND + 100 mV VDD - 100 mV 1 5 GND - 100 mV VDD + 30 mV 400 50 65 80 80 90 100 2.5 0.1 VDD 0.5 0.03 65 80 80 Unit Hz min nom Hz max nom Bits min Bits p-p V rms typ ppm of FSR max V typ nV/C typ V typ ppm/C typ dB min V nom V nom V min V max nA max pA/C typ V min V max nA/V typ pA/V/C typ dB min dB min dB min dB min dB min V nom V min V max A/V typ nA/V/C typ dB min dB min dB min 73 dB typ, 50 1 Hz, 60 1 Hz, FS[2:0] = 1004. 90 dB typ, 50 1 Hz, FS[2:0] = 1014. 90 dB typ, 60 1 Hz, FS[2:0] = 0114. 73 dB typ, 50 1 Hz, 60 1 Hz, FS[2:0] = 1004. 90 dB typ, 50 1 Hz, FS[2:0] = 1014. 90 dB typ, 60 1 Hz, FS[2:0] = 0114. AIN = 1 V. 100 dB typ. 50 1 Hz (FS[2:0] = 1014), 60 1 Hz (FS[2:0] = 0114). Update rate 20 Hz. 9.5 Hz update rate. 3.5 ppm typ. Test Conditions/Comments
100 dB typ, AIN = 1 V. Because AIN2 is single-ended, it can have a negative voltage of 100 mV minimum (see Page 18). Buffered mode. Buffered mode.
Unbuffered mode. Unbuffered mode. Current varies with input voltage.
Rev. 0 | Page 3 of 20
AD7787
Parameter LOGIC INPUTS All Inputs Except SCLK1 VINL, Input Low Voltage VINH, Input High Voltage SCLK Only (Schmitt-Triggered Input)1 VT(+) VT(-) VT(+) - VT(-) VT(+) VT(-) VT(+) - VT(-) Input Currents Input Capacitance LOGIC OUTPUTS VOH, Output High Voltage1 VOL, Output Low Voltage1 VOH, Output High Voltage1 VOL, Output Low Voltage1 Floating-State Leakage Current Floating-State Output Capacitance Data Output Coding POWER REQUIREMENTS5 Power Supply Voltage VDD - GND Power Supply Currents IDD Current6 AD7787B Unit Test Conditions/Comments
0.8 0.4 2.0 1.4/2 0.8/1.4 0.3/0.85 0.9/2 0.4/1.1 0.3/0.85 1 10 VDD - 0.6 0.4 4 0.4 1 10 Offset Binary
V max V max V min V min/V max V min/V max V min/V max V min/V max V min/V max V min/V max A max pF typ V min V max V min V max A max pF typ
VDD = 5 V. VDD = 3 V. VDD = 3 V or 5 V. VDD = 5 V. VDD = 5 V. VDD = 5 V. VDD = 3 V. VDD = 3 V. VDD = 3 V. VIN = VDD or GND. All Digital Inputs. VDD = 3 V, ISOURCE = 100 A. VDD = 3 V, ISINK = 100 A. VDD = 5 V, ISOURCE = 200 A. VDD = 5 V, ISINK = 1.6 mA.
2.5/5.25 75 145 80 160 1
V min/max A max A max A max A max A max 65 A typ, VDD = 3.6 V, unbuffered mode. 130 A typ, VDD = 3.6 V, buffered mode. 73 A typ, VDD = 5.25 V, unbuffered mode. 145 A typ, VDD = 5.25 V, buffered mode.
IDD (Power-Down Mode)
1 2
Specification is not production tested but is supported by characterization data at initial product release. Full-scale error applies to both positive and negative full scale and applies at the factory calibration conditions (VDD = 4 V). 3 The AD7787 can tolerate absolute analog input voltages down to GND - 200 mV but the leakage current will increase. 4 FS[2:0] are the three bits used in the filter register to select the output word rate. 5 Digital inputs equal to VDD or GND. 6 The current consumption can be further reduced by using the ADC in one of the low power modes (see Table 14).
Rev. 0 | Page 4 of 20
AD7787 TIMING CHARACTERISTICS
Sample tested during initial release to ensure compliance. All input signals are specified with tR = tF = 5 ns (10% to 90% of VDD) and timed from a voltage level of 1.6 V (see Figure 3 and Figure 4). VDD = 2.5 V to 5.25 V; GND = 0 V, REFIN = 2.5 V, CDIV1 = CDIV0 = 0, Input Logic 0 = 0 V, Input Logic 1 = VDD, unless otherwise noted. Table 2.
