9.2.1 Geophone Interface

A typical geophone front-end application is shown in Figure 81. The application diagram shows the ADS1284 operation with dual ±2.5-V analog supplies. The ADS1284 can also operate with a single 5-V analog supply.

1. Optional external diode clamps.

2. Optional reference noise filter.

Figure 81. Geophone Interface Application

The geophone input signal is filtered by both a differential filter (components C₄ and R₁ to R₄) and by common-mode filters (components C₂, C₃ and R₁, R₂). The differential filter removes high-frequency normal-mode components from the input signal. The common-mode filters remove high-frequency components that are common to both input leads. The input filters are not required for all applications; check the system requirements for each application.

Resistors R₅ and R₆ bias the signal input to the midsupply point (ground). For single-supply operation, set the bias to a low impedance midsupply point (AVDD / 2 = 2.5 V).

Optional diode clamps protect the ADS1284 inputs from high-level voltage transients and overloads. The diodes provide additional protection if possible high-level input transients and surges exceed the ADC internal ESD diode rating.

The REF5050 5-V reference provides the reference to the ADC. An optional filter network ®₇ and C₅) reduces the in-band reference noise for improved dynamic performance. However, the RC filter network increases the filter settling-time (from seconds to possibly minutes) depending on the dielectric absorption properties of capacitor C₅. Capacitor C₇ is mandatory and provides high-frequency bypassing of the reference inputs; place C₇ as close as possible to the ADS1284 pins. Resistor R₇ (1 kΩ) results in a 1% systematic gain error. Multiple ADCs can share a single reference, but if shared, use independent reference filters for each ADC.

As an alternative, the REF5045 (4.5 V) reference can be used. The REF5045 reference has the advantage of operating directly from the 5-V (total) power supply; however, the 4.5-V reference reduces signal range by 10% and results in a 1-dB loss of SNR.

Capacitor C₆ (10 nF) filters the PGA output glitches caused by sampling of the modulator. This capacitor also forms an antialias filter with a low-pass cutoff frequency of 26 kHz.