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ADS931
ADS 931 E
SBAS060A - MAY 2001
8-Bit, 33MHz Sampling ANALOG-TO-DIGITAL CONVERTER
TM
FEATURES
q +2.7V TO +5.5V SUPPLY OPERATION q LOW POWER: 69mW at +3V q ADJUSTABLE FULL SCALE RANGE WITH EXTERNAL REFERENCE q NO MISSING CODES q POWER DOWN q SSOP-28 PACKAGE
DESCRIPTION
The ADS931 is a high-speed pipelined Analog-to-Digital (A/D) converter that is specified to operate from standard +5V or +3V power supplies. This converter includes a high bandwidth track/hold and an 8-bit quantizer. The performance is specified with a single-ended input range of 1V to 2V when operating off of a +3V supply. This device also allows for standard input ranges such as 2V to 4V or 2V to 3V, when operating on +5V supplies. The full scale input range is set by an external reference. The ADS931 employs digital error correction techniques to provide excellent differential linearity for demanding imaging applications. Its low distortion and high SNR give the extra margin needed for telecommunications, video and test instrumentation applications. The ADS931 is available in an SSOP-28 package.
APPLICATIONS
q q q q q BATTERY POWERED EQUIPMENT CAMCORDERS PORTABLE TEST EQUIPMENT DIGITAL CAMERAS COMMUNICATIONS
CLK ADS931 Timing Circuitry
LVDD
IN
T/H
Pipeline A/D
Error Correction Logic
3-State Outputs
8-Bit Digital Data
Reference Ladder
REFT CM REFB
Pwrdn
OE
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
PRODUCTION DATA information is current as of publication date. Products conform to specifications per the terms of Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters.
Copyright (c) 1996, Texas Instruments Incorporated
www.ti.com
ABSOLUTE MAXIMUM RATINGS
+VS ....................................................................................................... +6V Analog Input ........................................................... (-0.3V) to (+VS +0.3V) Logic Input ............................................................. (-0.3V) to (+VS +0.3V) Case Temperature ......................................................................... +100C Junction Temperature .................................................................... +150C Storage Temperature ..................................................................... +150C
ELECTROSTATIC DISCHARGE SENSITIVITY
This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet its published specifications.
PACKAGE/ORDERING INFORMATION
PACKAGE DRAWING NUMBER 324 SPECIFIED TEMPERATURE RANGE -40C to +85C PACKAGE MARKING ADS931E ORDERING NUMBER(1) ADS931E ADS931E/1K TRANSPORT MEDIA Rails Tape and Reel
PRODUCT ADS931E
PACKAGE SSOP-28 Surface Mount
"
"
"
"
"
NOTES: (1) Models with a slash (/) are available only in Tape and Reel in the quantities indicated (e.g., /1K indicates 1000 devices per reel). Ordering 1000 pieces of "ADS931E/1K" will get a single 1000-piece Tape and Reel.
