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a
FEATURES 16-Bit Monotonicity over Temperature 2 LSBs Integral Linearity Error Microprocessor Compatible with Readback Capability Unipolar or Bipolar Output Multiplying Capability Low Power (100 mW Typical)
VREF + R
LC2MOS 16-Bit Voltage Output DAC AD7846
FUNCTIONAL BLOCK DIAGRAM
VCC VDD
AD7846
A2 16 SEGMENT SWITCH MATRIX
R R
RIN
R
A3 12-BIT DAC A1 12 DAC LATCH
VOUT
R VREF - 4
CS CONTROL LOGIC R/ W LDAC CLR
12 I/O LATCH
VSS
DB15 DB0
DGND
GENERAL DESCRIPTION
PRODUCT HIGHLIGHTS
The AD7846 is a 16-bit DAC constructed with Analog Devices' LC2MOS process. It has VREF+ and VREF- reference inputs and an on-chip output amplifier. These can be configured to give a unipolar output range (0 V to +5 V, 0 V to +10 V) or bipolar output ranges ( 5 V, 10 V). The DAC uses a segmented architecture. The 4 MSBs in the DAC latch select one of the segments in a 16-resistor string. Both taps of the segment are buffered by amplifiers and fed to a 12-bit DAC, which provides a further 12 bits of resolution. This architecture ensures 16-bit monotonicity. Excellent integral linearity results from tight matching between the input offset voltages of the two buffer amplifiers. In addition to the excellent accuracy specifications, the AD7846 also offers a comprehensive microprocessor interface. There are 16 data I/O pins, plus control lines (CS, R/W, LDAC and CLR). R/W and CS allow writing to and reading from the I/O latch. This is the readback function which is useful in ATE applications. LDAC allows simultaneous updating of DACs in a multiDAC system and the CLR line will reset the contents of the DAC latch to 00 . . . 000 or 10 . . . 000 depending on the state of R/W. This means that the DAC output can be reset to 0 V in both the unipolar and bipolar configurations. The AD7846 is available in 28-lead plastic, ceramic, and PLCC packages.
1. 16-Bit Monotonicity The guaranteed 16-bit monotonicity over temperature makes the AD7846 ideal for closed-loop applications. 2. Readback The ability to read back the DAC register contents minimizes software routines when the AD7846 is used in ATE systems. 3. Power Dissipation Power dissipation of 100 mW makes the AD7846 the lowest power, high accuracy DAC on the market.
REV. E
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 which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 World Wide Web Site: http://www.analog.com Fax: 781/326-8703 (c) Analog Devices, Inc., 2000
AD7846-SPECIFICATIONS
Parameter RESOLUTION UNIPOLAR OUTPUT Relative Accuracy @ +25C TMIN to TMAX Differential Nonlinearity Error Gain Error @ +25C TMIN to TMAX Offset Error @ +25C TMIN to TMAX Gain TC2 Offset TC2 BIPOLAR OUTPUT Relative Accuracy @ +25C TMIN to TMAX Differential Nonlinearity Error Gain Error @ +25C TMIN to TMAX Offset Error @ +25C TMIN to TMAX Bipolar Zero Error @ +25C TMIN to TMAX Gain TC2 Offset TC2 Bipolar Zero TC2 REFERENCE INPUT Input Resistance VREF+ Range VREF- Range OUTPUT CHARACTERISTICS Output Voltage Swing Resistive Load Capacitive Load Output Resistance Short Circuit Current DIGITAL INPUTS VIH (Input High Voltage) VIL (Input Low Voltage) IIN (Input Current) CIN (Input Capacitance)2 DIGITAL OUTPUTS VOL (Output Low Voltage) VOH (Output High Voltage) Floating State Leakage Current Floating State Output Capacitance2 POWER REQUIREMENTS3 VDD VSS VCC IDD ISS ICC Power Supply Sensitivity4 Power Dissipation J, A Versions 16 12 16 1 12 16 12 16 1 1 6 8 1 6 16 6 16 6 12 1 1 1 20 40 VSS + 6 to VDD - 6 VSS + 6 to VDD - 6 VSS + 4 to VDD - 3 2 1000 0.3 25 2.4 0.8 10 10 0.4 4.0 10 10 +11.4/+15.75 -11.4/-15.75 +4.75/+5.25 5 5 1 1.5 100
1 VOUT loaded with 2 k
K, B Versions 16 4 8 0.5 6 16 6 16 1 1 2 4 0.5 4 16 4 12 4 8 1 1 1 20 40 VSS + 6 to VDD - 6 VSS + 6 to VDD - 6 VSS + 4 to VDD - 3 2 1000 0.3 25 2.4 0.8 10 10 0.4 4.0 10 10 +11.4/+15.75 -11.4/-15.75 +4.75/+5.25 5 5 1 1.5 100 Bits
(VDD = +14.25 V to +15.75 V; VSS = -14.25 V to -15.75 V; VCC = +4.75 V to +5.25 V. , 1000 pF to 0 V; VREF+ = +5 V; RIN connected to 0 V. All specifications TMIN to TMAX, unless otherwise noted.)
