Chapter VII MATCHING IN-AMP CIRCUITS TO MODERN ADCs Calculating ADC Requirements The resolution of commercial ADCs is specified in bits. In an ADC, the available resolution equals (2n) – 1, where n is the number of bits. For example, an 8-bit converter provides a resolution of (28) – 1, which equals 255. In this case, the full-scale input range of the converter divided by 255 will equal the smallest signal it can resolve. For example, an 8-bit ADC with a 5 V full-scale input range will have a limiting resolution of 19.6 mV. In selecting an appropriate ADC to use, we need to find a device that has a resolution better than the measurement resolution but, for economy’s sake, not a great deal better. Table 7-1 provides input resolution and full-scale input range using an ADC with or without an in-amp preamplifier. Note that the system resolution specified in the figure refers to that provided by the converter together with the in-amp preamp (if used). Also, note that for any low level measurement, not only are low noise semiconductor devices needed, but also careful attention to component layout, grounding, power supply bypassing, and often, the use of balanced, shielded inputs. For many applications, an 8-bit or 10-bit converter is appropriate. The decision to use a high resolution converter alone, or to use a gain stage ahead of a lower resolution converter, depends on which is more important: component cost, or parts count and ease of assembly. One very effective way to raise system resolution is to amplify the signal first, to allow full use of the dynamic range of the ADC. However, this added gain ahead of the converter will also increase noise. Therefore, it is often useful to add low-pass filtering between the output of an in-amp (or other gain stage) and the input of the converter. Also, in most cases, the system bandwidth should not be set higher than that required to accurately measure the signal of interest. A good rule of thumb is to set the –3 dB corner frequency of the low-pass filter at 10 to 20 times the highest frequency that will be measured. Adding amplification before the ADC will also reduce the circuit’s full-scale input range, but it will lower the resolution requirements (and, therefore, the cost) of the ADC (see Figure 7-1). For example, using an in-amp with a gain of 10 ahead of an 8-bit, 5 V ADC will increase circuit resolution from 19.5 mV (5 V/256) to 1.95 mV. At the same time, the full-scale input range of the circuit will be reduced to 500 mV (5 V/10). Table 7-1. Typical System Resolutions vs. Converter Resolution and Preamp (IA) Gain Converter Type (2n) – 1 Converter Resolution System mV/Bit In-Amp FS Range Resolution (5 V/((2n) – 1)) Gain (V p-p) (mV p-p) 10-Bit 10-Bit 10-Bit 10-Bit 12-Bit 12-Bit 12-Bit 12-Bit 14-Bit 14-Bit 14-Bit 14-Bit 16-Bit 16-Bit 16-Bit 16-Bit 4.9 mV 4.9 mV 4.9 mV 4.9 mV 1.2 mV 1.2 mV 1.2 mV 1.2 mV 0.305 mV 0.305 mV 0.305 mV 0.305 mV 0.076 mV 0.076 mV 0.076 mV 0.076 mV 1023 1023 1023 1023 4095 4095 4095 4095 16,383 16,383 16,383 16,383 65,535 65,535 65,535 65,535 1 2 5 10 1 2 5 10 1 2 5 10 1 2 5 10 7-1 5 2.5 1 0.5 5 2.5 1 0.5 5 2.5 1 0.5 5 2.5 1 0.5 4.9 2.45 0.98 0.49 1.2 0.6 0.24 0.12 0.305 0.153 0.061 0.031 0.076 0.038 0.015 0.008 Matching ADI In-Amps with Some Popular ADCs Table 7-2 shows recommended ADCs