ADS801UG4

ADS801 ADS 801 U SBAS036B – MAY 1995 – REVISED FEBRUARY 2005 12-Bit, 25MHz Sampling ANALOG-TO-DIGITAL CONVERTER DESCRIPTION FEATURES ● ● ● ● ● NO MISSING CODES LOW POWER: 270mW INTERNAL REFERENCE WIDEBAND TRACK-AND-HOLD: 65MHz SINGLE +5V SUPPLY The ADS801 is a low-power, monolithic 12-bit, 25MHz Analog-to-Digital (A/D) converter utilizing a small geometry CMOS process. This complete converter includes a 12-bit quantizer, wideband track-and-hold, reference, and three-state outputs. It operates from a single +5V power supply and can be configured to accept either single-ended or differential input signals. APPLICATIONS The ADS801 employs digital error correction to provide excellent Nyquist differential linearity performance for demanding imaging applications. Its low distortion, high SNR, and high oversampling capability give it the extra margin needed for telecommunications, instrumentation, and video applications. ● ● ● ● IF AND BASEBAND DIGITIZATION DIGITAL COMMUNICATIONS TEST INSTRUMENTATION CCD IMAGING Copiers Scanners Cameras ● VIDEO DIGITIZING ● GAMMA CAMERAS This high-performance A/D converter is specified over temperature for AC and DC performance at a 25MHz sampling rate. The ADS820 is available in an SO-28 package. CLK MSBI OE Error Correction Logic 3-State Outputs Timing Circuitry IN Pipeline A/D Converter T/H IN 12-Bit Digital Data +3.25V REFT CM REFB +1.25V 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. All trademarks are the property of their respective owners. Copyright © 1995-2005, Texas Instruments Incorporated 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. www.ti.com ELECTROSTATIC DISCHARGE SENSITIVITY ABSOLUTE MAXIMUM RATINGS(1) +VS ....................................................................................................... +6V Analog Input ............................................................ 0V to (+VS + 300mV) 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. Logic Input ............................................................... 0V to (+VS + 300mV) Case Temperature ......................................................................... +100°C Junction Temperature .................................................................... +150°C Storage Temperature .................................................................... +125°C 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. External Top Reference Voltage (REFT) ................................. +3.4V Max External Bottom Reference Voltage (REFB) ............................ +1.1V Min NOTE: (1) Stresses above these ratings may permanently damage the device. PACKAGE/ORDERING INFORMATION(1) SPECIFIED TEMPERATURE RANGE PACKAGE MARKING PRODUCT PACKAGE-LEAD PACKAGE DESIGNATOR ORDERING NUMBER TRANSPORT MEDIA, QUANTITY ADS801U SO-28 DW –40°C to +85°C ADS801U ADS801U Rails, 28 ADS801U " " " " ADS801U/1K Tape and Reel, 1000 NOTE: (1) For the most current package and ordering information, see the Package Option Addendum at the end of this document, or see the TI website at www.ti.com. ELECTRICAL CHARACTERISTICS At TA = +25°C, VS = +5V, Sampling Rate = 25MHz, and with a 50% duty cycle clock having a 2ns rise-and-fall time, unless otherwise noted. ADS800U PARAMETER Resolution Specified Temperature Range ANALOG INPUT Differential Full-Scale Input Range Common-Mode Voltage Analog Input Bandwidth (–3dB) Small-Signal Full-Power Input Impedance DIGITAL INPUT Logic Family Convert Command CONDITIONS f = 10MHz No Missing Codes Integral Linearity Error at f = 500kHz Spurious-Free Dynamic Range (SFDR) f = 500kHz (–1dBFS input) f = 10MHz (–1dBFS input) TYP MAX UNITS –40 12 +85 Bits °C Both Inputs, 180° Out-of-Phase +1.25 +3.25 V –20dBFS(1) Input 0dBFS Input +25°C +25°C fS = 2.5MHz +25°C Full ∆ +VS = ±5% ∆ +VS = ±5% +2.25 V 400 65 1.25 || 4 MHz MHz MΩ || pF TTL/HCT Compatible CMOS Falling Edge Start Conversion ±0.6 ±1.0 ±85 0.03 ±2.1 0.05 +25°C Full +25°C CONVERSION CHARACTERISTICS Sample Rate Data Latency DYNAMIC CHARACTERISTICS Differential Linearity Error f = 500kHz MIN TAMBIENT ACCURACY(2) Gain Error Gain Tempco Power-Supply Rejection of Gain Input Offset Error Power-Supply Rejection of Offset TEMP 10k ±1.5 ±2.5 0.15 ±3.0 0.15 25M Sample/s Convert Cycle ±1.0 ±1.0 ±1.0 ±1.0 LSB LSB LSB LSB LSB LSB 6.5 ±0.3 ±0.4 ±0.3 ±0.4 Tested ±1.7 +25°C 0°C to +85°C +25°C 0°C to +85°C 0°C to +85°C Full +25°C Full +25°C Full 63 62 57 55 77 73 61 61 % % ppm/°C %FSR/% % %FSR/% dBFS dBFS dBFS dBFS NOTES: (1) dBFS refers to dB below Full-Scale. (2) Percentage accuracies are referred to the internal A/D converter Full-Scale Range of 4Vp-p. (3) IMD is referred to the larger of the two input signals. If referred to the peak envelope signal (≈0dB), the intermodulation products will be 7dB lower. (4) No “rollover” of bits. 2 ADS801 www.ti.com SBAS036B ELECTRICAL CHARACTERISTICS (Cont.) At TA = +25°C, VS = +5V, Sampling Rate = 25MHz, and with a 50% duty cycle clock having a 2ns rise-and-fall time, unless otherwise noted. ADS800U PARAMETER CONDITIONS DYNAMIC CHARACTERISTICS (Cont.) 2-Tone Intermodulation Distortion (IMD)(3) f = 4.4MHz and 4.5MHz (–7dBFS each tone) f = 10MHz (–1dBFS input) Signal-to-(Noise + Distortion) (SINAD) f = 500kHz (–1dBFS input) f = 10MHz (–1dBFS input) OUTPUTS Logic Family Logic Coding Logic Levels NTSC or PAL NTSC or PAL 1.5x Full-Scale Input Logic Selectable Logic LOW, CL = 15pF max Logic HIGH, CL = 15pF max 3-State Enable Time 3-State Disable Time POWER-SUPPLY REQUIREMENTS Supply Voltage: +VS Supply Current: +IS Power Consumption MIN +25°C Full Signal-to-Noise Ratio (SNR) f = 500kHz (–1dBFS input) Differential Gain Error Differential Phase Error Aperture Delay Time Aperture Jitter Over-Voltage Recovery Time(4) TEMP MAX UNITS –64 –63 dBc dBc +25°C Full +25°C Full 64 61 62 58 66 64 65 64 dB dB dB dB +25°C Full +25°C Full +25°C +25°C +25°C +25°C +25°C 63 60 56 54 66 63 59 58 0.5 0.1 2 7 2 dB dB dB dB % degrees ns ps rms ns TTL/HCT Compatible CMOS Falling Edge 0 0.4 V Full Full +2.5 Full Operating Operating Operating Operating Operating TYP Full +25°C Full +25°C Full Thermal Resistance, θJA SO-28 +4.75 +VS V 20 2 40 10 ns ns +5.0 54 54 270 270 +5.25 65 68 325 340 V mA mA mW mW 75 °C/W NOTES: (1) dBFS refers to dB below Full-Scale. (2) Percentage accuracies are referred to the internal A/D converter Full-Scale Range of 4Vp-p. (3) IMD is referred to the larger of the two input signals. If referred to the peak envelope signal (≈0dB), the intermodulation products will be 7dB lower. (4) No “rollover” of bits. ADS801 SBAS036B www.ti.com 3 PIN DESCRIPTIONS PIN CONFIGURATION Top View SO PIN DESIGNATOR 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 GND B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 GND +VS CLK +VS OE 19 MSBI 20 21 +VS REFB GND 1 28 GND B1 2 27 IN B2 3 26 IN B3 4 25 GND B4 5 24 +VS B5 6 23 REFT B6 7 22 CM B7 8 21 REFB B8 9 20 +VS B9 10 19 MSBI B10 11 18 OE B11 12 17 +VS B12 13 16 CLK 22 CM GND 14 15 +VS 23 REFT 24 25 26 27 28 +VS GND IN IN GND ADS801 DESCRIPTION Ground Bit 1, Most Significant Bit (MSB) Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 Bit 8 Bit 9 Bit 10 Bit 11 Bit 12, Least Significant Bit (LSB) Ground +5V Power Supply Convert Clock Input, 50% Duty Cycle +5V Power Supply HIGH: High-Impedance State. LOW or Floating: Normal Operation. Internal