9.2.2 Detailed Design Procedure

Referring to Figure 32, the TLV8544 4-channel op amp is powered directly by a 3.3-V CR2032 coin battery. The first two amplifier stages of the TLV8544 implement active filter functionality. The remaining two amplifiers of the TLV8544 are used for building a window comparator. The comparator flags the detection of a motion event to an ultra-low-power wireless microcontroller on the same board. Due to the higher gain in the filter stages and higher output noise from the sensor, it is necessary to optimize the placement of the high-frequency filter pole and the window comparator thresholds to avoid false detection.

The first two amplifiers (A and B) in the circuit are used in identical active bandpass filters with corner frequencies of 0.7 and 10.6 Hz. Each filter stage has a gain of about 220 V/V to account for the reduced sensitivity of the sensor due to the low current biasing of the PIR sensor. Considering the 8-kHz unity gain bandwidth (UGBW) product of the TLV8544, the bandwidth of each stage is limited to approximately 36 Hz. The above choice of cutoff frequencies give a relatively wide bandwidth to detect a person running in the field of view, yet narrow enough to limit the peak-to-peak noise at the output of the filters.

Amplifier A is a noninverting gain/filter stage providing the high input impedance needed to prevent loading of the sensor. The DC gain of the stage due to the presence of C6 is unity. Therefore, the sensor output provides the bias voltage needed at the A stage to avoid clipping of the lower cycle of the input signal. Diodes D1 and D2 limit the output signal, avoiding overdriving of the second stage and consequently placing a large charge on coupling capacitor C4, which helps with the recovery time.