for use with the latest generation of ADI in-amps. Table 7-2. Recommended ADCs for Use with ADI In-Amps ADI In-Amp AD8221AR ADI In-Amp AD620AR Small Signal BW: Noise (eNI): VOS: In-Amp Gain: Maximum Output Voltage Swing: CMR: Nonlinearity: Supply Voltage: Supply Current: 0.01% Settling Time for 5 V Step: 0.001% Settling Time for 5 V Step: 562 kHz 8 nV/√Hz 60 V max 10 Small Signal BW: Noise (eNI): VOS: In-Amp Gain: Maximum Output Voltage Swing: CMR: Nonlinearity: Supply Voltage: Supply Current: 0.01% Settling Time for 5 V Step: 800 kHz 9 nV/√Hz 125 V max 10 3.9 V 90 dB (dc to 60 Hz) 10 ppm max 5 V 1 mA max 5 s 6 s Recommended ADI ADC#1 AD7685, AD7687 Resolution: 16 bits Input Range: 0 V to 5 V Sampling Rate: Up to 250 kSPS S/D Supply: 3 V or 5 V Power: 1.7 mW @ 2.5 V and 6 mW typ @ 5 V Comments: Same package, the AD7685 can be driven through a simple RC from the AD8221 directly. The REF pin can be driven to fit the ADC range. Recommended ADI ADC#2 AD7453/AD7457 Resolution: 12 bits Input Range: 0 V to VDD Sampling Rate: 555 kSPS/100 kSPS S/D Supply: 3 V or 5 V Power: 0.3 mA @ 100 kSPS Comments: Single channel, pseudo differential inputs in a SOT-23 package 7-2 3.9 V 73 dB (dc to 60 Hz) 40 ppm max 5 V 1.3 mA max 7 s Recommended ADI ADC#1 AD7610, AD7663 Resolution: 16 bits Input Range: Multiple, such as 10 V, 5 V, ... Sampling Rate: Up to 250 kSPS S/D Supply: 5V Power: 2.7 mA @ 100 kSPS Comments: Allow more and larger input ranges Recommended ADI ADC#2 AD7895 Resolution: 12 bits Input Range: Multiple, such as 10 V, 2.5 V, 0 V to 2.5 V Sampling Rate: 200 kSPS S/D Supply: 5V Power: 2.2 mA @ 100 kSPS Comments: Allows a bipolar or unipolar input with a single supply Table 7-2. Recommended ADCs for Use with ADI In-Amps (continued) ADI In-Amp AD8225 Fixed Gain of 5 Small Signal BW: Noise (eNI): VOS: In-Amp Gain: Maximum Output Voltage Swing: CMR: Nonlinearity: Supply Voltage: Supply Current: 0.01% Settling Time for 5 V Step: 0.001% Settling Time for 5 V Step: 900 kHz 8 nV/√Hz 125 V max 5 Recommended ADI ADC#1 AD7866 Resolution: 12 bits Input Range: 0 V to VREF V or 0 V to 2 VREF V Sampling Rate: 1 MSPS for both ADCs S/D Supply: Single, 2.7 V to 5.25 V Power: 24 mW max at 1 MSPS with 5 V supply 11.4 mW max at 1 MSPS with 3 V supply Comments: Dual, 2-channel, simultaneous sampling ADC with a serial interface 4 V 90 dB (dc to 60 Hz) 10 ppm max 5 V 1.2 mA max 3.2 s Recommended ADI ADC#2 AD7862/AD7864 Resolution: 12 bits Input Range: 0 V to +2.5 V, 0 V to +5 V, 2.5 V, 5 V, 10 V Sampling Rate: 600 kSPS for one channel S/D Supply: Single, 5 V Power: 90 mW typ Comments: 4-channel, simultaneous sampling ADC with a parallel interface 4 s Recommended ADI ADC#1 AD7661 Resolution: 16 bits Input Range: 0 V to 2.5 V Sampling Rate: Up to 100 kSPS S/D Supply: 5V Power: 8 mA @ 100 kSPS with reference Comments: Provide a reference voltage Recommended ADI ADC#3 AD7863/AD7865 Resolution: 14 bits Input Range: 0 V to +2.5 V, 0 V to +5 V, 2.5 V, 5 V, 10 V Sampling Rate: 175 kSPS for both channels/ 360 kSPS for one channel, respectively S/D Supply: Single, 5 V Power: 70 mW typ/115 mV typ, respectively Comments: 2-/4-channel, respectively, simultaneous sampling ADC with a parallel interface Recommended ADI ADC#2 AD7940 Resolution: 14 bits Input Range: 0 V to VDD Sampling Rate: 100 kSPS S/D Supply: 