pull-down resistors. Most Significant Bit Inversion, HIGH: MSB inverted for complementary output. LOW or Floating: Straight output. Internal pull-down resistors. +5V Power Supply Bottom Reference Bypass. For external bypassing of internal +1.25V reference. Common-Mode Voltage. It is derived by (REFT + REFB)/2. Top Reference Bypass. For external bypassing of internal +3.25V reference. +5V Power Supply Ground Input Complementary Input Ground TIMING DIAGRAM tCONV Convert Clock tL tD tH DATA LATENCY (6.5 Clock Cycles) Hold Hold Hold Hold Hold Hold Track “N + 1” Track “N + 2” Track “N + 3” Track “N + 4” Track “N + 5” Track “N + 6” Track (1) Internal Track-and-Hold Track Hold “N” t2 Output Data Data Valid N–8 Data Valid N–7 Data Valid N–6 N–5 N–4 N–3 N–2 N–1 N t1 Data Invalid SYMBOL tCONV tL tH tD t1 t2 NOTE: (1) “ 4 DESCRIPTION MIN Convert Clock Period Clock Pulse LOW Clock Pulse HIGH Aperture Delay Data Hold Time, CL = 0pF New Data Delay Time, CL = 15pF max 40 19 19 TYP MAX UNITS 100µs ns ns ns ns ns ns 20 20 2 3.9 12.5 ” indicates the portion of the waveform that will stretch out at slower sample rates. ADS801 www.ti.com SBAS036B TYPICAL CHARACTERISTICS At TA = +25°C, VS = +5V, Sampling Rate = 25MHz, and with a 50% duty cycle clock having a 2ns rise-and-fall time, unless otherwise noted. SPECTRAL PERFORMANCE SPECTRAL PERFORMANCE 0 0 fIN = 10MHz –20 –20 –40 –40 Amplitude (dB) Amplitude (dB) fIN = 500kHz –60 –80 –100 –60 –80 –100 –120 –120 0 2.5 5.0 7.5 10.0 0 12.5 2.5 5.0 Frequency (MHz) 7.5 DIFFERENTIAL LINEARITY ERROR 12.5 DIFFERENTIAL LINEARITY ERROR 2.0 2.0 fIN = 500kHz fIN = 10MHz 1.0 1.0 DLE (LSB) DLE (LSB) 10.0 Frequency (MHz) 0 –1.0 0 –1.0 –2.0 –2.0 0 1024 2048 3072 4096 0 1024 2048 Code 3072 4096 Code 2-TONE INTERMODULATION INPUT FREQUENCY vs DYNAMIC PERFORMANCE 80 0 f1 = 4.5MHz 75 f2 = 4.4MHz SFDR SFDR, SNR (dB) Amplitude (dB) –20 –40 –60 –80 70 65 SNR 60 55 –100 50 –120 0.0 3.13 6.25 9.38 12.50 0.1 Frequency (MHz) 10 100 Frequency (MHz) ADS801 SBAS036B 1 www.ti.com 5 TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, VS = +5V, Sampling Rate = 25MHz, and with a 50% duty cycle clock having a 2ns rise-and-fall time, unless otherwise noted. SWEPT POWER SFDR OUTPUT NOISE HISTOGRAM (NO SIGNAL) 100 800k fIN = 10MHz 80 SFDR (dBFS) Counts 600k 400k 60 40 200k 20 0.0 0 N–2 N–1 N N+1 –50 N+2 –40 –30 –20 Code SWEPT POWER SNR fIN = 500kHz 60 2.0 ILE (LSB) SNR (dB) 10 4.0 fIN = 10MHz 40 20 0 –2.0 0 –4.0 –50 –40 –30 –20 –10 0 10 0 1024 2048 3072 4096 Input Amplitude (dBm) Code DYNAMIC PERFORMANCE vs SINGLE-ENDED FULL-SCALE INPUT RANGE DYNAMIC PERFORMANCE vs DIFFERENTIAL FULL-SCALE INPUT RANGE 65 70 SFDR (fIN = 10MHz) Dynamic Range (dB) SNR (fIN = 10MHz) Dynamic Range (dB) 0 INTEGRAL LINEARITY ERROR 80 60 SFDR (fIN = 10MHz) 55 50 65 SNR (fIN = 10MHz) 60 55 1 2 3 4 5 1 Single-Ended Full-Scale Input Range (Vp-p) 6 –10 Input Amplitude (dBm) 2 3 4 5 Differential Full-Scale Input Range (Vp-p) ADS801 www.ti.com SBAS036B TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, VS = +5V, Sampling Rate = 25MHz, and with a 50% duty cycle clock having a 2ns rise-and-fall time, unless otherwise noted. SPURIOUS-FREE DYNAMIC RANGE (SFDR) vs TEMPERATURE DIFFERENTIAL LINEARITY ERROR vs TEMPERATURE 0.5 80 fIN = 500kHz 75 fIN = 500kHz SFDR (dBFS) DLE (LSBs) 0.4 0.3 fIN = 10MHz 70 65 fIN = 10MHz 0.2 60 0.1 55 –50 –25 0 25 50 75 100 –50 0 25 50 75 Temperature (°C) SIGNAL-TO-NOISE RATIO vs TEMPERATURE SIGNAL-TO-(NOISE + DISTORTION) vs TEMPERATURE 100 70 70 65 68 fIN = 500kHz fIN = 500kHz SINAD (dB) SNR (dB) –25 Temperature (°C) 66 64 fIN = 10MHz 60 fIN = 10MHz 55 50 45 62 –50 –25 0 25 50 75 –50 100 –25 0 Temperature (°C) 25 50 75 100 Temperature (°C) POWER DISSIPATION vs TEMPERATURE SUPPLY CURRENT vs TEMPERATURE 52 265 260 IQ (mA) Power (mW) 51 50 255 250 49 245 –50 –25 0 25 50 75 100 –50 Temperature (°C) 0 25 50 75 100 Temperature (°C) ADS801 SBAS036B –25 www.ti.com 7 TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, VS = +5V, Sampling Rate = 25MHz, and with a 50% duty cycle clock having a 2ns rise-and-fall time, unless otherwise noted. GAIN ERROR vs TEMPERATURE OFFSET ERROR vs TEMPERATURE –1.50 Offset (% FSR) Gain (% FSR) –0.05 –0.55 –1.05 –1.55 –1.75 –2.0 –2.25 –50 –25 0 25 50 75 100 –50 –25 0 Temperature (°C) 25 50 75 100 Temperature (°C) TRACK-MODE SMALL-SIGNAL INPUT BANDWIDTH Track-Mode Input Response (dB) 1 0 –1 –2 –3 –4 –5 10k 100k 1M 10M 100M 1G Frequency (Hz) 8 ADS801 www.ti.com SBAS036B THEORY OF OPERATION Op Amp Bias The ADS801 is a high-speed, sampling A/D converter with pipelining. It uses a fully differential architecture and digital error correction to ensure 12-bit resolution. The differential track-and-hold circuit is shown in Figure 1. The switches are controlled by an internal clock that is a non-overlapping 2phase signal, φ1 and φ2. At the sampling time, the input signal is sampled on the bottom plates of the input capacitors. In the next clock phase, φ2, 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. The track-and-hold circuit can also convert a single-ended input signal into a fully differential signal for the quantizer. The pipelined quantizer architecture has 11 stages with each stage containing a 2-bit quantizer and a 2-bit Digital-toAnalog Converter (DAC), as shown in Figure 2. Each 2-bit quantizer stage converts on the edge of the sub-clock, which is twice the frequency of the externally applied clock. The output of each quantizer is fed into its own delay line to time- IN IN φ1 φ1 CH φ2 CI IN IN φ1 φ2 OUT φ1 OUT φ1 CI φ2 CH φ1 φ1 Input Clock (50%) Op Amp Bias VCM Internal Non-Overlapping Clock φ1 φ2 φ1 FIGURE 1. Input Track-and-Hold Configuration with Timing Signals. Digital Delay Input T/H 2-Bit Flash STAGE 1 VCM 2-Bit DAC + Σ – x2 B1 (MSB) Digital Delay B2 STAGE 2 B3 2-Bit DAC Digital Error Correction 2-Bit Flash + Σ – x2 B4 B5 B6 B7 B8 B9 B10 Digital Delay B11 B12 (LSB) 2-Bit Flash STAGE 10 2-Bit DAC + Σ – x2 STAGE 11 2-Bit Flash Digital Delay FIGURE 2. Pipeline A/D Converter Architecture. ADS801 SBAS036B www.ti.com 9 align it with the data created from the following quantizer stages. This aligned data is fed into a digital error correction circuit that can adjust the output data based on the information found on the redundant bits. This technique gives the ADS801 excellent differential linearity and ensures no missing codes at the 12-bit level. Since there are two pipeline stages per external clock cycle, there is a 6.5 clock cycle data latency from the start convert signal to the valid output data. The output data is available in Straight Offset Binary (SOB) or Binary Two’s Complement (BTC) format. THE ANALOG INPUT AND INTERNAL REFERENCE The analog input of the ADS801 can be configured in various ways and driven with different circuits, depending on the nature of the signal and the level of performance desired. The ADS801 has an internal reference that sets the full-scale input range of the A/D converter. The differential input range has each input centered around the common-mode of +2.25V, with each of the two inputs having a full-scale range of +1.25V to +3.25V. Since each input is 2Vp-p and 180° out-of-phase with the other, a 4V differential input signal to the quantizer results. As shown in Figure 3, the positive full-scale reference (REFT) and the negative