Unit Test Conditions/Comments VREF - = 0 V, VOUT = 0 V to +10 V 1 LSB = 153 V All Grades Guaranteed Monotonic VOUT Load = 10 M
LSB typ LSB max LSB max LSB typ LSB max LSB typ LSB max ppm FSR/C typ ppm FSR/C typ LSB typ LSB max LSB max LSB typ LSB max LSB typ LSB max LSB typ LSB max ppm FSR/C typ ppm FSR/C typ ppm FSR/C typ k min k max Volts Volts
VREF- = -5 V, VOUT = -10 V to +10 V 1 LSB = 305 V All Grades Guaranteed Monotonic VOUT Load = 10 M VOUT Load = 10 M
Resistance from VREF+ to VREF - Typically 30 k
V max k min pF max typ mA typ V min V max A max pF max Volts max Volts min A max pF max V min/V max V min/V max V min/V max mA max mA max mA max LSB/V max mW typ ISINK = 1.6 mA ISOURCE = 400 A DB0-DB15 = 0 to VCC To 0 V To 0 V To 0 V or Any Power Supply
VOUT Unloaded VOUT Unloaded
VOUT Unloaded
NOTES 1 Temperature ranges as follows: J, K Versions: 0C to +70C; A, B Versions: -40C to +85C 2 Guaranteed by design and characterization, not production tested. 3 The AD7846 is functional with power supplies of 12 V. See Typical Performance Curves. 4 Sensitivity of Gain Error, Offset Error and Bipolar Zero Error to V DD, VSS variations. Specifications subject to change without notice.
-2-
REV. E
AD7846 AC PERFORMANCE CHARACTERISTICS
Parameter Output Settling Time
1
These characteristics are included for design guidance and are not subject to test. (VREF+ = +5 V; VDD = +14.25 V to +15.75 V; VSS = -14.25 V to -15.75 V; VCC = +4.75 V to +5.25 V; RIN connected to 0 V.)
Limit at TMIN to TMAX (All Versions) 6 9 7 70 0.5 10 50
Unit s max s max V/s typ nV-secs typ mV pk-pk typ nV-secs typ nV/Hz typ
Test Conditions/Comments To 0.006% FSR. VOUT loaded. VREF- = 0 V. Typically 3.5 s. To 0.003% FSR. VOUT loaded. VREF- = -5 V. Typically 6.5 s. DAC alternately loaded with 10 . . . 0000 and 01 . . . 1111. VOUT unloaded. VREF- = 0 V, VREF+ = 1 V rms, 10 kHz sine wave. DAC loaded with all 0s. DAC alternately loaded with all 1s and all 0s. CS High. Measured at VOUT. DAC loaded with 0111011 . . . 11. VREF+ = VREF- = 0 V.
Slew Rate Digital-to-Analog Glitch Impulse AC Feedthrough Digital Feedthrough Output Noise Voltage Density 1 kHz-100 kHz
NOTES 1 LDAC = 0. Settling time does not include deglitching time of 2.5 s (typ). Specifications subject to change without notice.
TIMING CHARACTERISTICS
Parameter t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 t12 0 60 0 60 0 120 10 60 0 70 0 70 130
(VDD = +14.25 V to +15.75 V; VSS = -14.25 V to -15.75 V; VCC = +4.75 V to +5.25 V)
Unit ns min ns min ns min ns min ns min ns max ns min ns max ns min ns min ns min ns min ns min Test Conditions/Comments R/W to CS Setup Time CS Pulsewidth (Write Cycle) R/W to CS Hold Time Data Setup Time Data Hold Time Data Access Time Bus Relinquish Time CLR Setup Time CLR Pulsewidth CLR Hold Time LDAC Pulsewidth CS Pulsewidth (Read Cycle)
Limit at TMIN to TMAX (All Versions)
NOTES 1 Timing specifications are sample tested at +25C to ensure compliance. All input control signals are specified with t R = tF = 5 ns (10% to 90% of +5 V) and timed from a voltage level of 1.6 V. 2 t6 is measured with the load circuits of Figure 1 and defined as the time required for an output to cross 0.8 V or 2.4 V. 3 t7 is defined as the time required for an output to change 0.5 V when loaded with the circuits of Figure 2. Specifications subject to change without notice.
5V 3k DBN 3k DGND 100pF DBN 100pF DGND
R/ W
t1
t3 t2
t1 t 12
t3
5V 0V 5V
a. High Z to VOH
b. High Z to VOL
CS
5V 3k
Figure 1. Load Circuits for Access Time (t6)
t4
DATA
DATA VALID
t5 t9 t 10 t8
t6
DATA VALID
t7 tt99 t 10 t 11
0V 5V
DBN 3k DGND 10pF
DBN 10pF DGND
t8
CLR
0V 5V 0V 5V 0V
a. VOH to High Z
b. VOL to High Z
LDAC
Figure 2. Load Circuits for Bus Relinquish Time (t7)
Figure 3. Timing Diagram
REV. E
-3-
AD7846
ABSOLUTE MAXIMUM RATINGS 1
VDD to DGND . . . . . . . . . . . . . . . . . . . . . . . . . -0.4 V to +17 V VCC to DGND . . . . . . . . . . . . . . . -0.4 V, VDD + 0.4 V or +7 V (Whichever Is Lower) VSS to DGND . . . . . . . . . . . . . . . . . . . . . . . . . +0.4 V to -17 V VREF+ to DGND . . . . . . . . . . . . . . . . VDD + 0.4 V, VSS - 0.4 V VREF- to DGND . . . . . . . . . . . . . . . . VDD + 0.4 V, VSS - 0.4 V VOUT to DGND2 . . . . . . . . VDD + 0.4 V, VSS - 0.4 V or 10 V (Whichever Is Lower) RIN to DGND . . . . . . . . . . . . . . . . . . VDD + 0.4 V, VSS - 0.4 V Digital Input Voltage to DGND . . . . . . -0.4 V to VCC + 0.4 V Digital Output Voltage to DGND . . . . . -0.4 V to VCC + 0.4 V Power Dissipation (Any Package) To +75C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1000 mW Derates above +75C . . . . . . . . . . . . . . . . . . . . . 10 mW/C Operating Temperature Range J, K Versions . . . . . . . . . . . . . . . . . . . . . . . . . 0C to +70C A, B Versions . . . . . . . . . . . . . . . . . . . . . . . -25C to +85C Storage Temperature Range . . . . . . . . . . . . -65C to +150C Lead Temperature (Soldering) . . . . . . . . . . . . . . . . . . +300C
NOTES 1 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 indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Only one Absolute Maximum Rating may be applied at any one time. 2 V OUT may be shorted to DGND, V DD, VSS, VCC provided that the power dissipation of the package is not exceeded.