3 V or 5 V Power: 0.83 mA @ 100 kSPS Comments: Single channel in an SOT-23 ADI In-Amp AD623AR Small Signal BW: Noise (eNI): VOS: In-Amp Gain: Maximum Output Voltage Swing: CMR: Nonlinearity: Supply Voltage: Supply Current: 0.01% Settling Time for 5 V Step: 100 kHz 35 nV/√Hz 200 V max 10 Recommended ADI ADC#4 AD7890/AD7891/AD7892 Resolution: 12 bits Input Range: 0 V to +2.5 V, 0 V to +4.096 V, 0 V to +5 V, 2.5 V, 5 V10 V Sampling Rate: 117/500/600 kSPS, respectively S/D Supply: Single, 5 V Power: 30/85/60 mW typ, respectively Comments: 8-/8-/1-channel, respectively 4.5 V 90 dB (dc to 60 Hz) 50 ppm typ 5 V 0.55 mA max 20 s 7-3 Table 7-2. Recommended ADCs for Use with ADI In-Amps (continued) ADI In-Amp AD627AR Small Signal BW: Noise (eNI): VOS: In-Amp Gain: Maximum Output Voltage Swing: CMR: Nonlinearity: Supply Voltage: Supply Current: 0.01% Settling Time for 5 V Step: 30 kHz 38 nV/√Hz 200 V max 10 4.9 V 77 dB (dc to 60 Hz) 100 ppm max 5 V 85 A max 135 s Recommended ADI ADC#1 AD7923/AD7927 Resolution: 12 bits Input Range: 0 V to VREF or 0 V to 2 VREF Sampling Rate: 200 kSPS S/D Supply: Single, 2.7 V to 5.25 V Power: 3.6 mW max @ 200 kSPS with a 3 V supply Comments: 8-/4-channel ADCs, respectively, with a serial interface and channel sequencer ADI In-Amp JFET In-Amp AD8220AR Small Signal BW: Noise (eNI): VOS: In-Amp Gain: Maximum Output Voltage Swing: CMRR: Nonlinearity: Supply Voltage: Supply Current: 0.01% Settling Time for 5 V step: 1000 kHz 15 nV/√Hz 1 mV max 10 64.8 V 110 dB (dc to 60 Hz) 10 ppm max Dual, 65 V 1 mA max 5 ms Recommended ADI ADC#1 AD7610/AD7663 Resolution: 16 bits 62.5 V, 65 V, 610 V Input Range: Sampling Rate: 250 kSPS for both ADCs 65 V to 615 V and 5 V S/D Supply: Recommended ADI ADC#2 AD7920 Resolution: 12 bits Input Range: 0 to VDD Sampling Rate: 250 kSPS S/D Supply: 2.35 V or 5.25 V Power: 3 mW typ @ 250 kSPS with 3 V supply Comments: Single channel, serial ADC in 6-lead SC70 7-4 Recommended ADC#2 AD7321, AD7323, and AD7327 Resolution: 13 bits 62.5 V, 65 V, 610 V Input Range: Sampling Rate: 500 kSPS 65 V to 615 V and +5 V S/D Supply: Power: 17 mW max at 0.5 MSPS with 615 V, and 5 V supply Recommended ADI ADC#3 AD7898-3 Resolution: 12 bits 62.5 V Input Range: Sampling Rate: 220 kSPS S/D Supply: 5V Power: 22.5 mW max at 220 kSPS with 5 V supply Table 7-2. Recommended ADCs for Use with ADI In-Amps (continued) ADI In-Amp Zero Drift In-Amp AD8230RZ ADI In-Amp AD8250/AD8251 High Speed Programmable Gain In-Amp Small Signal BW: Noise (eNI): VOS: In-Amp Gain: Maximum Output Voltage Swing: CMRR: Nonlinearity: Supply Voltage: Supply Current: 2 kHz 240 nV/√Hz 10 mV max 10 Small Signal BW: Noise (eNI): VOS: In-Amp Gain: Maximum Output Voltage Swing: CMRR: Nonlinearity: Supply Voltage: Supply Current: 0.01% Settling Time for 5 V step: 64.7 V 120 dB (dc to 60 Hz) 20 ppm max 65 V 3.5 mA max Recommended ADI ADC#1 AD7942 Resolution: 14 bits Input Range: 5V Sampling Rate: 250 kSPS S/D Supply: 2.7 V to 5.25 V Power: 1.25 mW, 2.5 V supply 10 MHz 13 nV/√Hz 100 mV 10 VCC – 1.2 V, VCC + 1.2 V 100 dB (dc to 60 Hz) 40 ppm max Dual, 65 V to 612 V 3 mA typ 0.5 ms Recommended ADI ADC#1 AD7685, AD7687 Resolution: 16 bits Input Range: 5V Sampling Rate: 250 kSPS S/D Supply: Single, 2.5 V to 5 V Power: 4 mW at 0.1 kSPS, 5 V supply Recommended ADI ADC#2 AD7321 Resolution: 