ZHCSH75A September 2017 – December 2017 ADS7142
PRODUCTION DATA.
The analog signal conditioning circuit is shown in the schematic in Figure 120. The first stage of the amplifier filter acts as a bandpass filter while the second stage applies an inverting gain. Components R10 and C5 serve as a low-pass filter to stabilize the supply voltage at the input to the sensor. Resistor R5 sets the bias current in the JFET output transistor of the PIR motion sensor. To save power, R5 is larger than recommended and essentially current starves the sensor. This comes at the expense of decreased sensitivity and higher output noise at the sensor output, which is a fair tradeoff for increased battery lifetime. Some of the loss in sensitivity at the sensor output can be compensated by a gain increase in the filter stages. Stage 1 of Figure 120 is arranged as a non-inverting gain filter stage. This provides a high-impedance load to the sensor so its bias point remains fixed. Because this stage has an effective DC gain of one due to C2, the sensor output bias voltage provides the DC bias for the first filter stage. Feedback diodes D1 and D2 provide clamping so that the op amps in both filter stages stay out of saturation for motion events which are close to the sensor. Stage 1 has a low and high cutoff frequency of 0.7 Hz and 10.6 Hz respectively and a gain of 220. Stage 2 is arranged as an inverting summer gain stage and is AC-coupled to Stage 1. A DC bias of VCC/2 is connected to the non-inverting input of the amplifier in this stage. Due to the higher gain in the filter stages and higher output noise from the sensor, care must be taken to optimize the placement of the high-frequency filter pole and the window comparator thresholds to avoid false detection.