ELECTRICAL CHARACTERISTICS
At TA = full specified temperature range unless otherwise noted. +VS = LVDD = +3V, specified single-ended input (1V to 2V) and sampling rate = 30MHz, unless otherwise specified. The input range is 2.25V to 3.25V when specified for +5V operation. ADS931E PARAMETER RESOLUTION SPECIFIED TEMPERATURE RANGE ANALOG INPUT Specified Full Scale Input Range(1) Common-mode Voltage Analog Input Bias Current Input Impedance DIGITAL INPUTS Logic Family High Input Voltage, VIH Low Input Voltage, VIL High Input Current, IIH Low Input Current, IIL Input Capacitance CONVERSION CHARACTERISTICS Start Conversion Sample Rate Data Latency DYNAMIC CHARACTERISTICS Differential Linearity Error f = 500kHz f = 12.5MHz No Missing Codes Integral Nonlinearity Error, f = 500kHz Spurious Free Dynamic Range(2) f = 500kHz (-1dBFS input) f = 12.5MHz (-1dBFS input) Two-Tone Intermodulation Distortion(4) f = 3.6MHz and 3.5MHz (-7dBFS each tone) Signal-to-Noise Ratio (SNR) f = 500kHz (-1dBFS input) f = 12.5MHz (-1dBFS input) Signal-to-(Noise + Distortion) (SINAD) f = 500kHz (-1dBFS input) f = 12.5MHz (-1dBFS input) Effective Number of Bits Differential Gain Error Differential Phase Error VS = +3V and +5V Ambient Air 1Vp-p +1.0 +1.5 1 1.25 || 5 TTL/HCT Compatible CMOS +2.0 10 10 5 Rising Edge of Convert Clock 10k 5 33M Samples/s Clk Cycles LVDD +0.8 V V A A pF CONDITIONS MIN TYP 8 -40 to +85 +2.0 MAX UNITS Bits C V V A M || pF
VS = +3V and +5V VS = +3V and +5V VS = +3V and +5V 43
0.7 0.7 Guaranteed 1.0 49 49 54
1.0
LSB LSB LSB dBFS(3) dBFS dBFS dB dB dB dB Bits % degrees
2.5
VS = +3V and +5V 44 VS = +3V and +5V 40 f = 12MHz(5) NTSC, PAL NTSC, PAL 45 45 7.2 2.3 1 48 48
2
ADS931
SBAS060A
ELECTRICAL CHARACTERISTICS (Cont.)
At TA = full specified temperature range unless otherwise noted. +VS = LVDD = +3V, specified single-ended input (1V to 2V) and sampling rate = 30MHz, unless otherwise specified. The input range is 2.25V to 3.25V when specified for +5V operation. ADS931E PARAMETER Output Noise Aperture Delay Time Aperture Jitter Analog Input Bandwidth Small Signal Full Power Overvoltage Recovery Time(6) DIGITAL OUTPUTS Logic Family Logic Coding High Output Voltage, VOH Low Output Voltage, VOL 3-State Enable Time 3-State Disable Time Internal Pull-Down Power-Down Enable Time Power-Down Disable Time Internal Pull-Down ACCURACY Gain Error Input Offset(7) Power Supply Rejection (Gain) External REFT Voltage Range External REFB Voltage Range Reference Input Resistance POWER SUPPLY REQUIREMENTS Supply Voltage: +VS Supply Current: +IS Power Dissipation Power Dissipation (Power Down) Thermal Resistance, JA SSOP-28 CONDITIONS Input AC-Grounded MIN TYP 0.2 2 7 350 100 2 TTL/HCT Compatible CMOS Straight Offset Binary 2.4 OE = L OE = H Pwrdn = L Pwrdn = H fS = 2.5MHz, VS = +3V and +5V Referred to Ideal Midscale VS = +10% REFB +0.5 0.8 2.4 6.5 75 2 1 3.5 25 VS-0.8 REFT -0.5 4 +5.5 29 87 %FS mV dB V V k V mA mW mW mW mW C/W 20 2 50 133 18 50 VDD 0.4 40 10 V V ns ns k ns ns k MAX UNITS LSBs rms ns ps rms MHz MHz ns
-20dBFS Input 0dBFS Input CL = 15pF
Operating VS = +3V VS = +3V VS = +5V VS = +3V VS = +5V
+2.7
+3.0 23 69 154 10 15 89
NOTES: (1) The single-ended input range is set by REFB and REFT values. (2) Spurious Free Dynamic Range refers to the magnitude of the largest harmonic. (3) dBFS is dB relative to full scale. (4) Two-tone intermodulation distortion is referred to as the largest fundamental tone. (5) Based on (SINAD -1.76)/6.02. (6) No "Rollover" of bits. (7) Offset deviation from ideal negative full scale.
