Precision Single-Supply Differential ADC Driver for Industrial-Level Signals (CN0180)

Standard single-ended industrial signal levels (±5 V, ±10 V, or 0 V to +10 V) are not directly compatible with the differential input range of modern high-performance 16-bit or 18-bit single-supply SAR ADCs. Appropriate interface driver circuits are required to attenuate, level-shift, and differentially convert industrial signals to have the correct amplitude and common-mode voltage to match the ADC input requirements. Although appropriate interface circuits can be designed using resistor networks and dual op amps, errors in the ratio matching of the resistors and errors between the amplifiers will result in errors at the final output. Especially at low power levels, achieving the required output phase matching and settling time can be very difficult.

The circuit shown in Figure 1 uses the AD8475 differential amplifier to perform attenuation, level shifting, and differential conversion without requiring any external components. Its ac and dc performance is compatible with the 18-bit, 1 MSPS AD7982 PulSAR® ADC and other 16/18-bit members of the family that can sample at rates up to 4 MSPS.

The AD8475 is a fully differential attenuating amplifier with integrated precision thin film gain setting resistors that provides precision attenuation (0.4× or 0.8×), common-mode level shifting, single-ended to differential conversion, and input overvoltage protection. It consumes only 15 mW when powered from a single 5 V supply. The 18-bit, 1 MSPS AD7982 consumes only 7 mW, 30 times less than competing products. The total power consumption of the combination is only 22 mW.

Figure 1. Single-ended-to-differential ADC driver (Simplified schematic: decoupling and all connections not shown)

Circuit Description

The AD8475 attenuating amplifier and the AD7982 18-bit differential ADC can be used to process large voltage signals in high precision analog front-end systems when powered from a single 5 V supply.

The AD8475 attenuates the input signal by a factor of 0.4 through its integrated precision trim resistors. It supports voltages up to 25 V peak-to-peak on a single 5 V supply. At low frequencies, the differential rail-to-rail output requires only 50 mV of headroom. The AD8475 can be driven by a true differential input, or it can be driven by a single-ended input and provide single-ended to differential conversion, as shown in Figure 1.

The RC network between the AD8475 and the ADC forms a single-pole filter that reduces undesirable aliasing effects and high frequency noise. The filter has a common-mode bandwidth of 29.5 MHz (20 Ω, 270 pF) and a differential bandwidth of 3.1 MHz (40 Ω, 1.3 nF).

The AD7982 is an 18-bit, successive approximation register (SAR) ADC that operates from a single power supply (VDD). The I/O interface voltage, VIO, can be set from 1.8 V to 5 V, depending on the interface logic supply. The AD7892 has true differential inputs that accept voltages up to ±VREF. The ADR435 is a 5 V, low noise (8 μV p-p, 0.1 Hz to 10 Hz), high precision (±2 mV for B grade) reference voltage source that is used to provide the REF voltage for the AD7982 and the supply voltage for the differential driver AD8475.

The two 10 kΩ resistors connected to the output of the ADR435 form a voltage divider to set the common-mode voltage (VOCM) at the output of the AD8475 to 2.5 V. This centers the input differential signal around the optimum common-mode input voltage, maximizing the dynamic range of the ADC.

For a 20 V p-p single-ended input signal, each differential output produces a signal that swings from +0.5 V to +4.5 V and is 180° out of phase.

When a 20 kHz, 20 V p-p signal is input into the AD8475, the resulting SNR is 96.3 dBFS and the THD is −112.3 dBFS; both parameters are referenced to the full-scale range of the AD7982, as shown in the FFT plot in Figure 2.

The signal levels shown in the table below the FFT plot are measured at the input of the AD7982. The full-scale range at this point is 10 V p-p differential, which reflects 25 V p-p to the input of the AD8475. The input test signal is 20 V p-p, which produces a signal 2 dB below the full-scale input of the ADC.

Figure 2. FFT plot of a 20 kHz signal, 2 dB below full scale, sampled at 1 MSPS

Note that under these conditions, the AD8475 can swing to within ±500 mV of the supply rails at each output and still achieve excellent distortion performance due to its rail-to-rail output structure.

This circuit has been tested using the AD8475 evaluation board (EVAL-AD8475Z) and the Analog Devices, Inc., PulSAR evaluation board and software. The EVAL-AD8475Z is a customer evaluation board designed to simplify standalone testing of the AD8475’s performance and functionality. The board can also be quickly tested with any Analog Devices, Inc., SAR converter, and its output connector can be directly connected to the analog input connector (SMB) of the PulSAR evaluation board (EVAL-AD76xx-CBZ). ADC Converter Evaluation and Development Board (EVAL-CED1Z) The setup is connected to a PC via USB as shown in Figure 3.

The input signal to the AD8475 is provided by an Audio Precision® AP2700 source. The LabVIEW®-based PulSAR evaluation software can be used to control the Audio Precision input signal and can also be used to monitor the input and output of the ADC.

Figure 3. SAR ADC converter evaluation platform. Note: The AD8475 differential amplifier evaluation board is connected to the SMB analog input connector of the PulSAR ADC evaluation board.

The circuit has been proven to operate stably and with good accuracy using the component values ​​shown. Other Analog Devices analog-to-digital converters can be used in place of the AD7982 to achieve the highest performance required. The AD8475 is capable of driving up to 18-bit, 4 MSPS ADCs with minimal performance degradation. Faster 18-bit ADCs include the AD7984 (1.33 MSPS) and AD7986 (2 MSPS). Differential 16-bit ADCs include the AD7688 (500 kSPS) and AD7693 (500 kSPS).

The AD8475 can also be used directly to drive single-ended or pseudo differential input ADCs. The rail-to-rail outputs of the AD8475 can be driven to within 0.5 V of each supply rail without significant degradation in ac performance.

The AD8475 can also provide a 0.8x differential attenuation by driving the IN 0.8× input. The supply voltage is very flexible, operating from a single supply of 3 V to 10 V or dual supplies up to ±5 V.