Parameter t3 t4 Read Operation t1 Limit at TMIN, TMAX (B Version) 100 100 0 60 80 0 60 80 10 80 100 10 0 30 25 0 Unit ns min ns min ns min ns max ns max ns min ns max ns max ns min ns max ns max ns min ns min ns min ns min ns min Conditions/Comments SCLK High Pulse Width SCLK Low Pulse Width CS Falling Edge to DOUT/RDY Active Time VDD = 4.75 V to 5.25 V VDD = 2.5 V to 3.6 V SCLK Active Edge to Data Valid Delay2 VDD = 4.75 V to 5.25 V VDD = 2.5 V to 3.6 V Bus Relinquish Time after CS Inactive Edge SCLK Inactive Edge to CS Inactive Edge SCLK Inactive Edge to DOUT/RDY High CS Falling Edge to SCLK Active Edge Setup Time2 Data Valid to SCLK Edge Setup Time Data Valid to SCLK Edge Hold Time CS Rising Edge to SCLK Edge Hold Time
t21
t53, 4 t6 t7 Write Operation t8 t9 t10 t11
1 2
These numbers are measured with the load circuit of Figure 2 and defined as the time required for the output to cross the VOL or VOH limits. The SCLK active edge is the falling edge of SCLK. 3 These numbers are derived from the measured time taken by the data output to change 0.5 V when loaded with the circuit of Figure 2. The measured number is then extrapolated back to remove the effects of charging or discharging the 50 pF capacitor. This means that the times quoted in the timing characteristics are the true bus relinquish times of the part and, as such, are independent of external bus loading capacitances. 4 RDY returns high after a read of the ADC. In single conversion mode and continuous conversion mode, the same data can be read again, if required, while RDY is high, although care should be taken to ensure that subsequent reads do not occur close to the next output update. In continuous read mode, the digital word can be read only once.
Rev. 0 | Page 5 of 20
AD7787
ISINK (1.6mA WITH VDD = 5V, 100A WITH VDD = 3V)
TO OUTPUT PIN 50pF
1.6V
ISOURCE (200A WITH VDD = 5V, 100A WITH VDD = 3V)
Figure 2. Load Circuit for Timing Characterization
CS (I)
t1
DOUT/RDY (O) MSB LSB
t6 t5
t2 t3
SCLK (I)
t7
04477-0-002
t4
I = INPUT, O = OUTPUT
Figure 3. Read Cycle Timing Diagram
CS (I)
t8
SCLK (I)
t11
t9 t10
04477-0-004
DIN (I)
MSB
LSB
I = INPUT, O = OUTPUT
Figure 4. Write Cycle Timing Diagram
Rev. 0 | Page 6 of 20
04477-0-003
AD7787 ABSOLUTE MAXIMUM RATINGS
TA = 25C, unless otherwise noted. Table 3.
Parameter VDD to GND Analog Input Voltage to GND Reference Input Voltage to GND Total AIN/REFIN Current (Indefinite) Digital Input Voltage to GND Digital Output Voltage to GND Operating Temperature Range Storage Temperature Range Maximum Junction Temperature MSOP JA Thermal Impedance JC Thermal Impedance Lead Temperature, Soldering (10 sec) IR Reflow, Peak Temperature Rating -0.3 V to +7 V -0.3 V to VDD + 0.3 V -0.3 V to VDD + 0.3 V 30 mA -0.3 V to VDD + 0.3 V -0.3 V to VDD + 0.3 V -40C to +105C -65C to +150C 150C 206C/W 44C/W 300C 220C
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although this product features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
Rev. 0 | Page 7 of 20
AD7787 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
SCLK 1 CS 2 AIN1(+) 3
10 DIN
AD7787
9 8
DOUT/RDY
04477-0-005
VDD TOP VIEW AIN1(-) 4 (Not to Scale) 7 GND REFIN 5
6
AIN2
Figure 5. Pin Configuration
Table 4. Pin Function Descriptions
Pin No. 1 Mnemonic SCLK Function Serial Clock Input for Data Transfers to and from the ADC. The SCLK has a Schmitt-triggered input, making the interface suitable for opto-isolated applications. The serial clock can be continuous with all data transmitted in a continuous train of pulses. Alternatively, it can be a noncontinuous clock with the information being transmitted to or from the ADC in smaller batches of data. Chip Select Input. This is an active low logic input used to select the ADC. CS can be used to select the ADC in systems with more than one device on the serial bus or as a frame synchronization signal in communicating with the device. CS can be hardwired low, allowing the ADC to operate in 3-wire mode with SCLK, DIN, and DOUT used to interface with the device. Analog Input. AIN1(+) is the positive terminal of the differential analog input pair AIN1(+)/AIN1(-). Analog Input. AIN1(-) is the negative terminal of the differential analog input pair AIN1(+)/AIN1(-). Reference Input. REFIN can be anywhere between VDD and GND + 0.1 V. The nominal reference voltage is 2.5 V, but the part functions with a reference from 0.1 V to VDD. Analog Input. AIN2 is a single-ended analog input. Ground Reference Point. Supply Voltage, 2.5 V to 5.25 V. Serial Data Output/Data Ready Output. DOUT/RDY serves a dual purpose. It functions as a serial data output pin to access the output shift register of the ADC. The output shift register can contain data from any of the on-chip data or control registers. In addition, DOUT/RDY operates as a data ready pin, going low to indicate the completion of a conversion. If the data is not read after the conversion, the pin will go high before the next update occurs. The DOUT/RDY falling edge can be used as an interrupt to a processor, indicating that valid data is available. With an external serial clock, the data can be read using the DOUT/RDY pin. With CS low, the data/control word information is placed on the DOUT/RDY pin on the SCLK falling edge and is valid on the SCLK rising edge. The end of a conversion is also indicated by the RDY bit in the status register. When CS is high, the DOUT/RDY pin is three-stated, but the RDY bit remains active. Serial Data Input to the Input Shift Register on the ADC. Data in this shift register is transferred to the control registers within the ADC; the register selection bits of the communications register identifying the appropriate register.