ADS931
SBAS060A
3
PIN CONFIGURATION
Top View SSOP
PIN DESCRIPTIONS
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 DESIGNATOR +VS LVDD NC NC Bit 8 (LSB) Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 (MSB) GND GND CLK OE Pwrdn +VS GND GND LpBy REFT NC REFB LnBy CM IN +VS DESCRIPTION Analog Supply Output Logic Driver Supply Voltage No Connection No Connection Data Bit 8 (D7) LSB Data Bit 7 (D6) Data Bit 6 (D5) Data Bit 5 (D4) Data Bit 4 (D3) Data Bit 3 (D2) Data Bit 2 (D1) Data Bit 1 (D0) MSB Analog Ground Analog Ground Convert Clock Input Output Enable, Active Low Power Down Pin Analog Supply Analog Ground Analog Ground Positive Ladder Bypass Reference Voltage Top No Connection Reference Voltage Bottom Negative Ladder Bypass Common-Mode Pin Analog Input Analog Supply
+VS LVDD NC NC Bit 8 (LSB) Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 (MSB) GND GND
1 2 3 4 5 6 7 ADS931 8 9 10 11 12 13 14
28 27 26 25 24 23 22 21 20 19 18 17 16 15
+VS IN CM LnBy REFB NC REFT LpBy GND GND +VS Pwrdn OE CLK
TIMING DIAGRAM
N+1 Analog In N tD Clock tCONV
N+2 N+3
N+4 N+5 tL tH N+6
N+7
5 Clock Cycles t2 Data Out N-5 N-4 N-3 N-2 N-1 N t1 N+1 N+2
Data Invalid
SYMBOL tCONV tL tH tD t1 t2
DESCRIPTION Convert Clock Period Clock Pulse Low Clock Pulse High Aperture Delay Data Hold Time, CL = 0pF New Data Delay Time, CL = 15pF max
MIN 30 14 14 3.9
TYP
MAX 100s
UNITS ns ns ns ns ns ns
15 15 2 12
4
ADS931
SBAS060A
TYPICAL CHARACTERISTICS
At TA = +25C, VS = +3V, specified single-ended input (-1dBFS) and sampling rate = 30MHz, unless otherwise specified.
SPECTRAL PERFORMANCE 0 fIN = 500kHz -20
Amplitude (dB) Amplitude (dB)
SPECTRAL PERFORMANCE 0 fIN = 3.58MHz -20
-40
-40
-60
-60
-80
-80
-100 0 5 10 Frequency (MHz) 15
-100 0 5 10 Frequency (MHz) 15
SPECTRAL PERFORMANCE 0 fIN = 12.5MHz -20
Amplitude (dB)
SPECTRAL PERFORMANCE 0 -20 fIN = 12.5MHz VS = +5V REFT = +3.25V REFB = +2.25V
Amplitude (dB)
0 5 10 Frequency (MHz) 15
-40 -60 -80 -100 -120 0 3 6 9 12 15 Frequency (MHz)
-40
-60
-80
-100
UNDERSAMPLING PERFORMANCE 0 -20
Amplitude (dB)
TWO-TONE INTERMODULATION 0 f1 = 3.6MHz at -7dB f2 = 3.5MHz at -7dB 2f1 - f2 55.7dBFS 2f2 - f1 54.6dBFS
fIN = 20MHz fS = 16MHz
-20
-40 -60 -80 -100 -120 0 1.6 3.2 4.8 6.4 8.0 Frequency (MHz)
Magnitude (dBFS)
-40
-60
-80
-100 0 2 4 6 8 10 Frequency (MHz)
ADS931
SBAS060A
5
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25C, VS = +3V, specified single-ended input (-1dBFS) and sampling rate = 30MHz, unless otherwise specified.
DIFFERENTIAL LINEARITY ERROR 2.0 fIN = 500kHz 1.0 1.0 2.0
DIFFERENTIAL LINEARITY ERROR fIN = 12.5MHz
DLE (LSB)
DLE (LSB)
0
0
-1.0
-1.0
-2.0 0 64 128 Output Code 192 256
-2.0 0 64 128 Output Code 192 256
INTEGRAL LINEARITY ERROR 4.0 fIN = 500kHz 100
SWEPT POWER SFDR
dBFS
SFDR (dBFS, dBc)
2.0 ILE (LSB)
80
60
0
40
-2.0
20 dBc -4.0 0 64 128 Output Code 192 256 0 -50 -40 -30 -20 -10 0 Input Amplitude (dBFS)
DYNAMIC PERFORMANCE vs INPUT FREQUENCY 50 SFDR 0.7 0.9
DIFFERENTIAL LINEARITY ERROR vs TEMPERATURE
SFDR, SNR (dB)
DLE (LSB)
48
0.5
SNR 46
0.3
fIN = 500kHz fIN = 12.5MHz
44 0.1 1 10 Frequency (MHz) 100
0.1 -50 -25 0 25 50 75 100 Temperature (C)
6
ADS931
SBAS060A
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25C, VS = +3V, specified single-ended input (-1dBFS) and sampling rate = 30MHz, unless otherwise specified.