ORDERING GUIDE
Model AD7846JN AD7846KN AD7846JP AD7846KP AD7846AP AD7846AQ AD7846BP
Temperature Range 0C to +70C 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -40C to +85C
Relative Accuracy 16 LSB 8 LSB 16 LSB 8 LSB 16 LSB 16 LSB 8 LSB
Package Description Plastic DIP Plastic DIP Plastic Leaded Chip Carrier (PLCC) Plastic Leaded Chip Carrier (PLCC) Plastic Leaded Chip Carrier (PLCC) Ceramic DIP Plastic Leaded Chip Carrier (PLCC)
Package Options N-28A N-28A P-28A P-28A P-28A Q-28 P-28A
CAUTION ESD (electrostatic discharge) sensitive device. The digital control inputs are diode protected; however, permanent damage may occur on unconnected devices subject to high energy electrostatic fields. Unused devices must be stored in conductive foam or shunts. The protective foam should be discharged to the destination socket before devices are removed. TERMINOLOGY LEAST SIGNIFICANT BIT Offset Error
WARNING!
ESD SENSITIVE DEVICE
This is the analog weighting of 1 bit of the digital word in a DAC. For the AD7846, 1 LSB = (VREF+ - VREF-)/216.
Relative Accuracy
This is the error present at the device output with all 0s loaded in the DAC. It is due to op amp input offset voltage and bias current and the DAC leakage current.
Bipolar Zero Error
Relative accuracy or endpoint nonlinearity is a measure of the maximum deviation from a straight line passing through the endpoints of the DAC transfer function. It is measured after adjusting for both endpoints (i.e., offset and gain errors are adjusted out) and is normally expressed in least significant bits or as a percentage of full-scale range.
Differential Nonlinearity
When the AD7846 is connected for bipolar output and 10 . . . 000 is loaded to the DAC, the deviation of the analog output from the ideal midscale of 0 V is called the bipolar zero error.
Digital-to-Analog Glitch Impulse
Differential nonlinearity is the difference between the measured change and the ideal change between any two adjacent codes. A specified differential nonlinearity of 1 LSB over the operating temperature range ensures monotonicity.
Gain Error
This is the amount of charge injected from the digital inputs to the analog output when the inputs change state. This is normally specified as the area of the glitch in either pA-secs or nV-secs depending upon whether the glitch is measured as a current or a voltage.
Multiplying Feedthrough Error
This is an ac error due to capacitive feedthrough from either of the VREF terminals to VOUT when the DAC is loaded with all 0s.
Digital Feedthrough
Gain error is a measure of the output error between an ideal DAC and the actual device output with all 1s loaded after offset error has been adjusted out. Gain error is adjustable to zero with an external potentiometer. -4-
When the DAC is not selected (i.e., CS is held high), high frequency logic activity on the digital inputs is capacitively coupled through the device to show up as noise on the VOUT pin. This noise is digital feedthrough. REV. E
AD7846
PIN CONFIGURATIONS Pin DIP
DB2
1 28 27 26 25 24
PIN FUNCTION DESCRIPTION Mnemonic DB2-DB0 VDD VOUT RIN Description Data I/O pins. DB0 is LSB. Positive supply for analog circuitry. This is +15 V nominal. DAC output voltage pin. Input to summing resistor of DAC output amplifier. This is used to select output voltage ranges. See Table I. VREF+ Input. The DAC is specified for VREF+ = +5 V. VREF- Input. For unipolar operation connect VREF- to 0 V and for bipolar operation connect it to -5 V. The device is specified for both conditions. Negative supply for the analog circuitry. This is -15 V nominal. Data I/O pins. DB15 is MSB. Ground pin for digital circuitry. Positive supply for digital circuitry. This is +5 V nominal. R/W input. This can be used to load data to the DAC or to read back the DAC latch contents. Chip select input. This selects the device. Clear input. The DAC can be cleared to 000 . . . 000 or 100 . . . 000. See Table II. Asynchronous load input to DAC. Data I/O pins.