2
CS
3 4 5 6 7 8 9
AIN1(+) AIN1(-) REFIN AIN2 GND VDD DOUT/RDY
10
DIN
Rev. 0 | Page 8 of 20
AD7787 TYPICAL PERFORMANCE CHARACTERISTICS
0 -10 -20 -30
VDD = 3V VREF = 2.048V 8 1.1875Hz UPDATE RATE TA = 25C 7 RMS NOISE = 1.25F
9
-50
OCCURRENCE
04477-0-012
-40
6 5 4 3 2 1 0 8388592 CODE
04477-0-014
dB
-60 -70 -80 -90
-100 -110 -120 0 20 40 60 80 100 FREQUENCY (Hz) 120 140 160
8388616
Figure 6. Frequency Response with 16.6 Hz Update Rate
100 VDD = 3V VREF = 2.048V 9.5Hz UPDATE RATE TA = 25C RMS NOISE = 1V
Figure 9. Noise Histogram for Clock Divide-by-8 Mode (CDIV0 = CDIV1 = 1)
8388616
80
OCCURRENCE
40
20
04477-0-010
04477-0-013
CODE
60
0 8388591
VDD = 3V, VREF = 2.048V 1.1875Hz UPDATE RATE TA = 25C, RMS NOISE = 1.25F 8388592 0 20 40 60 READ NO. 80 100
CODE
8388619
Figure 7. Noise Distribution Histogram (CDIV1 = CDIV0 = 0)
8388619
Figure 10. Noise Plot in Clock Divide-by-8 Mode (CDIV0 = CDIV1 = 1)
3.0 VDD = 5V UPDATE RATE = 16.6Hz TA = 25C
2.5
RMS NOISE (V)
2.0
CODE
1.5
1.0
8388591
VDD = 3V, VREF = 2.048V, 9.5Hz UPDATE RATE TA = 25C, RMS NOISE = 1V 0 200 400 600 READ NO. 800
04477-0-011
0 0 0.5 1.0 1.5 2.0 2.5 3.0 VREF (V) 3.5 4.0 4.5 5.0
1000
Figure 8. Typical Noise Plot with 16.6 Hz Update Rate (CDIV1 = CD1V0 = 0)
Figure 11. RMS Noise vs. Reference Voltage
Rev. 0 | Page 9 of 20
04477-0-015
0.5
AD7787 ON-CHIP REGISTERS
The ADC is controlled and configured via a number of on-chip registers, which are described on the following pages. In the following descriptions, set implies a Logic 1 state and cleared implies a Logic 0 state, unless otherwise noted.
COMMUNICATIONS REGISTER (RS1, RS0 = 0, 0)
The communications register is an 8-bit write-only register. All communications to the part must start with a write operation to the communications register. The data written to the communications register determines whether the next operation is a read or write operation and to which register this operation takes place. For read or write operations, once the subsequent read or write operation to the selected register is complete, the interface returns to where it expects a write operation to the communications register. This is the default state of the interface and, on power-up or after a reset, the ADC is in this default state waiting for a write operation to the communications register. In situations where the interface sequence is lost, a write operation of at least 32 serial clock cycles with DIN high returns the ADC to this default state by resetting the entire part. Table 5 outlines the bit designations for the communications register. CR0 through CR7 indicate the bit location, CR denoting the bits are in the communications register. CR7 denotes the first bit of the data stream. The number in the parenthesis indicates the power-on/reset default status of that bit.
CR7 WEN (0)
CR6 0 (0)
CR5 RS1 (0)
CR4 RS0 (0)
CR3 R/W (0)
CR2 CREAD (0)
CR1 CH1 (0)
CR0 CH0 (0)
Table 5. Communications Register Bit Designations
Bit Location CR7 Bit Name WEN Description Write Enable Bit. A 0 must be written to this bit so that the write to the communications register actually occurs. If a 1 is the first bit written, the part does not clock on to subsequent bits in the register. It stays at this bit location until a 0 is written to this bit. Once a 0 is written to the WEN bit, the next seven bits are loaded to the communications register. This bit must be programmed to Logic 0 for correct operation. Register Address Bits. These address bits are used to select which of the ADC's registers are being selected during this serial interface communication (see Table 6). A 0 in this bit location indicates that the next operation is a write to a specified register. A 1 in this position indicates that the next operation is a read from the designated register. Continuous Read of the Data Register. When this bit is set to 1 (and the data register is selected), the serial interface is configured so that the data register can be continuously read, i.e., the contents of the data register are placed on the DOUT pin automatically when the SCLK pulses are applied. The communications register does not have to be written to for data reads. To enable continuous read mode, the instruction 00111100 (Channel AIN1) or 00111101 (Channel AIN2) must be written to the communications register. To exit the continuous read mode, the instruction 001110XX must be written to the communications register while the RDY pin is low. While in continuous read mode, the ADC monitors activity on the DIN line so that it can receive the instruction to exit continuous read mode. Additionally, a reset occurs if 32 consecutive 1s are seen on DIN. Therefore, DIN should be held low in continuous read mode until an instruction is to be written to the device. These bits are used to select the analog input channel. Channel AIN1 or AIN2 can be selected or an internal short (AIN1(-)/AIN1(-)) can be selected. Alternatively, the power supply can be selected, i.e., the ADC can measure the voltage on the power supply, which is useful for monitoring power supply variation. To perform this measurement, the power supply voltage is divided by 5 and then applied to the modulator for conversion. The ADC uses a 1.17 V 5% on-chip reference as the reference source when this channel is selected. Any change in channel resets the filter and a new conversion is started.