SPURIOUS FREE DYNAMIC RANGE (SFDR) vs TEMPERATURE 54
SIGNAL-TO-NOISE RATIO vs TEMPERATURE 50 fIN = 12.5MHz
52 fIN = 12.5MHz
SFDR (dBFS)
SNR (dB)
50
fIN = 500kHz
48
48
fIN = 500kHz 46
46
44 -50 -25 0 25 50 75 100 Temperature (C)
44 -50
-25
0
25
50
75
100
Temperature (C)
POWER DISSIPATION vs TEMPERATURE 66
GAIN ERROR vs TEMPERATURE 2.6
Gain Error (%FS)
64
2.5
Power (mW)
62
2.4
60
2.3
58 -50 -25 0 25 50 75 100 Temperature (C)
2.2 -50 -25 0 25 50 75 100 Temperature (C)
OFFSET ERROR vs TEMPERATURE 9 12 10 8
Offset Error (mV) Counts (x 105)
OUTPUT NOISE HISTOGRAM (DC Input)
8 6 4 2
7
6
5 -50 -25 0 25 50 75 100 Temperature (C)
0 N-2
N-1
N Output Code
N+1
N+2
ADS931
SBAS060A
7
THEORY OF OPERATION
The ADS931 is a high-speed sampling A/D converter that utilizes a pipeline architecture. The fully differential topology and digital error correction guarantee 8-bit resolution. The track-and-hold circuit is shown in Figure 1. The switches are controlled by an internal clock which has a non-overlapping two phase signal, 1 and 2. At the sampling time the input signal is sampled on the bottom plates of the input capacitors. In the next clock phase, 2, the bottom plates of the input capacitors are connected together and the feedback capacitors are switched to the op amp output. At this time the charge redistributes between CI and CH, completing one track-and-hold cycle. The differential output is a held DC representation of the analog input at the sample time. In the normal mode of operation, the complementary input is tied to the common-mode voltage. In this case, the track-andhold circuit converts a single-ended input signal into a fully differential signal for the quantizer. Consequently, the input signal gets amplified by a gain or two, which improves the signal-to-noise performance. Other parameters such as smallsignal and full-power bandwidth, and wideband noise are also defined in this stage.
Op Amp Bias 1
VCM 1 CH 2
CI IN IN (Opt.) 1 1 2 CI CH 1 Input Clock (50%) Op Amp Bias Internal Non-overlapping Clock 1 2 1 VCM 1 1
OUT OUT
2
FIGURE 1. Input Track-And-Hold Configuration with Timing Signals.
IN
Input T/H 2-Bit Flash STAGE 1 2-Bit DAC
Digital Delay
x2
+
-
Digital Delay
2-Bit Flash STAGE 2
2-Bit DAC B1 (MSB)
Digital Error Correction
x2
+
-
B2 B3 B4 B5 B6 B7 B8 (LSB)
Digital Delay
2-Bit Flash STAGE 6
2-Bit DAC
x2
+
-
STAGE 7
2-Bit Flash
Digital Delay
FIGURE 2. Pipeline A/D Architecture. 8
ADS931
SBAS060A
The pipelined quantizer architecture has 7 stages with each stage containing a two-bit quantizer and a two bit digitalto-analog converter, as shown in Figure 2. Each two-bit quantizer stage converts on the edge of the sub-clock, which is the same frequency of the externally applied clock. The output of each quantizer is fed into its own delay line to time-align it with the data created from the subsequent quantizer stages. This aligned data is fed into a digital error correction circuit which can adjust the output data based on the information found on the redundant bits. This technique provides the ADS931 with excellent differential linearity and guarantees no missing codes at the 8-bit level. To accommodate a bipolar signal swing, the ADS931 operates with a common-mode voltage (VCM) which is derived from the external references. Due to the symmetric resistor ladder inside the ADS931, VCM is situated between the top and bottom reference voltage. Equation 1 can be used for calculating the common-mode voltage level. VCM = (REFT +REFB)/2 (1) At the same time, the two external reference voltage levels define the full-scale input range for the ADS931. This makes it possible for the input range to be adapted to the signal swing of the front end.
