Referring to Figure 9-1, Schottky diodes (BAS70 or equivalent) protect the ADC inputs from voltage overloads. The ADC inputs are protected from ESD events by the optional ESD protection diodes (TVS0701). The geophone signal is level-shifted to mid-supply by driving the input termination resistors (R₁ and R₂) common point to 2.5 V. The level-shift voltage is derived from the reference voltage and is buffered by the OPA391 op amp. The input termination resistors also provide the input bias current return path for the ADC inputs.

The input signal is filtered to reduce out-of-band noise. The filter is comprised of common-mode and differential sections. The common-mode section filters noise common to both inputs, consisting of R₃, R₄, C₁, and C₂. The differential section filters differential noise, consisting of R₃ through R₆ and C₃. The resistor values are kept low to reduce thermal noise.

The REF6241 4.096-V voltage reference is used. The ADC must be programmed to match the value of the reference voltage. The optional noise filter consisting of R₇ and C₃ reduces reference noise. Resistor R₇ increases gain error resulting from the impedance of the reference input.

The AVDD1 power supply voltage = 5 V, with AVSS connected to AGND. The AVDD2 voltage is 2.5 V to minimize power consumption. If IOVDD = 1.8V, connect the CAPD pin to IOVDD.

Besides the power supply pins, place additional capacitors at certain pins. Capacitors are required between CAPP – CAPN, REFP – REFN, and at the CAPBP, CAPBN, CAPI, CAPR, CAPC, and CAPD pins with the capacitance values given in Figure 9-1. The CAPP – CAPN, CAPBP, and CAPBN capacitors are C0G type.

The DAC1282 provides a low-distortion signal to test THD performance, and through the DAC1282 dc test mode, test geophone impulse response. Increase the value of the DAC1282 capacitors CAPP and CAPN to 10 nF to optimize the ADS1285 THD test performance. See the DAC1282 data sheet for additional circuit details.