1-3 4
DB3 DB4 DB5 LDAC CLR CS
DB1 2 DB0 3 VDD 4 VOUT 5 RIN 6 VREF+ 7
5 6
AD7846
23 22
R/W TOP VIEW VREF- 8 (Not to Scale) 21 VCC VSS 9 DB15 10 DB14 11 DB13 12 DB12 13 DB11 14
20 19 18 17 16 15
7 8
VREF+ VREF-
DGND DB6 DB7 DB8 DB9 DB10
9
VSS
10-19 DB15-DB6 20 DGND 21 VCC PLCC
DB0 DB1 DB2 DB3 DB4 DB5 VDD
22
R/W
4
3
2
1
28 27 26 PIN 1 IDENTIFIER 25 24
VOUT 5 RIN 6 VREF+ 7 VREF- 8 VSS 9 DB15 10 DB14 11
12 13 14 15
LDAC CLR CS R/W VCC DGND DB6
23 24 25
CS CLR LDAC
AD7846
TOP VIEW
(Not to Scale)
23 22 21 20 19
26-28 DB5-DB3
Table I. Output Voltage Ranges
16
17
18
DB13
DB12
DB11
DB10
DB9
DB8
DB7
Output Range 0 V to +5 V 0 V to +10 V +5 V to -5 V +5 V to -5 V +10 V to -10 V
VREF+ +5 V +5 V +5 V +5 V +5 V
VREF- 0V 0V -5 V 0V -5 V
RIN VOUT 0V VOUT +5 V 0V
REV. E
-5-
AD7846-Typical Performance Curves
8 VDD = +15V VSS = -15V VREF+ = +1Vrms VREF- = 0V
VOUT - V p-p 20 30 VDD = +15V VSS = -15V VREF+ = 5V SINE WAVE VREF- = 0V GAIN = +2
6
VOUT - mV p-p
4
10
2
0 102
103
104 105 FREQUENCY - Hz
106
0 101
102
103 104 105 FREQUENCY - Hz
106
107
Figure 4. AC Feedthrough. VREF+ = 1 V rms, 10 kHz Sine Wave
Figure 5. AC Feedthrough vs. Frequency
Figure 6. Large Signal Frequency Response
500
NOISE SPECTRAL DENSITY - nV/ Hz
400
VREF+ = VREF- = 0V GAIN = +1 DAC LOADED WITH ALL 1s
VOUT 50mV/DIV
VOUT
50mV/DIV
300
200
LDAC
5V/DIV
100
DATA 5V/DIV
DATA
5V/DIV
0 102
103
104 105 FREQUENCY - Hz
106
0.5 s/DIV
1 s/DIV
Figure 7. Noise Spectral Density
Figure 8. Digital-to-Analog Glitch Impulse Without Internal Deglitcher (10 . . . 000 to 011 . . . 111 Transition)
Figure 9. Digital-to-Analog Glitch Impulse With Internal Deglitcher (10 . . . 000 to 011 . . . 111 Transition)
Figure 10. Pulse Response (Large Signal)
Figure 11. Pulse Response (Small Signal)
Figure 12. Spectral Response of Digitally Constructed Sine Wave
-6-
REV. E
AD7846
4.0 3.5 3.0
INL - LSBs
1.0
TA = +25 C VREF+ = +5V VREF- = 0V GAIN = +1
DNL - LSBs
0.8
TA = +25 C VREF+ = +5V VREF- = 0V GAIN = +1
2.5 2.0 1.5
0.6
0.4
0.2
1.0 0.5 11 12 13 14 VDD/VSS - Volts 15 16
0 11 12 13 14 VDD/VSS - Volts 15 16
Figure 13. Typical Linearity vs. VDD/VSS
Figure 14. Typical Monotonicity vs. VDD/VSS
Table II. Control Logic Truth Table
CIRCUIT DESCRIPTION Digital Section
Figure 15 shows the digital control logic and on-chip data latches in the AD7846. Table II is the associated truth table. The D/A converter has two latches that are controlled by four signals: CS, R/W, LDAC and CLR. The input latch is connected to the data bus (DB15-DB0). A word is written to the input latch by bringing CS low and R/W low. The contents of the input latch may be read back by bringing CS low and R/W high. This feature is called "readback" and is used in system diagnostic and calibration routines. Data is transferred from the input latch to the DAC latch with the LDAC strobe. The equivalent analog value of the DAC latch contents appears at the DAC output. The CLR pin resets the DAC latch contents to 000 . . . 000 or 100 . . . 000, depending on the state of R/W. Writing a CLR loads 000 . . . 000 and reading a CLR loads 100 . . . 000. To reset a DAC to 0 V in a unipolar system the user should exercise CLR while R/W is low; to reset to 0 V in a bipolar system exercise the CLR while R/W is high.