of the converter will start to degrade due to large variations of the input's switch ON resistance over the input voltage. Therefore, the signal swing should remain approximately 0.5V away from each rail during normal operation. DRIVING THE ANALOG INPUTS AC-COUPLED DRIVER Figure 3 shows an example of an ac-coupled, single-ended interface circuit using a high speed op amp which operates on dual supplies (OPA650, OPA658). The mid-point reference voltage, (VCM), biases the bipolar, ground-referenced input signal. The capacitor C1 and resistor R1 form a highpass filter with the -3dB frequency set at f-3dB = 1/(2 R1 C1) (2)
The values for C1 and R1 are not critical in most applications and can be set freely. The values shown in Figure 3 correspond to a corner frequency of 1.6kHz.
+VS +5V VIN OPA65x R1 1k VCM 402 C1 RS 50 0.1F IN ADS931 CM
APPLICATIONS
SIGNAL SWING AND COMMON-MODE CONSIDERATIONS The ADS931 is primarily designed and specified for a +3V single supply voltage. However, due to its supply range of +2.7V to +5.25V, it is well suited for +5V applications. The nominal input signal swing is 1Vp-p, situated between +1V and +2V. This means that the signal swings 0.5V around a common-mode voltage of +1.5V when using a 3V rail, or typically +2.75V on a 5V supply. In some applications, it might be advantageous to increase the input signal swing. For example, increasing it to 2Vp-p may improve the achievable signal-to-noise performance. However, consideration should be given to keeping the signal swing within the linear range of operation of the driving circuitry to avoid any excessive distortion. In extreme situations, the performance
+5V C1 0.1F VIN
OPA680
-5V 402
0.1F
FIGURE 3. AC-Coupled, Single-Ended Interface Circuit. Figure 4 depicts a circuit that can be used in single-supply applications. The mid-reference voltage biases the op amp up to the appropriate common-mode voltage, for example VCM = +1.5V. With the use of capacitor CG, the DC gain for the non-inverting op amp input is set to +1V/V. As a result, the transfer function is modified to VOUT = VIN {(1 + RF/RG) + VCM}
+5V RS 50 IN 22pF VCM
ADS931
(3)
R1 1k
CM
RF 402 RG 402 CG 0.1F NOTE: (1) See text for discussion. RP(1) 402
0.1F
FIGURE 4. +5V Single-Supply Interface Circuit Example Using the Voltage Feedback Amplifier OPA680.