full-scale (REFB) are brought out for external bypassing. In addition, the common-mode voltage (CM) may be used as a reference to provide the appropriate offset for the driving circuitry. However, care must be taken not to appreciably load this reference node. For more information regarding external references, single-ended input, and ADS801 drive circuits, refer to the applications section. DIGITAL OUTPUT DATA The 12-bit output data is provided at CMOS logic levels. The standard output coding is Straight Offset Binary (SOB) where a full-scale input signal corresponds to all “1s” at the output, as shown in Table I. This condition is met with pin 19 “LO” or Floating, due to an internal pull-down resistor. By applying a logic “HI” voltage to this pin, a Binary Two’s Complement (BTC) output will be provided where the most significant bit is inverted. The digital outputs of the ADS801 can be set to a high-impedance state by driving OE (pin 18) with a logic HIGH. Normal operation is achieved with pin 18 LOW or Floating, due to internal pull-down resistors. This function is provided for testability purposes and is not meant to drive digital buses directly or be dynamically changed during the conversion process. OUTPUT CODE DIFFERENTIAL INPUT(1) SOB PIN 19 FLOATING or LOW BTC PIN 19 HIGH 111111111111 111111111111 111111111110 111000000000 110000000000 101000000000 100000000001 100000000000 011111111111 011000000000 010000000000 001000000000 000000000001 000000000000 011111111111 011111111111 011111111110 011000000000 010000000000 001000000000 000000000001 000000000000 111111111111 111000000000 110000000000 101000000000 100000000001 100000000000 +FS (IN = +3.25V, IN = +1.25V) +FS – 1LSB +FS – 2LSB +3/4 Full-Scale +1/2 Full-Scale +1/4 Full-Scale +1LSB Bipolar Zero (IN = IN = +2.25V) –1LSB –1/4 Full-Scale –1/2 Full-Scale –3/4 Full-Scale –FS + 1LSB –FS (IN = +1.25V, IN = +3.25V) NOTE: (1) In the single-ended input mode, +FS = +4.25V and –FS = +0.25V. TABLE I. Coding Table for the ADS801. ADS801 +3.25V 23 APPLICATIONS REFT 0.1µF DRIVING THE ADS801 2kΩ +2.25V 22 The ADS801 has a differential input with a common-mode of +2.25V. For AC-coupled applications, the simplest way to create this differential input is to drive the primary winding of a transformer with a single-ended input. A differential output is created on the secondary if the center tap is tied to the common-mode voltage of +2.25V, as per Figure 4. This transformer-coupled input arrangement provides good high- To Internal Comparators CM 2kΩ 21 0.1µF REFB +1.25V FIGURE 3. Internal Reference Structure. 22 CM CLOCK REQUIREMENTS 0.1µF The CLK pin accepts a CMOS level clock input. The rising and falling edges of the externally applied convert command clock controls the various interstage conversions in the pipeline. Therefore, the duty cycle of the clock should be held at 50% with low jitter and fast rise-and-fall times of 2ns or less. This is particularly important when digitizing a highfrequency input and operating at the maximum sample rate. Deviation from a 50% duty cycle will effectively shorten some of the interstage settling times, thus degrading the SNR and DNL performance. 