CR6 CR5 to CR4 CR3 CR2
0 RS1 to RS0 R/W CREAD
CR1 to CR0
CH1 to CH0
Table 6. Register Selection
RS1 0 0 0 1 1 RS0 0 0 1 0 1 Register Communications Register during a Write Operation Status Register during a Read Operation Mode Register Filter Register Data Register
Rev. 0 | Page 10 of 20
Register Size 8-Bit 8-Bit 8-Bit 8-Bit 24-Bit
AD7787
Table 7. Channel Selection
CH1 0 0 1 1 CH0 0 1 0 1 Channel AIN1(+) - AIN1(-) AIN2 AIN1(-) - AIN1(-) VDD Monitor
STATUS REGISTER (RS1, RS0 = 0, 0; POWER-ON/RESET = 0x8C)
The status register is an 8-bit read-only register. To access the ADC status register, the user must write to the communications register, select the next operation to be a read, and load Bits RS1 and RS0 with 0s. Table 8 outlines the bit designations for the status register. SR0 through SR7 indicate the bit locations, SR denoting the bits are in the status register. SR7 denotes the first bit of the data stream. The number in the parenthesis indicates the power-on/reset default status of that bit.
SR7 RDY (1)
SR6 ERR (0)
SR5 0 (0)
SR4 0 (0)
SR3 1 (1)
SR2 1 (1)
SR1 CH1 (0)
SR0 CH0 (0)
Table 8. Status Register Bit Designations
Bit Location SR7 Bit Name RDY Description Ready Bit for ADC. Cleared when data is written to the ADC data register. The RDY bit is set automatically after the ADC data register has been read or a period of time before the data register is updated with a new conversion result to indicate to the user not to read the conversion data. It is also set when the part is placed in power-down mode. The end of a conversion is indicated by the DOUT/RDY pin also. This pin can be used as an alternative to the status register for monitoring the ADC for conversion data. ADC Error Bit. This bit is written to at the same time as the RDY bit. Set to indicate that the result written to the ADC data register has been clamped to all 0s or all 1s. Error sources include overrange, underrange. Cleared by a write operation to start a conversion. These bits are automatically cleared. These bits are automatically set. These bits indicate which channel is being converted by the ADC.
SR6
ERR
SR5 to SR4 SR3 to SR2 SR1 to SR0
0 1 CH1 to CH0
Rev. 0 | Page 11 of 20
AD7787
MODE REGISTER (RS1, RS0 = 0, 1; POWER-ON/RESET = 0x02)
The mode register is an 8-bit register from which data can be read or to which data can be written. This register is used to configure the ADC for unipolar or bipolar mode, to enable or disable the buffer, or to place the device into power-down mode. Table 9 outlines the bit designations for the mode register. MR0 through MR7 indicate the bit locations, MR denoting the bits are in the mode register. MR7 denotes the first bit of the data stream. The number in the parenthesis indicates the power-on/reset default status of that bit. Any write to the setup register resets the modulator and filter and sets the RDY bit.
MR7 MD1 (0)
MR6 MD0 (0)
MR5 0 (0)
MR4 0 (0)
MR3 BO (0)
MR2 U/B (0)
MR1 BUF (1)
MR0 0 (0)
Table 9. Mode Register Bit Designations
Bit Location MR7 to MR6 Bit Name MD1 to MD0 Description Mode Select Bits. These bits select between continuous conversion mode, single conversion mode, and standby mode. In continuous conversion mode, the ADC continuously performs conversions and places the result in the data register. RDY goes low when a conversion is complete. The user can read these conversions by placing the device in continuous read mode whereby the conversions are automatically placed on the DOUT line when SCLK pulses are applied. Alternatively, the user can instruct the ADC to output the conversion by writing to the communications register. After power-on, the first conversion is available after a period 2/fADC while subsequent conversions are available at a frequency of fADC. In single conversion mode, the ADC is placed in power-down mode when conversions are not being performed. When single conversion mode is selected, the ADC powers up and performs a single conversion, which occurs after a period 2/fADC. The conversion result is placed in the data register, RDY goes low, and the ADC returns to power-down mode. The conversion remains in the data register and RDY remains active (low) until the data is read or another conversion is performed (see Table 10). These bits must be programmed with a Logic 0 for correct operation. Burnout Current Enable Bit. When this bit is set to 1 by the user, the 100 nA current sources in the signal path are enabled. When BO = 0, the burnout currents are disabled. The burnout currents can be enabled only when the buffer is active. Unipolar/Bipolar Bit. Set by user to enable unipolar coding, i.e., zero differential input results in 0x000000 output and a full-scale differential input results in 0xFFFFFF output. Cleared by the user to enable bipolar coding. Negative full-scale differential input results in an output code of 0x000000, zero differential input results in an output code of 0x800000, and a positive full-scale differential input will result in an output code of 0xFFFFFF. Configures the AD7787 for buffered or unbuffered mode of operation. If cleared, the ADC operates in unbuffered mode, lowering the power consumption of the device. If set, the device operates in buffered mode, allowing the user to place source impedances on the front end without contributing gain errors to the system. This bit must be programmed with a Logic 0 for correct operation.