R/W CLR
CS 1 0 0 X X X
R/W LDAC CLR X 0 1 X 0 1 X X X 0 X X X X X 1 0 0
Function 3-State DAC I/O Latch in HighZ State DAC I/O Latch Loaded with DB15-DB0 Contents of DAC I/O Latch Available on DB15-DB0 Contents of DAC I/O Latch Transferred to DAC Latch DAC Latch Loaded with 000 . . . 000 DAC Latch Loaded with 100 . . . 000
D/A Conversion
Figure 16 shows the D/A section of the AD7846. There are three DACs, each of which have their own buffer amplifiers. DAC1 and DAC2 are 4-bit DACs. They share a 16-resistor string but have their own analog multiplexers. The voltage reference is applied to the resistor string. DAC3 is a 12-bit voltage mode DAC with its own output stage. The 4 MSBs of the 16-bit digital code drive DAC1 and DAC2 while the 12 LSBs control DAC3. Using DAC1 and DAC2, the MSBs select a pair of adjacent nodes on the resistor string and present that voltage to the positive and negative inputs of DAC3. This DAC interpolates between these two voltages to produce the analog output voltage. To prevent nonmonotonicity in the DAC due to amplifier offset voltages, DAC1 and DAC2 "leap-frog" along the resistor string. For example, when switching from Segment 1 to Segment 2, DAC1 switches from the bottom of Segment 1 to the top of Segment 2 while DAC2 stays connected to the top of Segment 1. The code driving DAC3 is automatically complemented to compensate for the inversion of its inputs. This means that any linearity effects due to amplifier offset voltages remain unchanged when switching from one segment to the next and 16-bit monotonicity is ensured if DAC3 is monotonic. So, 12-bit resistor matching in DAC3 guarantees overall 16-bit monotonicity. This is much more achievable than the 16-bit matching which a conventional R-2R structure would have needed. -7-
DAC 16 DB15 RST DB15 SET DB14-DB0 RST DB15-DB0 LATCHES
LDAC
16 3-STATE I/O LATCH 16 DB15 DB0
CS
Figure 15. Input Control Logic
REV. E
AD7846
VREF+ SEGMENT 16 R
DAC1 S1 S3 S2 S4
DAC2
RIN
R DAC3 A1 12 BIT DAC A3 VOUT
S15 S17
S14 S16 A2 DB11-DB0
DB15-DB12
DB15-DB12 SEGMENT 1
VREF-
Figure 16. D/A Conversion
Output Stage
+15V +5V
The output stage of the AD7846 is shown in Figure 17. It is capable of driving a 2 k/1000 pF load. It also has a resistor feedback network which allows the user to configure it for gains of one or two. Table I shows the different output ranges that are possible. An additional feature is that the output buffer is configured as a track-and-hold amplifier. Although normally tracking its input, this amplifier is placed in a hold mode for approximately 2.5 s after the leading edge of LDAC. This short state keeps the DAC output at its previous voltage while the AD7846 is internally changing to its new value. So, any glitches that occur in the transition are not seen at the output. In systems where the LDAC is tied permanently low, the deglitching will not be in operation. Figures 8 and 9 show the outputs of the AD7846 without and with the deglitcher.
RIN 10k
C1 1F
4 VDD VREF+ R1 10k
VCC VOUT VOUT (0V TO +10V)
AD586
AD7846*
VREF- VSS
RIN DGND
SIGNAL GROUND *ADDITIONAL PINS OMITTED FOR CLARITY
-15V
Figure 18. Unipolar Binary Operation
Table III. Code Table for Figure 18
10k C1
Binary Number in DAC Latch MSB LSB 1111 1111 1111 1111 1000 0000 0000 0000 0000 0000 0000 0001 0000 0000 0000 0000
Analog Output (VOUT) +10 (65535/65536) V +10 (32768/65536) V +10 (1/65536) V 0
VOUT DAC3
ONE SHOT
NOTE 1 LSB = 10 V/216 = 10 V/65536 = 152 V.
LDAC
Figure 17. Output Stage
UNIPOLAR BINARY OPERATION
Figure 18 shows the AD7846 in the unipolar binary circuit configuration. The DAC is driven by the AD586, +5 V reference. Since RIN is tied to 0 V, the output amplifier has a gain of 2 and the output range is 0 V to +10 V. If a 0 V to +5 V range is required, RIN should be tied to VOUT, configuring the output stage for a gain of 1. Table III gives the code table for the circuit of Figure 18.
Offset and gain may be adjusted in Figure 18 as follows: To adjust offset, disconnect the VREF- input from 0 V, load the DAC with all 0s and adjust the VREF- voltage until VOUT = 0 V. For gain adjustment, the AD7846 should be loaded with all 1s and R1 adjusted until VOUT = 10 (65535)/(65536) = 9.999847 V. If a simple resistor divider is used to vary the VREF- voltage, it is important that the temperature coefficients of these resistors match that of the DAC input resistance (-300 ppm/C). Otherwise, extra offset errors will be introduced over temperature. Many circuits will not require these offset and gain adjustments. In these circuits, R1 can be omitted. Pin 5 of the AD586 may be left open circuit and Pin 8 (VREF- ) of the AD7846 tied to 0 V.
-8-
REV. E
AD7846
BIPOLAR OPERATION Other Output Voltage Ranges
Figure 19 shows the AD7846 set up for 10 V bipolar operation. The AD588 provides precision 5 V tracking outputs which are fed to the VREF+ and VREF- inputs of the AD7846. The code table for Figure 19 is shown in Table IV.
+15V R1 39k 4 VDD VREF+ VCC VOUT VOUT (-10V TO +10V) +15V +5V
In some cases, users may require output voltage ranges other than those already mentioned. One example is systems which need the output voltage to be a whole number of millivolts (i.e., 1 mV, 2 mV, etc.). If the AD689 (8.192 V reference) is used with the AD7846 as in Figure 20, then the LSB size is 125 V. This makes it possible to program whole millivolt values at the Output. Table V shows the code table for Figure 20.