ADS931
SBAS060A
9
Again, the input coupling capacitor C1 and resistor R1 form a high-pass filter. At the same time, the input impedance is defined by R1. Although many high-speed op amps operate on single supply voltages down to +3V, their ac-performance is often lower when compared to their +5V acperformance. This is especially true at signal frequencies of 5MHz or higher, where noticeable degradation is exhibited that will limit the performance of the system. If possible, the op amp and A/D converter pair should be supplied with +5V and the common-mode voltage set to +2.5V, which is usually the preferred dc bias level for single-supply op amps. Keeping the signal swing within 1Vp-p prevents the op amp from exhibiting excessive distortion caused by its slew-rate limitations. Depending on the selected amplifier, the use of a pull-up or pull-down resistor (RP) located directly at its output may considerably improve the distortion performance. Resistor RS isolates the op amp's output from the capacitive load to avoid gain peaking or even oscillation. It can also be used to establish a defined bandwidth in order to roll off the high frequency noise. The value of RS is usually set between 10 and 100. DC-COUPLED INTERFACE CIRCUIT Shown in Figure 5 is a single-supply, DC-coupled circuit which can be set in a gain of -1V/V or higher. Depending on the gain, the divider ratio set by resistors R1 and R2 must be adjusted to yield the correct common-mode voltage for the ADS931. With a +3V supply, the input signal of the ADS931 is 1Vp-p, typically centered around the common-mode voltage of +1.5V, which can be derived from the external references. EXTERNAL REFERENCE The ADS931 requires external references on pin 22 (REFT) and pin 24 (REFB). Internally those pins are connected by the resistor ladder, which has a nominal resistance of 4k
(15%). In order to establish a correct voltage drop across the ladder, the external reference circuit must be able to supply typically 250A of current. With this current the fullscale input range of the ADS931 is set between +1V and +2V, or 1Vp-p. In general, the voltage drop across REFT and REFB determines the input full-scale range (FSR) of the ADS931. Equation 4 can be used to calculate the span. FSR = REFT - REFB (4)
Depending on the application, several options are possible to supply the external reference voltages to the ADS931 without degrading the typical performance. LOW-COST SOLUTION The easiest way to achieve the required reference voltages is to place the reference ladder of the ADS931 between the supply rails, as shown in Figure 6. Two additional resistors (RT, RB) are necessary to set the correct current through the ladder. The table in Figure 6 lists the value for several possible configurations, however depending on the desired full-scale swing and supply voltage, different resistor values might be selected. The trade-offs, when selecting this reference circuit, are the variations in the reference voltages due to component tolerances, temperature drift and power supply variations. In any case, it is recommended to bypass the reference ladder with at least 0.1F ceramic capacitors, as shown in Figure 6. The purpose of the capacitors is twofold. They will bypass most of the high frequency noise which results from feedthrough of the clock and switching noise from the sample and hold stages. Secondly, they serve as a charge reservoir to supply instantaneous current to internal nodes. HIGH ACCURACY SOLUTION For those application demanding a higher level of dc accuracy and drift a reference circuit with a precision reference element might be used (see Figure 7). A stable +1.2V
+5V RF RIN VIN A1 RS IN 22pF CM R1 0.1F C2 R2 REFB REFT ADS931
External Top Reference
2k
RCM1
2k
RCM2
External Bottom Reference
FIGURE 5. Single-Ended, DC-Coupled Interface Circuit.
10
ADS931
SBAS060A
+3V 10F 0.1F RT 4k +VS 0.1F VIN 2k +1.5V 0.1F CM 1k LnBy +VS 3V 5V 5V 5V RT 4k 12k 8k 3k RB 4k 12k 8k 3k REFT REFB 2V 2V 3V 3.5V 1V 1V 2V 1.5V 0.1F 1k REFB 0.1F RB 4k IN 1k ADS931 1k LpBy 0.1F REFT 0.1F
FIGURE 6. Low-cost Solution to Supply External Reference Voltages.
+VS 10 +VS 1/2 A1 RF1 10k RG1 REF1004 +1.2V 5k
Top Reference
3k 1/2 A1
10
Bottom Reference
RF2
RG2
FIGURE 7. High Accuracy Solution to Supply External Reference Voltages.
reference voltage is established by a two terminal bandgap reference diode, the REF1004-1.2. Using a general-purpose single-supply dual operational amplifier (A1), like an OPA2237, OPA2234 or MC34072, the two required reference voltages for the ADS931 can be generated by setting each op amp to the appropriate gain; for example: set REFT to +2V and REFB to +1V.
