AN4368 datasheet(7/18 Pages) STMICROELECTRONICS

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AN4368

Three-stage architecture for detecting motion

DocID025334 Rev 1

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3.3

Stage 3

The third and last architectural stage allows the user to perform a window comparator. As a

consequence, the signal is perfectly conditioned to go to a microcontroller. Figure 5 shows

the schematic of this stage.

Figure 5: Schematic of the third architectural stage

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

When a heat source is detected, the output of the op-amp U3 and/or U4 is in low state. The

divider bridge, composed of the resistors R6, R7, R8, and R9, is used to set the voltage

reference of these devices. The TSU101 op-amp is an input/output rail to rail op-amp, and

there is no constraint on the input common mode voltage. Thus, while the voltage

references of U3 and U4 are within the Vcc range the window comparator works.

For the following example, the reference of U3 is set to 0.84 * Vcc:

When the signal (Vout2) is above this reference (2.77 V if Vcc = 3.3 V), the output of U3 is

in low state, close to ground.

U4 is used to detect when the signal is below a certain reference. In this example, the

voltage reference is set to 530 mV thanks to the divider bridge composed of R6, R7, R8, and

R9. The voltage reference calculation is:

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