13 bits 62.5 V Input Range: Sampling Rate: 500 kSPS 65 V to 615 V, S/D Supply: 2.7 V to 5.25 V Power: 17 mW max at 500 kSPS with 615 V, 5 V supply Recommended ADI ADC#2 AD7327, AD7323, and AD7321 Resolution: 13 bits/12 bits 62.5 V Input Range: Sampling Rate: 0.5 MSPS 65 V to 615 V, single, 5 V S/D Supply: Power: 17 mW max at 500 kSPS with 615 V, 5 V supply NOTE: Specifications are preliminary. Refer to www.analog.com. 7-5 Table 7-2. Recommended ADCs for Use with ADI In-Amps (continued) ADI In-Amp Zero Drift In-Amp AD8553RM ADI In-Amp Zero Drift In-Amp AD8555AR/AD8556ARZ Small Signal BW: Noise (eNI): VOS: In-Amp Gain: Maximum Output Voltage Swing: CMRR: Nonlinearity: Supply Voltage: Supply Current: 1 kHz 150 nV/√Hz 50 mV max 10 Small Signal BW: Noise (eNI): VOS: In-Amp Gain: Maximum Output Voltage Swing: CMRR: Nonlinearity: Supply Voltage: Supply Current: 0.1% Settling Time for 4 V step: 150 kHz 32 nV√Hz 10 mV max 10 0.075 V to 4.925 V 120 dB (dc to 60 Hz) 600 ppm max Single, 5 V 1.3 mA max Recommended ADI ADC#1 AD7476 Resolution: 12 bits Input Range: 0 to VDD Sampling Rate: 1 MSPS S/D Supply: 2.35 V to 5.25 V Power: 3.6 mW max at 1 MSPS with 3 V supply 15 mW max at 1 MSPS with 5 V supply 30 mV to 4.94 V 100 dB (G = 70, dc to 200 Hz) 1000 ppm typ Single, 5 V 2.5 mA max 8 ms Recommended ADI ADC#1 AD7685 Resolution: 16 bits Input Range: 5V Sampling Rate: 250 kSPS S/D Supply: Single, 2.5 V to 5 V Power: 4 mW at 0.1 SPS with 5 V supply Recommended ADI ADC#2 AD7466 Resolution: 12 bits Input Range: 0 to VDD Sampling Rate: 100 kSPS S/D Supply: 1.6 V to 3.6 V Power: 0.62 mW max at 100 kSPS with 3 V supply 0.12 mW max at 100 kSPS with 1.6 V supply Recommended ADI ADC#2 AD7476 Resolution: 12 bits Input Range: 0 to VDD Sampling Rate: 1 MSPS S/D Supply: 2.35 V to 5.25 V Power: 3.6 mW max at 1 MSPS with 3 V supply 15 mW max at 1 MSPS with 5 V supply Recommended ADI ADC#3 AD7476A Resolution: 12 bits Input Range: 0 to VDD Sampling Rate: 1 MSPS S/D Supply: 2.7 V to 5.25 V Power: 3.6 mW max at 1 MSPS with 3 V supply 12.5 mW max at 1 MSPS with 5 V supply 7-6 PIXEL #2 PIXEL #1 PIXEL LEVEL REFERENCE LEVEL INSTRUMENTATION AMPLIFIER PIXEL LEVEL INPUT NEED 12-BIT SAMPLEAND-HOLD ACCURACY @1MHz REFERENCE LEVEL INPUT 2MHz 500ns ADC DC CORRECTED OUTPUT AD7266, AD7322, ETC. TOTAL SETTLING TIME FOR SAMPLE-AND-HOLD AND IN-AMP MUST BE LESS THAN 500ns Figure 7-1. In-amp buffers ADC and provides dc correction. High Speed Data Acquisition As the speed and accuracy of modern data acquisition systems have increased, a growing need for high bandwidth instrumentation amplifiers has developed— particularly in the field of CCD imaging equipment where offset correction and input buffering are required. Here, double-correlated sampling techniques are often used for offset correction of the CCD imager. As shown in Figure 7-1, two sample-and-hold amplifiers monitor the pixel and reference levels, and a dc-corrected output is provided by feeding their signals into an instrumentation amplifier. Figure 7-2 shows how a single multiplexed high bandwidth in-amp can replace several slow speed nonmultiplexed buffers. The system benefits from the common-mode noise reduction and subsequent increase in dynamic range provided by the in-amp. HIGH SPEED IA SIGNAL INPUTS MUX ADC SIGNAL INPUTS MUX AD7266, AD7322, ETC. Figure 