CLOCK INPUT The clock input of the ADS931 is designed to accommodate either +3V or +5V CMOS logic levels. To drive the clock input with a minimum amount of duty cycle variation and support the maximum sampling rate (30MSPS), high speed or advanced CMOS logic should be used (HC/HCT, AC/ACT). When digitizing at high sampling rates, a 50%
ADS931
SBAS060A
11
duty cycle, along with fast rise and fall times (2ns or less), are recommended to meet the rated performance specifications. However, the ADS931 performance is tolerant to duty cycle variations of as much as 10%, which should not affect the performance. For applications operating with input frequencies up to Nyquist (fCLK/2) or undersampling applications, special consideration must be made to provide a clock with very low jitter. Clock jitter leads to aperture jitter (tA) which can be the ultimate limitation to achieving good SNR performance. Equation 5 shows the relationship between aperture jitter, input frequency and the signal-tonoise ratio: SNR = 20log10 [1/(2 fIN tA)] (5)
+VS
+LVDD
ADS931
Digital Output Stage
FIGURE 8. Independent Supply Connection for Output Stage. During power-down, the digital outputs are set in 3-state. With the clock applied, the converter does not accurately process the sampled signal. After removing the power-down condition, the output data from the following 5 clock cycles is invalid (data latency). DECOUPLING AND GROUNDING CONSIDERATIONS The ADS931 has several supply pins, one of which is dedicated to supply only the output driver (LVDD). The remaining supply pins are not divided into analog and digital supply pins (+VS) since they are internally connected on the chip. For this reason, it is recommended that the converter be treated as an analog component and to power it only from the analog supply. Digital supply lines often carry high levels of noise which can couple back into the converter and limit performance. Because of the pipeline architecture, the converter also generates high frequency transients and noise that are fed back into the supply and reference lines. This requires that the supply and reference pins be sufficiently bypassed. Figure 9 shows the recommended decoupling scheme for the analog supplies. In most cases, 0.1F ceramic chip capacitors are adequate to keep the impedance low over a wide frequency range. Their effectiveness largely depends on the proximity to the individual supply pin. Therefore, they should be located as close as possible to the supply pins. In addition, one larger bipolar capacitor (1F to 22F) should be placed on the PC board in proximity of the converter circuit.
DIGITAL OUTPUTS The digital outputs of the ADS931 are standard CMOS stages and designed to be compatible to both high speed TTL and CMOS logic families. The logic thresholds are for low-voltage CMOS: VOL = 0.4V, VOH = 2.4V, which allows the ADS931 to directly interface to 3V logic. The digital output driver of the ADS931 uses a dedicated digital supply pin (pin 2, LVDD), as shown in Figure 8. By adjusting the voltage on LVDD, the digital output levels will vary respectively. It is recommended to limit the fan-out to one in order to keep the capacitive loading on the data lines below the specified 15pF. If necessary, external buffers or latches may be used to provide the added benefit of isolating the A/D converter from any digital activities on the bus coupling back high frequency noise, which degrades the performance.
STRAIGHT OFFSET BINARY (SOB) PIN 12 FLOATING or LO 11111111 11111111 11111110 11100000 11000000 10100000 10000001 10000000 01111111 01100000 01000000 00100000 00000001 00000000
SINGLE-ENDED INPUT +FS (IN = REFT Voltage) +FS -1LSB +FS -2LSB +3/4 Full Scale +1/2 Full Scale +1/4 Full Scale +1LSB Bipolar Zero (IN +1.5V) -1LSB -1/4 Full Scale -1/2 Full Scale -3/4 Full Scale -FS +1LSB -FS (IN = REFB Voltage)
TABLE I. Coding Table for the ADS931.
+VS 1 GND 13 14
ADS931 +VS 18 GND 19 20 +VS 28
POWER-DOWN MODE The ADS931's low power consumption can be reduced even further by initiating a power-down mode. For this, the Power Down pin (pin 17) must be tied to a logic "High" reducing the current drawn from the supply by approximately 84%. In normal operation, the power-down mode is disabled by an internal pull-down resistor (50k).
0.1F
0.1F
0.1F
FIGURE 9. Recommended Bypassing for Analog Supply Pins.
12
ADS931
SBAS060A
PACKAGE OPTION ADDENDUM
www.ti.com
3-Oct-2003
PACKAGING INFORMATION
ORDERABLE DEVICE ADS931E ADS931E/1K STATUS(1) ACTIVE ACTIVE PACKAGE TYPE SSOP SSOP PACKAGE DRAWING DB DB PINS 28 28 PACKAGE QTY 48 1000
(1) The marketing status values are defined as follows: ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device.
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