9.2.1 Geophone Application

Figure 79 illustrates a typical geophone application circuit. The application shows the ADC operating on a bipolar ±2.5-V analog supply voltage. The power-supply voltages are provided by low-dropout (LDO) regulators to provide well-regulated, low-noise supply voltages to the ADC. The ADC also operates using with a unipolar 5-V analog supply voltage. The 6-V zener diode between AVDD and AVSS clamps the ADC supply voltage if the inputs are driven when the ADC supply voltage is off.

Resistors R₁ and R₂ bias the floating geophone to mid-supply. The resistors also provide a return path for the ADC input current. To prevent pickup of PCB ground-related noise, connect the resistors together first, then connect to ground. For unipolar-supply operation, make this connection to a low-impedance 2.5-V voltage.

The geophone signal is filtered both differentially, by components C₃ and R₃ through R₆, and common-mode filtered by components C₁, C₂ and R₃, R₄. The differential filter removes normal-mode noise. The common-mode filters remove noise that is common to both inputs. The differential filter high-cut frequency is 10 times lower to minimize the effects of component mismatch of the common-mode filter that otherwise can lead to degraded differential-filter performance. Adjust the filter components according to the application requirements. The protection diodes protect the ADC inputs from system-level ESD transients and signal overrange events.

A low-noise, low-power, 2.5-V voltage reference drives the ADC reference input. The voltage reference ground terminal is connected to –2.5 V. R₉ and C₅ form an optional 2-Hz noise filter to reduce voltage reference noise. Capacitor C₆ filters the reference sampling glitches. Place the capacitor directly at the ADC pins. Multiple ADCs can share a single reference but place a 0.1-µF capacitor at each ADC reference input.

Capacitor C₄ (10 nF), located at the CAPP and CAPN pins, filters modulator sampling glitches. The capacitor also provides a antialiasing filter with a high cutoff corner frequency of approximately 9.5 kHz. Resistors R₇ and R₈ provide an offset voltage to the modulator for idle tone reduction when operating in high-resolution mode. Use the internal offset in low-power mode operation. The resistors are not needed in this case.

Figure 79. Geophone Analog Interface