10 26 IN AC Input Signal ADS801 22pF Mini-Circuits TT1-6-KK81 or equivalent 27 IN 22pF FIGURE 4. AC-Coupled Single-Ended to Differential Drive Circuit Using a Transformer. ADS801 www.ti.com SBAS036B frequency AC performance. It is important to select a transformer that gives low distortion and does not exhibit core saturation at full-scale voltage levels. Since the transformer does not appreciably load the ladder, there is no need to buffer the Common-Mode (CM) output in this instance. In general, it is advisable to keep the current draw from the CM output pin below 0.5µA to avoid nonlinearity in the internal reference ladder. A FET input operational amplifier, such as the OPA130, can provide a buffered reference for driving external circuitry. The analog IN and IN inputs should be bypassed with 22pF capacitors to minimize track-and-hold glitches and to improve high input frequency performance. series with the input, CSH is the external capacitor from the input to ground, and CADC is the internal input capacitance of the A/D converter (typically 4pF). Figure 5 illustrates another possible low-cost interface circuit that utilizes resistors and capacitors in place of a transformer. Depending on the signal bandwidth, the component values should be carefully selected in order to maintain the product performance outlined. The input capacitors, CIN, and the input resistors, RIN, create a high-pass filter with the lower corner frequency at fC = 1/(2pRINCIN). The corner frequency can be reduced by either increasing the value of RIN or CIN. If the circuit operates with a 50Ω or 75Ω impedance level, the resistors are fixed and only the value of the capacitor can be increased. Usually AC-coupling capacitors are electrolytic or tantalum capacitors with values of 1µF or higher. It should be noted that these large capacitors become inductive with increased input frequency, which could lead to signal amplitude errors or oscillation. To maintain a low AC-coupling impedance throughout the signal band, a small value (e.g. 1µF) ceramic capacitor could be added in parallel with the polarized capacitor. If the signal needs to be DC-coupled to the input of the ADS801, an operational amplifier input circuit is required. In the differential input mode, any single-ended signal must be modified to create a differential signal. This can be accomplished by using two operational amplifiers; one in the noninverting mode for the input and the other amplifier in the inverting mode for the complementary input. The low distortion circuit in Figure 6 will provide the necessary input shifting required for signals centered around ground. It also employs a diode for output level shifting to ensure a low distortion +3.25V output swing. Other amplifiers can be used in place of the OPA842s if the lowest distortion is not necessary. If output level shifting circuits are not used, care must be taken to select operational amplifiers that give the necessary performance when swinging to +3.25V with a ±5V supply operational amplifier. Capacitors CSH1 and CSH2 are used to minimize current glitches resulting from the switching in the input track-andhold stage and to improve signal-to-noise performance. These capacitors can also be used to establish a low-pass filter and effectively reduce the noise bandwidth. In order to create a real pole, resistors RSER1 and RSER2 were added in series with each input. The cutoff frequency of the filter is determined by fC = 1/(2pRSER • (CSH + CADC)), where RSER is the resistor in Resistors R1 and R2 are used to derive the necessary common-mode voltage from the buffered top and bottom references. The total load of the resistor string should be selected so that the current does not exceed 1mA. Although the circuit in Figure 5 uses two resistors of equal value so that the common-mode voltage is centered between the top and bottom reference (+2.25V), it is not necessary to do so. In all cases the center point, VCM, should be bypassed to ground in order to provide a low-impedance AC ground. The ADS801 can also be configured with a single-ended input full-scale range of +0.25V to +4.25V