+15V +5V
+15V C1 1F R2 10k
AD588
VDD
VCC VOUT
AD7846*
RIN VREF- -15V VSS DGND SIGNAL GROUND
AD689
VREF+
VOUT (0V TO 8.192V)
AD7846*
VREF- SIGNAL GROUND
RIN
VSS
DGND
R3 100k -15V
*ADDITIONAL PINS OMITTED FOR CLARITY
*ADDITIONAL PINS OMITTED FOR CLARITY
-15V
Figure 19. Bipolar 10 V Operation
Table IV. Offset Binary Code Table for Figure 19
Figure 20. Unipolar Output with AD689
Table V. Code Table for Figure 20
Binary Number in DAC Latch MSB LSB 1111 1111 1111 1111 1000 0000 0000 0001 1000 0000 0000 0000 0111 1111 1111 1111 0000 0000 0000 0000
Analog Output (VOUT) +10 (32767/32768) V +10 (1/32768) V 0V -10 (1/32768) V -10 (32768/32768) V
Binary Number in DAC Latch MSB LSB 1111 1111 1111 1111 1000 0000 0000 0000 0000 0000 0000 1000 0000 0000 0000 0100 0000 0000 0000 0010 0000 0000 0000 0001
NOTE 1 LSB = 8.192 V/2l6 = 125 V.
Analog Output (VOUT) 8.192 V (65535/65536) = 8.1919 V 8.192 V (32768/65536) = 4.096 V 8.192 V (8/65536) = 0.001 V 8.192 V (4/65536) = 0.0005 V 8.192 V (2/65536) = 0.00025 V 8.192 V (1/65536) = 0.000125 V
NOTE 1 LSB = 10 V/215 = 10 V/32768 = 305 V.
Full scale and bipolar zero adjustment are provided by varying the gain and balance on the AD588. R2 varies the gain on the AD588 while R3 adjusts the +5 V and -5 V outputs together with respect to ground. For bipolar zero adjustment on the AD7846, load the DAC with 100 . . . 000 and adjust R3 until VOUT = 0 V. Full scale is adjusted by loading the DAC with all 1s and adjusting R2 until VOUT = 9.999694 V. When bipolar zero and full scale adjustment are not needed, R2 and R3 can be omitted, Pin 12 on the AD588 should be connected to Pin 11 and Pin 5 should be left floating. If a user wants a +5 V output range, there are two choices. By tying Pin 6 (RIN) of the AD7846 to VOUT (Pin 5), the output stage gain is reduced to unity and the output range is 5 V. If only a positive +5 V reference is available, bipolar 5 V operation is still possible. Tie VREF- to 0 V and connect RIN to VREF+. This will also give a 5 V output range. However, the linearity, gain, and offset error specifications will be the same as the unipolar 0 V to +5 V range.
Multiplying Operation
The AD7846 is a full multiplying DAC. To get four-quadrant multiplication, tie VREF- to 0 V, apply the ac input to VREF+ and tie RIN to VREF+. Figure 6 shows the Large Signal Frequency Response when the DAC is used in this fashion.
REV. E
-9-
AD7846
TEST APPLICATION
Figure 21 shows the AD7846 in an Automatic Test Equipment application. The readback feature of the AD7846 is very useful in these systems. It allows the designer to eliminate phantom memory used for storing DAC contents and increases system reliability since the phantom memory is now effectively on chip with the DAC. The readback feature is used in the following manner to control a data transfer. First, write the desired 16-bit word to the DAC input latch using the CS and R/W inputs. Verify that correct data has been received by reading back the latch contents. Now, the data transfer can be completed by bringing the asynchronous LDAC control line low. The analog equivalent of the digital word now appears at the DAC output. In Figure 21, each pin on the Device Under Test can be an
input or output. The AD345 is the pin driver for the digital inputs, and the AD9687 is the receiver for the digital outputs. The digital control circuitry determines the signal timing and format. DACs 1 and 2 set the pin driver voltage levels (VH and VL), and DACs 3 and 4 set the receiver voltage levels. The pin drivers used in ATE systems normally have a nonlinearity between input and output. The 16-bit resolution of the AD7846 allows compensation for these input/output nonlinearities. The dc parametrics shown in Figure 21 measure the voltage at the device pin and feed this back to the system processor. The pin voltage can thus be fine-tuned by incrementing or decrementing DACs 1 and 2 under system processor control.
DC PARAMETRICS
VH STORED DATA AND INHIBIT PATTERN PERIOD GENERATION AND DELAY COMPARE DATA AND DON'T CARE DATA +15V R1 39k DAC1 DAC2 DAC3 DAC4 COMPARE REGISTER D D INH INH VL
FORMATTER
AD345
DUT
AD9687
AD7846
VOUT VREF+
AD7846
VOUT VREF+
AD7846
VOUT VREF+
AD7846
VOUT VREF+
AD588
RIN VREF- DGND DB15 DB0
RIN VREF- DGND DB15 DB0
RIN VREF- DGND DB15 DB0
RIN VREF- DGND DB15 DB0
-15V
Figure 21. Digital Test System with 16-Bit Performance
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REV. E
AD7846
POSITION MEASUREMENT APPLICATION
Figure 22 shows the AD7846 in a position measurement application using an LVDT (Linear Variable Displacement Transducer), an AD630 synchronous demodulator and a comparator to make a 16-bit LVDT-to-Digital Converter. The LVDT is excited with a fixed frequency and fixed amplitude sine wave (usually 2.5 kHz, 2 V pk-pk). The outputs of the secondary coil are in antiphase and their relative amplitudes depend on the position of the core in the LVDT. The AD7846 output interpolates between these two inputs in response to the DAC input code. The AD630 is set up so that it rectifies the DAC output signal. Thus, if the output of the DAC is in phase with the VREF+ input, the inverting input to the comparator will be positive, and if it is in phase with VREF-, the output will be negative. By turning on each bit of the DAC in succession starting with the MSB, and deciding to leave it on or turn it off based on the comparator output, a 16-bit measurement of the core position is obtained.