7-2. Single high speed in-amp and mux replace several slow speed buffers. Previously, the low bandwidths of commonly available instrumentation amplifiers, plus their inability to drive 50 loads, restricted their use to low frequency applications—generally below 1 MHz. Some higher bandwidth amplifiers have been available, but these have been fixed-gain types with internal resistors. With these amplifiers, there was no access to the inverting and noninverting terminals of the amplifier. Using modern op amps and employing the complementary bipolar (CB) process, video bandwidth instrumentation amplifiers that offer both high bandwidths and impressive dc specifications may now be constructed. Common-mode rejection may be optimized by trimming or by using low cost resistor arrays. 7-7 The bandwidth and settling time requirements demanded of an in-amp buffering an ADC, and for the sample-and-hold function preceding it, can be quite severe. The input buffer must pass the signal along fast enough so that the signal is fully settled before the ADC takes its next sample. At least two samples per cycle are required for an ADC to unambiguously process an input signal (FS/2)—this is referred to as the Nyquist criteria. Therefore, a 2 MHz ADC, such as the AD7266 or AD7322, requires that the input buffer/sample-and-hold sections preceding it provide 12-bit accuracy at a 1 MHz bandwidth. Settling time is equally important: the sampling rate of an ADC is the inverse of its sampling frequency—for the 2 MHz ADC, the sampling rate is 500 ns. This means that for a total throughput rate of less than 1 s, these same input buffer/sample-and-hold sections must have a total settling time of less than 500 ns. and high speed at moderate gains. Circuit gain is set by resistor RG where gain = 1 + 2 R F /RG. The R F resistors should be kept at around 1 k to ensure maximum bandwidth. Operating at a gain of 10 (using a 222 resistor for RG ) the –3 dB bandwidth of this circuit is approximately 3.4 MHz. The ac common-mode rejection ratio (gain of 10, 1 V p-p common-mode signal applied to the inputs) is 60 dB from 1 Hz to 200 kHz and 43 dB at 2 MHz. And it provides better than 46 dB CMRR from 4 MHz to 7 MHz. The RFI rejection characteristics of this amplifier are also excellent—the change in dc offset voltage vs. common-mode frequency is better than 80 dB from 1 Hz up to 15 MHz. Quiescent supply current for this circuit is 15 mA. A High Speed In-Amp Circuit for Data Acquisition This circuit can be used to drive a modern, high speed ADC such as the AD871 or AD9240, and to provide very high speed data acquisition. The AD830 can also be used for many high speed applications. Figure 7-3 shows a discrete in-amp circuit using two AD825 op amps and an AMP03 differential (subtractor) amplifier. This design provides both high performance For lower speed applications requiring a low input current device, the AD823 FET input op amp can be substituted for the AD825. +VS 0.01MF –VIN AD825* VOUT 0.01MF –IN RF 25k6 2 25k6 7 +VIN RG 222k6 RF 1k6 AMP03 6 0.01MF AD825* SENSE 1k6 –VS +VS 5 4 +IN 3 25k6 25k6 1 +VS OUTPUT –VS REF 0.01MF –VS * REFER TO ANALOG DEVICES WEBSITE AT WWW.ANALOG.COM FOR THE LATEST OP AMP PRODUCTS AND SPECIFICATIONS. Figure 7-3. A High performance, high speed in-amp circuit. 7-8 ADC