by tying the complementary input to the common-mode reference voltage (see Figure 7). This configuration will result in increased even-order harmonics, especially at higher input frequencies. However, this tradeoff may be quite acceptable for time-domain applications. The driving amplifier must give adequate performance with a +0.25V to +4.25V output swing in this case. C1 0.1µF CIN 0.1µF R1 (6kΩ) RSER1(1) 49.9Ω +3.25V Top Reference IN RIN1 25Ω CIN 0.1µF RIN2 25Ω CSH1 22pF R3 1kΩ RSER2(1) 49.9Ω C2 0.1µF ADS8xx VCM R2 (6kΩ) IN CSH2 22pF +1.25V Bottom Reference C3 0.1µF NOTE: (1) Indicates optional component. FIGURE 5. AC-Coupled Differential Input Circuit. ADS801 SBAS036B www.ti.com 11 +5V 604Ω +5V 301Ω BAS16(1) Optional High Impedance Input Amplifier 301Ω 27 IN OPA842 301Ω 2.49kΩ 0.1µF +5V(2) 22pF 0.1µF –5V 604Ω DC-Coupled Input Signal +5V OPA842 604Ω OPA130 +5V –5V 24.9Ω ADS801 49.9Ω 2.49kΩ +2.25V 22 CM +5V 301Ω BAS16(1) 301Ω Input Level Shift Buffer 26 IN OPA842 0.1µF 22pF 604Ω –5V NOTES: (1) A Philips BAS16 diode or equivalent may be used. (2) Supply bypassing not shown. 301Ω FIGURE 6. A Low-Distortion, DC-Coupled, Single-Ended to Differential Input Driver Circuit. 2 • (REFTEXT – REFBEXT), with the common-mode being centered at (REFTEXT + REFBEXT)/2. Refer to the typical characteristics for expected performance versus full-scale input range. 22 CM 0.1µF ADS801 Single-Ended Input Signal The circuit in Figure 8 works completely on a single +5V supply. As a reference element, it uses micro-power reference REF1004-2.5 that is set to a quiescent current of 0.1mA. Amplifier A2 is configured as a follower to buffer the +1.25V generated from the resistor divider. To provide the necessary current drive, a pull-down resistor (RP) is added. 26 IN 27 IN 22pF Full-Scale = +0.25V to +4.25V with internal references. FIGURE 7. Single-Ended Input Connection. EXTERNAL REFERENCES AND ADJUSTMENT OF FULL-SCALE RANGE The internal reference buffers are limited to approximately 1mA of output current. As a result, these internal +1.25V and +3.25V references may be overridden by external references that have at least 18mA (at room temperature) of output drive capability. In this instance, the common-mode voltage will be set halfway between the two references. This feature can be used to adjust the gain error, improve gain drift, or to change the full-scale input range of the ADS801. Changing the fullscale range to a lower value has the benefit of easing the swing requirements of external input amplifiers. The external references can vary as long as the value of the external top reference (REFTEXT) is less than or equal to +3.4V, the value of the external bottom reference (REFBEXT) is greater than or equal to +1.1V, and the difference between the external references are greater than or equal to 1.5V. For the differential configuration, the full-scale input range will be set to the external reference values that are selected. For the single-ended mode, the input range is 12 Amplifier A1 is configured as an adjustable-gain stage, with a range of approximately 1 to 1.32. The pull-up resistor again relieves the op amp from providing the full current drive. The value of the pull-up, pull-down resistors is not critical and can be varied to optimize power consumption. The need for pullup, pull-down resistors depends only on the drive capability of the selected drive amplifier, and thus can be omitted. PC-BOARD LAYOUT AND BYPASSING A well-designed, clean pc-board layout will assure proper operation and clean spectral response. Proper grounding and bypassing, short lead lengths, and the use of ground planes are particularly