Resistors R3 and R4 provide a pathway for the input bias current of the OPA1678 while maintaining the high input impedance of the circuit. The contact microphone capacitance and the required
low-frequency response determine the values of R3 and R4. The –3-dB frequency formed by the microphone capacitance and amplifier input impedance is shown in Equation 4:

Equation 4.

A piezo element with 8 nF of capacitance was selected for this design because the 9-kHz resonance is towards the upper end of the audible bandwidth, and is less likely to affect the frequency response of many musical instruments. The minimum value for resistors R3 and R4 is then calculated with Equation 5:

Equation 5.

1-MΩ resistors are selected for R3 and R4 to make sure the circuit meets the design requirements for –3-dB bandwidth. The center point of resistors R3 and R4 is biased to half the supply voltage through the voltage divider formed by R5 and R6. This sets the input common-mode voltage of the circuit to a value within the input voltage range of the OPA1678. Piezo elements can produce very large voltages if the elements are struck with sufficient force. To prevent damage, the input of the OPA1678 is protected by a transient voltage suppressor (TVS) diode placed across the preamplifier inputs. The TPD1E1B04 TVS was selected due to low capacitance and the 6.4-V clamping voltage does not clamp the desired low amplitude vibration signals. Resistors R14 and R15 limit current flow into the amplifier inputs in the event that the internal protection diodes of the amplifier are forward-biased.