MR5 to MR4 MR3
0 BO
MR2
U/B
MR1
BUF
MR0
0
Table 10. Operating Modes
MD1 0 0 1 1 MD0 0 1 0 1 Mode Continuous Conversion Mode (Default) Reserved Single Conversion Mode Power-Down Mode
Rev. 0 | Page 12 of 20
AD7787
FILTER REGISTER (RS1, RS0 = 1, 0; POWER-ON/RESET = 0x04)
The filter register is an 8-bit register from which data can be read or to which data can be written. This register is used to set the output word rate. Table 11 outlines the bit designations for the filter register. FR0 through FR7 indicate the bit locations, FR denoting the bits are in the filter register. FR7 denotes the first bit of the data stream. The number in the parenthesis indicates the power-on/reset default status of that bit.
FR7 0 (0)
FR6 0 (0)
FR5 CDIV1 (0)
FR4 CDIV0 (0)
FR3 0 (0)
FR2 FS2 (1)
FR1 FS1 (0)
FR0 FS0 (0)
Table 11. Filter Register Bit Designations
Bit Location FR7 to FR6 FR5 to FR4 Bit Name 0 CLKDIV1 to CDIV0 Description These bits must be programmed with a Logic 0 for correct operation. These bits are used to operate the AD7787 in the lower power modes. The clock is internally divided and the power is reduced. In the low power modes, the update rates will scale with the clock frequency so that dividing the clock by 2 causes the update rate to be reduced by a factor of 2 also. 00 Normal Mode 01 Clock Divided by 2 10 Clock Divided by 4 11 Clock Divided by 8 This bit must be programmed with a Logic 0 for correct operation. These bits set the output word rate of the ADC. The update rate influences the 50 Hz/60 Hz rejection and the noise. Table 12 shows the allowable update rates when normal power mode is used. In the low power modes, the update rate is scaled with the clock frequency. For example, if the internal clock is divided by a factor of 2, the corresponding update rates are divided by 2 also.
FR3 FR2 to FR0
0 FS2 to FS0
Table 12. Update Rates
FS2 0 0 0 0 1 1 1 1 FS1 0 0 1 1 0 0 1 1 FS0 0 1 0 1 0 1 0 1 fADC (Hz) 120 100 33.3 20 16.6 16.7 13.3 9.5 f3dB (Hz) 28 24 8 4.7 4 4 3.2 2.3 RMS Noise (V) 40 25 3.36 1.6 1.5 1.5 1.2 1.1 Rejection 25 dB @ 60 Hz 25 dB @ 50 Hz 80 dB @ 60 Hz 65 dB @ 50 Hz/60 Hz (Default Setting) 80 dB @ 50 Hz 67 dB @ 50 Hz/60 Hz
DATA REGISTER (RS1, RS0 = 1, 1; POWER-ON/RESET = 0x000000)
The conversion result from the ADC is stored in this data register. This is a read-only register. On completion of a read operation from this register, the RDY bit/pin is set.
Rev. 0 | Page 13 of 20
AD7787 ADC CIRCUIT INFORMATION
OVERVIEW
The AD7787 is a low power ADC that incorporates an - modulator, a buffer, and an on-chip digital filter intended for the measurement of wide dynamic range, low frequency signals, such as those in pressure transducers, weigh scales, and temperature measurement applications. The part has one differential input and one single-ended input. The inputs can be operated in buffered or unbuffered mode. Buffering the input channel means that the part can accommodate significant source impedances on the analog input .The device requires an external reference of 2.5 nominal. Figure 12 shows the basic connections required to operate the part.
POWER SUPPLY 0.1F 10F
of 2.5 V. These numbers are typical and generated with a differential input voltage of 0 V. The peak-to-peak resolution figures represent the resolution for which there is no code flicker within a six-sigma limit. The output noise comes from two sources. The first is the electrical noise in the semiconductor devices (device noise) used in the implementation of the modulator. The second is quantization noise, which is added when the analog input is converted into the digital domain. The device noise is at a low level and is independent of frequency. The quantization noise starts at an even lower level but rises rapidly with increasing frequency to become the dominant noise source. Table 13. Typical Peak-to-Peak Resolution (Effective Resolution) vs. Update Rate
Update Rate 9.5 13.3 16.7 16.6 20 33.3 100 120 Peak-to-Peak Resolution 19.5 19 19 19 18.5 17.5 14.5 14 Effective Resolution 22 21.5 21.5 21.5 21 20 17 16.5
VDD REFIN IN+
AD7787
OUT- OUT+ AIN+ AIN- 10k AIN2 GND THERMISTOR
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CS DOUT/RDY MICROCONTROLLER SCLK DIN
IN-
REDUCED CURRENT MODES
The AD7787 has a current consumption of 160 A maximum when operated with a 5 V power supply, the buffer enabled, and the clock operating at its maximum speed. The clock frequency can be divided by a factor of 2, 4, or 8 before being applied to the modulator and filter, resulting in a reduction in the current consumption of the AD7787. Bits CDIV1 and CDIV0 in the filter register are used to enter these low power modes (see Table 14). When the internal clock is reduced, the update rate is also reduced. For example, if the filter bits are set to give an update rate of 16.6 Hz when the AD7787 is operated in full power mode, the update rate equals 8.3 Hz in divide-by-2 mode. In the low power modes, there may be some degradation in the ADC performance.
