AN4368

Three-stage architecture for detecting motion

DocID025334 Rev 1

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Three-stage architecture for detecting motion

To detect motion, a three-stage architecture based on op-amps is used to condition the

signal of a PIR sensor. This architecture is explained below.

3.1

Stage 1

The first architectural stage amplifies the signal. It cancels the DC part of the signal and

filters the high frequency noise that could lead to false detections. The schematic of this

first architectural stage is shown in Figure 3.

Figure 3: Schematic of the first architectural stage

1.

C = capacitor, D = diode, R = resistor, U = op-amp

higher frequencies because usually we are detecting human motion.

Since we do not amplify the DC part of the signal, the Vio, which is the input offset voltage

of the op-amp, has no importance in this application.

higher than the noise level. The DC bias of the first architectural stage is determined by the

sensor. To avoid saturation, the gain must not be too big because amplification is made

around this common mode voltage (which is not Vcc/2).

STMicroelectronics’ TSU101 op-amp is a perfect fit for this architectural stage. As well as

having very low consumption, the TSU101 operates with a typical current consumption of

only 600 nA at Vcc = 3.3 V.