ASIN t LVDT x ASIN t VREF+ VOUT RIN
In a multiple DAC system, the double buffering of the AD7846 allows the user to simultaneously update all DACs. In Figure 24, a 16-bit word is loaded to the input latches of each of the DACs in sequence. Then, with one instruction to the appropriate address, CS4 (i.e., LDAC) is brought low, updating all the DACs simultaneously.
ADDRESS BUS
ADDRESS DECODE ALE 16-BIT LATCH
CS
AD7846*
LDAC
8086
DEN RD WR AD0-AD15 DATA BUS
R/W CLR DB0-DB15 +5V
CS
AD7846*
LDAC R/W CLR DB0-DB15
SIGNAL GROUND
AD7846*
-(1-x)ASIN t VREF- DGND DB15 DB0
+5V
*ADDITIONAL PINS OMITTED FOR CLARITY
CS
PROCESSOR DATA BUS
AD7846*
LDAC R/W
R1 100k C1 1F
AD630*
*LINEAR CIRCUITRY OMITTED FOR CLARITY
CLR DB0-DB15
+5V
TO PROCESSOR PORT
Figure 24. AD7846-to-8086 Interface: Multiple DAC System
AD7846-to-MC68000 Interface
Figure 22. AD7846 in Position Measurement Application
MICROPROCESSOR INTERFACING AD7846-to-8086 Interface
Interfacing between the AD7846 and MC68000 is accomplished using the circuit of Figure 25. The following routine writes data to the DAC latches and then outputs the data via the DAC latch. 1000 MOVE.W #W, D0 The desired DAC data, W, is loaded into Data Register 0. W may be any value between 0 and 65535 (decimal) or 0 and FFFF (hexadecimal). The data, W, is transferred between D0 and the DAC register. Control is returned to the System Monitor using these two instructions.
Figure 23 shows the 8086 16-bit processor interfacing to the AD7846. The double buffering feature of the DAC is not used in this circuit since LDAC is permanently tied to 0 V. AD0- AD15 (the 16-bit data bus) are connected to the DAC data bus (DB0-DB15). The 16-bit word is written to the DAC in one MOV instruction and the analog output responds immediately. In this example, the DAC address is D000H.
ADDRESS BUS
MOVE.W D0, $E000
ADDRESS DECODE ALE 16-BIT LATCH +5V
CS LDAC CLR
MOVE.W #228, D7 TRAP #14
8086
DEN RD WR AD0-AD15 DATA BUS *LINEAR CIRCUITRY OMITTED FOR CLARITY
AD7846*
R/W
DB0-DB15
Figure 23. AD7846-to-8086 Interface Circuit
REV. E
-11-
AD7846
A1-A23 ADDRESS BUS
ANALOG SUPPLY +15V 0V -15V
CS +5V CLR LDAC
DIGITAL SUPPLY +5V DGND
MC68000
DS DTACK ADDRESS DECODE
AD7846*
R/W D0-D15 DATA BUS *LINEAR CIRCUITRY OMITTED FOR CLARITY R/W DB0-DB15
R1 SIGNAL GROUND
R2
Figure 25. AD7846-to-MC68000 Interface
DIGITAL FEEDTHROUGH
AD588*
R3
AD7846*
R4 RL
VOUT (+5V TO -5V)
In the preceding interface configurations, most digital inputs to the AD7846 are directly connected to the microprocessor bus. Even when the device is not selected, these inputs will be constantly changing. The high frequency logic activity on the bus can feed through the DAC package capacitance to show up as noise on the analog output. To minimize this Digital Feedthrough isolate the DAC from the noise source. Figure 26 shows an interface circuit which isolates the DAC from the bus.
R5 *ADDITIONAL PINS OMITTED FOR CLARITY
Figure 27. AD7846 Grounding
A1-A15
ADDRESS BUS
ADDRESS DECODE MICROPROCESSOR +5V
CS CLR LDAC
R1 to R5 represent lead and track resistances on the printed circuit board. R1 is the resistance between the Analog Power Supply ground and the Signal Ground. Since current flowing in R1 is very low (bias current of AD588 sense amplifier), the effect of R1 is negligible. R2 and R3 represent track resistance between the AD588 outputs and the AD7846 reference inputs. Because of the Force and Sense outputs on the AD588, these resistances will also have a negligible effect on accuracy. R4 is the resistance between the DAC output and the load. If RL is constant, then R4 will introduce a gain error only which can be trimmed out in the calibration cycle. R5 is the resistance between the load and the analog common. If the output voltage is sensed across the load, R5 will introduce a further gain error which can be trimmed out. If, on the other hand, the output voltage is sensed at the analog supply common, R5 appears as part of the load and therefore introduces no errors.