Figure 12. Basic Connection Diagram
The output rate of the AD7787 (fADC) is user programmable with the settling time equal to 2 x tADC. Normal mode rejection is the major function of the digital filter. Table 12 lists the available update rates from the AD7787. Simultaneous 50 Hz and 60 Hz rejection is optimized when the update rate equals 16.6 Hz as notches are placed at both 50 Hz and 60 Hz with this update rate (see Figure 6).
NOISE PERFORMANCE
Table 13 shows the output rms noise, rms resolution, and peakto-peak resolution (rounded to the nearest 0.5 LSB) for the different update rates and input ranges for the AD7787. The numbers given are for the bipolar input range with a reference
Table 14. Low Power Mode Selection
CDIV[1:0] 00 10 10 11 Clock 1 1/2 1/4 1/8 Typ Current, Buffered (A) 146 87 56 41 Typ Current, Unbuffered (A) 75 45 30 25 50 Hz/60 Hz Rejection (dB) 65 64 75 86
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AD7787
DIGITAL INTERFACE
As previously outlined, the AD7787's programmable functions are controlled using a set of on-chip registers. Data is written to these registers via the part's serial interface and read access to the on-chip registers is also provided by this interface. All communications with the part must start with a write to the communications register. After power-on or reset, the device expects a write to its communications register. The data written to this register determines whether the next operation is a read operation or a write operation and also determines to which register this read or write operation occurs. Therefore, write access to any of the other registers on the part begins with a write operation to the communications register followed by a write to the selected register. A read operation from any other register (except when continuous read mode is selected) starts with a write to the communications register followed by a read operation from the selected register. The AD7787's serial interface consists of four signals: CS, DIN, SCLK, and DOUT/RDY. The DIN line is used to transfer data into the on-chip registers while DOUT/RDY is used for accessing data from the on-chip registers. SCLK is the serial clock input for the device and all data transfers (either on DIN or DOUT/RDY) occur with respect to the SCLK signal. The DOUT/ RDY pin operates as a data-ready signal as well as a DOUT pin. Each time a conversion is available in the output register, DOUT/RDY goes low. DOUT/RDY resets high when a read operation from the data register is completed. It also goes high prior to the updating of the data register to indicate when not to read from the device to ensure that a data read is not attempted while the register is being updated. CS is used to select a device. It can be used to decode the AD7787 in systems where several components are connected to the serial bus. Figure 3 and Figure 4 show timing diagrams for interfacing to the AD7787 with CS being used to decode the part. Figure 3 shows the timing for a read operation from the AD7787's output shift register, while Figure 4 shows the timing for a write operation to the input shift register. In all modes, except continuous read mode, it is possible to read the same word from the data register several times even though the DOUT/RDY line returns high after the first read operation. However, care must be taken to ensure that the read operations have been completed before the next output update occurs. In continuous read mode, the data register can only be read once. The serial interface can operate in 3-wire mode by tying CS low. In this case, the SCLK, DIN, and DOUT/RDY lines are used to communicate with the AD7787. The end of the conversion can be monitored using the RDY bit in the status register. This scheme is suitable for interfacing to microcontrollers. If CS is required as a decoding signal, it can be generated from a port pin. For microcontroller interfaces, it is recommended that SCLK idle high between data transfers. The AD7787 can be operated with CS being used as a frame synchronization signal. This scheme is useful for DSP interfaces. In this case, the first bit (MSB) is effectively clocked out by CS, because CS would normally occur after the falling edge of SCLK in DSPs. The SCLK can continue to run between data transfers, provided the timing numbers are obeyed. The serial interface can be reset by writing a series of 1s to the DIN input. If a Logic 1 is written to the AD7787 line for at least 32 serial clock cycles, the serial interface is reset. In 3-wire systems, this ensures that the interface can be reset to a known state if the interface gets lost due to a software error or some glitch in the system. Reset returns the interface to the state in which it is expecting a write to the communications register. This operation resets the contents of all registers to their poweron values. The AD7787 can be configured to continuously convert or to perform a single conversion (see Figure 13 through Figure 15).
CS
DIN
0x10
0x82
0x10
0x82
DOUT/RDY
DATA
DATA
SCLK
Figure 13. Single Conversion
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AD7787
Single Conversion Mode
In single conversion mode, the AD7787 is placed in shutdown mode between conversions. When a single conversion is initiated by setting MD1 to 1 and MD0 to 0 in the mode register, the AD7787 powers up, performs a single conversion, and then returns to shutdown mode. When a single conversion is initiated, the AD7787's oscillator requires 1 ms to power up and settle. The AD7787 then performs a conversion which requires 2 x tADC. DOUT/RDY is high while the conversion is being performed and goes low to indicate the completion of the conversion. When the data word has been read from the data register, DOUT/RDY goes high. If CS is low, DOUT/RDY remains high until another conversion is initiated and completed. The data register can be read several times, if required, even when DOUT/RDY has gone high.
