Three-stage architecture for detecting motion

AN4368

8/18

DocID025334 Rev 1

(see Table 1). If only one digital output is required, the user may use a NAND gate

Table 1: NAND truth table

Vout2

Output NAND gate

Vout2 < 0.16 * Vcc

High

Low

High

0.16 * Vcc < Vout2 < 0.84 * Vcc

High

High

Low

Vout2 > 0.84 * Vcc

Low

High

High

3.4

The best op-amp for each architectural stage

The schematics show that a quad op-amp is a good choice for the current application: one

op-amp for stage 1, a second op-amp for stage 2 and two additional op-amps for stage 3.

In this respect, the TSU104 is ideal as only one active component is needed for signal

conditioning. The one drawback of the TSU104 is that it uses two op-amps for a

comparator function but, this is not an issue at such low speeds.

However, it may be better to use an open drain comparator (TS332) whose outputs are

connected together as described in Figure 6.

Figure 6: Window comparator schematic

1.

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

As long as there is no detection, the output is in high state and it minimizes consumption

through the pull-up resistor.

The global schematic that shows the three architectural stages together is available in

Section 8: "Appendix A".