important for high-frequency circuits. Multilayer pc-boards are recommended for best performance, but if carefully designed, a two-sided pc-board with large, heavy ground planes can give excellent results. It is recommended that the analog and digital ground pins of the ADS801 be connected directly to the analog ground plane. In our experience, this gives the most consistent results. The A/D converter power-supply commons should be tied together at the analog ground plane. Power supplies should be bypassed with 0.1µF ceramic capacitors as close to the pin as possible. ADS801 www.ti.com SBAS036B +5V RP 220Ω A1 1/2 OPA2234 +5V Top Reference +2.5V to +3.25V 2kΩ 10kΩ 6.2kΩ 10kΩ REF1004 +2.5V 10kΩ(1) 0.1µF A2 1/2 OPA2234 +1.25V 10kΩ Bottom Reference RP 220Ω 10kΩ(1) NOTE: (1) Use parts alternatively for adjustment capability. FIGURE 8. Optional External Reference to Set the Full-Scale Range Utilizing a Dual, Single-Supply Op Amp. DYNAMIC PERFORMANCE TESTING DYNAMIC PERFORMANCE DEFINITIONS The ADS801 is a high-performance converter and careful attention to test techniques is necessary to achieve accurate results. Highly accurate phase-locked signal sources allow high-resolution FFT measurements to be made without using data windowing functions. A low-jitter signal generator, such as the HP8644A for the test signal, phase-locked with a lowjitter HP8022A pulse generator for the A/D converter clock, gives excellent results. Low-pass filtering (or bandpass filtering) of test signals is absolutely necessary to test the low distortion of the ADS801. Using a signal amplitude slightly lower than full-scale will allow a small amount of “headroom” so that noise or DC-offset voltage will not overrange the A/D converter and cause clipping on signal peaks. 1. 10 log 2. Sinewave Signal Power Noise + Harmonic Power (first 15 harmonics) Signal-to-Noise Ratio (SNR): 10 log 3. Sinewave Signal Power Noise Power Intermodulation Distortion (IMD): 10 log Highest IMD Pr oduct Power ( to 5th − order ) Sinewave Signal Power IMD is referenced to the larger of the test signals f1 or f2. Five “bins” either side of peak are used for calculation of fundamental and harmonic power. The “0” frequency bin (DC) is not included in these calculations, as it is of little importance in dynamic signal processing applications. ADS801 SBAS036B Signal-to-Noise-and-Distortion Ratio (SINAD): www.ti.com 13 FIGURE 9. ADS801 Interface Schematic with AC-Coupling and External Buffers. 14 www.ti.com ADS801 SBAS036B R2 50Ω AC Input Signal Mini-Circuits TT1-6-KK81 or equivalent 0.1µF 0.1µF Ext Clk 22pF 22pF (1) 0.1µF 0.1µF GND IN IN GND +VS REFT CM REFB +VS MSBI OE +VS CLK +VS 28 27 26 25 24 23 22 21 20 19 18 17 16 15 ADS801 1 2 3 4 5 6 7 8 9 10 11 12 13 14 GND MSB LSB GND NOTE: (1) All capacitors should be located as close to the pins as the manufacturing process will allow. Ceramic X7R surface-mount capacitors or equivalent are recommended. 0.1µF R1 50Ω 0.1µF 0.1µF +5V Dir 19 1 G+ 3 2 17 18 4 5 15 16 6 14 8 7 12 13 9 2 11 19 1 18 –541 4 16 G+ 5 15 Dir 6 14 3 7 13 17 9 8 12 –541 11 PACKAGE OPTION ADDENDUM www.ti.com 10-Dec-2020 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Pins Package Drawing Qty Eco Plan (2) Lead finish/ Ball material MSL Peak Temp Op Temp (°C) Device Marking (3) (4/5) (6) ADS801U ACTIVE SOIC DW 28 20 RoHS & Green NIPDAU Level-2-260C-1 YEAR -40 to 85 ADS801U (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. (2) RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may reference these types of products as "Pb-Free". RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption. Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of