AN4368

Conditioning the signal

DocID025334 Rev 1

13/18

Figure 10 shows the output voltage of stage 2 where the signal has been further amplified.

In this case, the Vout1 signal has been amplified by -52. Clear peaks can be detected. This

figure allows us to see how common mode voltage has changed with amplification. In

Figure 8: "Output voltage of the PIR sensor", common mode voltage (Vicm) was 1.1 V,

whereas below, common mode voltage is half Vcc.

Figure 10: Output voltage of stage 2

The signal generated by the sensor is now sufficiently amplified to set a threshold

detection. The divider bridges and the op-amps used as comparators create a window

comparator. In our case (see Section 3.3: "Stage 3"), we have a low threshold voltage of

0.53 V and a high threshold voltage of 2.77 V (Vcc - 0.53 V). At the output of this

comparator window, a NAND gate could be added to use only one microcontroller input.