Continuous Conversion Mode
This is the default power-up mode. The AD7787 will continuously converts with the RDY pin in the status register going low each time a conversion is complete. If CS is low, the DOUT/RDY line also goes low when a conversion is complete. To read a conversion, the user can write to the communications register, indicating that the next operation is a read of the data register. The digital conversion is placed on the DOUT/RDY pin as soon as SCLK pulses are applied to the ADC. DOUT/RDY returns high when the conversion is read. The user can read this register additional times, if required. However, the user must ensure that the data register is not being accessed at the completion of the next conversion or the new conversion word is lost.
CS
0x38 DIN
0x38
DOUT/RDY
DATA
DATA
SCLK
Figure 14. Continuous Conversion
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AD7787
Continuous Read Mode
Rather than write to the communications register each time a conversion is complete to access the data, the AD7787 can be placed in continuous read mode. By writing 00111100 (Channel AIN1) or 00111101 (Channel AIN2) to the communications register, the user only needs to apply the appropriate number of SCLK cycles to the ADC, and the 24-bit word is automatically placed on the DOUT/RDY line when a conversion is complete. When DOUT/RDY goes low to indicate the end of a conversion, sufficient SCLK cycles must be applied to the ADC, and the data conversion is placed on the DOUT/RDY line. When the conversion is read, DOUT/RDY returns high until the next conversion is available. In this mode, the data can only be read once. Also, the user must ensure that the data-word is read before the next conversion is complete. If the user has not read the conversion before the completion of the next conversion, or if insufficient serial clocks are applied to the AD7787 to read the word, the serial output register is reset when the next conversion is complete, and the new conversion is placed in the output serial register. To exit the continuous read mode, the instruction 001110XX must be written to the communications register while the RDY pin is low. While in the continuous read mode, the ADC monitors activity on the DIN line so that it can receive the instruction to exit the continuous read mode. Additionally, a reset occurs if 32 consecutive 1s are seen on DIN. Therefore, DIN should be held low in continuous read mode until an instruction is written to the device.
CS
DIN
0x3C
DOUT/RDY
DATA
DATA
DATA
SCLK
Figure 15. Continuous Read
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AD7787 CIRCUIT DESCRIPTION
ANALOG INPUT CHANNEL
The AD7787 has two analog input channels that are connected to the on-chip buffer amplifier when the device is operated in buffered mode and directly to the modulator when the device is operated in unbuffered mode. In buffered mode (the BUF bit in the mode register is set to 1), the input channel feeds into a high impedance input stage of the buffer amplifier. Therefore, the input can tolerate significant source impedances and is tailored for direct connection to external resistive-type sensors, such as strain gauges or resistance temperature detectors (RTDs). When BUF = 0, the part is operated in unbuffered mode. This results in a higher analog input current. Note that this unbuffered input path provides a dynamic load to the driving source. Therefore, resistor/capacitor combinations on the input pins can cause dc gain errors, depending on the output impedance of the source that is driving the ADC input. Table 15 shows the allowable external resistance/capacitance values for the unbuffered mode such that no gain error at the 20-bit level is introduced. Table 15. External R-C Combination for No 20-Bit Gain Error
C (pF) 50 100 500 1000 5000 R () 16.7 k 9.6 k 2.2 k 1.1 k 160
The voltage on AIN2 is referenced to GND. Therefore, when bipolar mode is selected and the part is operated in unbuffered mode, the voltage on AIN2 can vary from GND - 100 mV to +2.5 V. In unipolar mode, the voltage on AIN2 can vary from 0 V to 2.5 V. The bipolar/unipolar option is chosen by programming the U/B bit in the mode register.
DATA OUTPUT CODING
When the ADC is configured for unipolar operation, the output code is natural (straight) binary with a zero differential input voltage resulting in a code of 00...00, a midscale voltage resulting in a code of 100...000, and a full-scale input voltage resulting in a code of 111...111. The output code for any analog input voltage can be represented as
Code = 2 N x (AIN /VREF ) When the ADC is configured for bipolar operation, the output code is offset binary with a negative full-scale voltage resulting in a code of 000...000, a zero differential input voltage resulting in a code of 100...000, and a positive full-scale input voltage resulting in a code of 111...111. The output code for any analog input voltage can be represented as Code = 2 N-1 x [(AIN /VREF ) + 1] where AIN is the analog input voltage and N = 24.
The absolute input voltage range in buffered mode is restricted to a range between GND + 100 mV and VDD - 100 mV. Care must be taken in setting up the common-mode voltage so that these limits are not exceeded. Otherwise, there is degradation in linearity and noise performance. The absolute input voltage in unbuffered mode includes the range between GND - 100 mV and VDD + 30 mV resulting from being unbuffered. The negative absolute input voltage limit does allow the possibility of monitoring small true bipolar signals with respect to GND.
REFERENCE INPUT
The AD7787 has a single-ended reference that is 2.5 V nominal, but the AD7787 is functional with reference voltages from 0.1 V to VDD. In applications where the excitation (voltage or current) for the transducer on the analog input also drives the reference voltage for the part, the effect of the low frequency noise in the excitation source is removed because the application is ratiometric. If the AD7787 is used in a nonratiometric application, a low noise reference should be used. Recommended 2.5 V reference voltage sources for the AD7787 include the ADR381 and ADR391, which are low noise, low power references. In a system that operates from a 2.5 V power supply, the reference voltage source requires some headroom. In this case, a 2.048 V reference, such as the ADR380 or ADR390, can be used, requiring only 300 mV of headroom. Also note that the reference input provides a high impedance, dynamic load. Because the input impedance of the reference input is dynamic, resistor/capacitor combinations on this input can cause dc gain errors, depending on the output impedance of the source driving the reference inputs.