Printed Circuit Board Layout
R/W DIR DATA BUS B BUS G A BUS
R/W
AD7846*
DB0-DB15
D0-D15
2 74LS245 *LINEAR CIRCUITRY OMITTED FOR CLARITY
Figure 26. AD7846 Interface Circuit Using Latches to Minimize Digital Feedthrough
Note that to make use of the AD7846 readback feature using the isolation technique of Figure 26, the latch needs to be bidirectional.
APPLICATION HINTS Noise
Figure 28 shows the AD7846 in a typical application with the AD588 reference, producing an output analog voltage in the 10 volts range. Full scale and bipolar zero adjustment are provided by potentiometers R2 and R3. Latches (2 x 74LS245) isolate the DAC digital inputs from the active microprocessor bus and minimize digital feedthrough. The printed circuit board layout for Figure 28 is shown in Figures 29 and 30. Figure 29 is the component side layout while Figure 30 is the solder side layout. The component overlay is shown in Figure 31. In the layout, the general grounding guidelines given in Figure 27 are followed. The AD588 and AD7846 are as close as possible, and the decoupling capacitors for these are also kept as close to the device pins as possible.
In high resolution systems, noise is often the limiting factor. With a 10 volt span, a 16-bit LSB is 152 V (-96 dB). Thus, the noise floor must stay below -96 dB in the frequency range of interest. Figure 7 shows the noise spectral density for the AD7846.
Grounding
As well as noise, the other prime consideration in high resolution DAC systems is grounding. With an LSB size of 152 V and a load current of 5 mA, 1 LSB of error can be introduced by series resistance of only 0.03 . Figure 27 below shows recommended grounding for the AD7846 in a typical application.
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REV. E
AD7846
+15V J1 C1 10 F C5 10 F C6 0.1 F C7 0.1 F DB15 C2 0.1 F DB14 DB13 C12 1F VREF+ DB12 DB11 DB10 2 3 4 5 6 7 8 9 10 1 +5V VREF- -15V DB7 18 DB6 19 VSS DB5 26 DB4 27 DB3 28 DB2 1 R3 100k DGND DB1 2 DB0 3 RIN 2 3 4 5 6 7 8 9 10 1 19 74LS245 20 18 17 16 15 14 13 12 11 C12/A12 C13/A13 C14/A14 C15/A15 C16/A16 C17/A17 C18/A18 C19/A19 C20/A20 C21/A21 C22/A22 C23/A23 C32/A32 19 74LS245 20 18 17 16 15 14 13 12 11 C4/A4 C5/A5 C6/A6 C7/A7 C8/A8 C9/A9 C10/A10 C11/A11 +5V C31/A31
R1 39k
AD588
R2 100k
AD7846
DB9 DB8
C4 0.1 F
C3 10 F
R/W CS
VOUT (+10V TO -10V)
VOUT
CLR LDAC
Figure 28. Schematic for AD7846 Board
REV. E
-13-
AD7846
Figure 29. PCB Component Side Layout for Figure 28
Figure 30. PCB Solder Side Layout for Figure 30
-14-
REV. E
AD7846
Figure 31. Component Overlay for Circuit of Figure 28
REV. E
-15-
AD7846
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
28-Lead Ceramic DIP (Q-28)
0.005 (0.13) MIN
28
0.100 (2.54) MAX
15
0.610 (15.49) 0.500 (12.70)
1 14
PIN 1 1.490 (37.85) MAX 0.225 (5.72) MAX
0.015 (0.38) MIN 0.150 (3.81) MIN 15 0
0.620 (15.75) 0.590 (14.99)
0.018 (0.46) 0.008 (0.20)
0.200 (5.08) 0.026 (0.66) 0.110 (2.79) 0.070 (1.78) SEATING 0.125 (3.18) 0.014 (0.36) 0.090 (2.29) 0.030 (0.76) PLANE
28-Lead Plastic DIP (N-28A)
1.450 (36.83) 1.440 (35.576)
28 15
0.550 (13.97) 0.530 (13.462)
1 14
PIN 1 0.200 (5.080) MAX SEATING PLANE
0.160 (4.06) 0.140 (3.56)
0.606 (15.39) 0.594 (15.09)
0.020 (0.508) 0.015 (0.381)
0.100 (2.54) BSC
0.065 (1.65) 0.045 (1.14)
15 0
0.012 (0.306) 0.008 (0.203)
28-Lead Plastic Leaded Chip Carrier (PLCC) (P-28A)
0.180 (4.57) 0.165 (4.19)
0.048 (1.21) 0.042 (1.07)
4 5 PIN 1 IDENTIFIER
26 25
0.021 (0.53) 0.013 (0.33) 0.050 (1.27) BSC 0.032 (0.81) 0.026 (0.66) 0.430 (10.92) 0.390 (9.91)
TOP VIEW
(PINS DOWN) 11 12 19 18
0.020 (0.50) R
0.456 (11.58) SQ 0.450 (11.43) 0.495 (12.57) SQ 0.485 (12.32)
0.040 (1.01) 0.025 (0.64) 0.110 (2.79) 0.085 (2.16)
-16-
REV. E
PRINTED IN U.S.A.
0.048 (1.21) 0.042 (1.07)
0.056 (1.42) 0.042 (1.07)
0.025 (0.63) 0.015 (0.38)
C1245c-1-6/00 (rev. E) 01201

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