A pyroelectric suppressor circuit for preventing undesirable thermally induced signals generated by a piezoelectric physical activity sensor from reaching processor circuitry within an implantable medical device is provided. The thermally induced signals typically have frequencies below a frequency in the range from about 0.1 mHz to about 10 mHz. The suppressor circuit provides a high-pass filter that rejects signals that have frequencies associated with thermally induced signals. Signals having frequencies greater than a frequency in the range from about 0.1 mHz to about 10 mHz, which correspond to patient activity, are passed on to processing circuitry of the implantable medical device.