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the AD7787 can accept either unipolar or bipolar input voltage ranges. Unipolar and bipolar signals on the AIN1(+) input are referenced to the voltage on the AIN(-) input. For example, if AIN1(-) is 2.5 V and the ADC is configured for unipolar mode, the input voltage range on the AIN1(+) pin is 2.5 V to 5 V when REFIN = 2.5 V. If the ADC is configured for bipolar mode, the analog input range on the AIN1(+) input is 0 V to 5 V.
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AD7787
Reference voltage sources like those previously recommended, e.g., ADR391, will typically have low output impedances and are, therefore, tolerant to having decoupling capacitors on REFIN without introducing gain errors in the system. Deriving the reference input voltage across an external resistor means that the reference input sees a significant external source impedance. External decoupling on the REFIN pin is not recommended in this type of circuit configuration. possible to the paths the currents took to reach their destinations. Avoid forcing digital currents to flow through the AGND sections of the layout. The AD7787's ground plane should be allowed to run under the AD7787 to prevent noise coupling. The power supply lines to the AD7787 should use as wide a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. Fast switching signals, such as clocks, should be shielded with digital ground to avoid radiating noise to other sections of the board, and clock signals should never be run near the analog inputs. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other. This reduces the effects of feedthrough through the board. A microstrip technique is by far the best, but it is not always possible with a double-sided board. In this technique, the component side of the board is dedicated to ground planes, while signals are placed on the solder side. Good decoupling is important when using high resolution ADCs. VDD should be decoupled with 10 F tantalum in parallel with 0.1 F capacitors to GND. To achieve the best from these decoupling components, they should be placed as close as possible to the device, ideally right up against the device. All logic chips should be decoupled with 0.1 F ceramic capacitors to DGND.
VDD MONITOR
Along with converting external voltages, the AD7787 can monitor the voltage on the VDD pin. When the CH1 and CH0 bits in the communications register are set to 1, the voltage on the VDD pin is internally attenuated by 5 and the resultant voltage is applied to the - modulator using an internal 1.17 V reference for the analog-to-digital conversion. This is useful because variations in the power supply voltage can be monitored.
GROUNDING AND LAYOUT
The digital filter provides rejection of broadband noise on the power supply, except at integer multiples of the modulator sampling frequency. The digital filter also removes noise from the analog and reference inputs, provided that these noise sources do not saturate the analog modulator. As a result, the AD7787 is more immune to noise interference than a conventional high resolution converter. However, because the resolution of the AD7787 is so high, and the noise levels from the AD7787 are so low, care must be taken with regard to grounding and layout. The printed circuit board that houses the AD7787 should be designed such that the analog and digital sections are separated and confined to certain areas of the board. A minimum etch technique is generally best for ground planes because it gives the best shielding. It is recommended that the AD7787's GND pin be tied to the AGND plane of the system. In any layout, it is important that the user keep in mind the flow of currents in the system, ensuring that the return paths for all currents are as close as
APPLICATIONS
Battery Monitoring
In battery monitoring, the battery current and voltage are measured. The current is passed through a 100 resistor. Because the current is from -200 A to +2000 A, the result is a voltage from -20 mV to +200 mV. Channel AIN1 of the AD7787 can be connected directly to the shunt resistor to measure this current. The battery voltage can vary from 12 V to 42 V with peaks up to 60 V. This voltage is attenuated using an external resistor network before being applied to the AD7787. The buffers onboard the AD7787 mean that channel AIN2 can be connected directly to the high impedance attenuator circuit without introducing gain errors.
GND
VDD
REFIN
AD7787
DOUT/RDY AIN1(+) RSHUNT AIN1(-) 100 AIN2 MUX - ADC SERIAL INTERFACE AND LOGIC CONTROL DIN SCLK CS
- + 12V OR 42V (60V PEAK) ATTENUATION CIRCUIT
-200A TO +2000A
Figure 16. Battery Monitoring
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AD7787 OUTLINE DIMENSIONS
3.00 BSC
10
6
3.00 BSC
1 5
4.90 BSC
PIN 1 0.50 BSC 0.95 0.85 0.75 0.15 0.00 0.27 0.17 COPLANARITY 0.10 COMPLIANT TO JEDEC STANDARDS MO-187BA 1.10 MAX 8 0 0.80 0.60 0.40
SEATING PLANE
0.23 0.08
Figure 17. 10-Lead Mini Small Outline Package [MSOP] (RM-10) Dimensions shown in millimeters
ORDERING GUIDE
Models AD7787BRM AD7787BRM-REEL EVAL-AD7787EB Temperature Range -40C to +105C -40C to +105C Package Description 10-Lead Mini Small Outline Package (MSOP) 10-Lead Mini Small Outline Package (MSOP) Evaluation Board Package Option RM-10 RM-10 Branding C1T C1T
(c) 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D04477-0-4/04(0)
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