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I'm new to opamps and I could use some help amplifying a PIR sensor and output to a microcontroller.

Minimum component count without compromising performance (highly integrated solutions).

Here is a provisional circuit I drew:

enter image description here

It uses a instrumentation amplifier as pre-amp and a duel comparator for window detector.

I know people usually use quad general purpose opamp (1 pre-amp, 2 comparator, 1 post-amp) for this but they require loads of external components.

  • Q1 is IRA-S210ST01.
  • Does Ref pin need buffering ? perhaps a unity-gain amp ? but it would increase component count.

Any comment is greatly appreciated !

Update: enter image description here

added ref buffer.

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2 Answers 2

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The typical input resistance of the REF pin is given as 20k. The external resistance connected to this pin is 200k (R4+R5). This means that the net resistance across the REF pin will be 18.2k which means the reference voltage will shift.

So, you should either

  • buffer the reference voltage, or
  • decrease the divider resistors.

The first option will increase the total power consumption and can be a bit problematic if your circuit will be supplied from a battery. The other option will increase the complexity.

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  • \$\begingroup\$ Thanks for the comment on the ref buffer. Could you please comment on the comparator part as well ? \$\endgroup\$
    – 7E10FC9A
    Oct 22, 2021 at 10:03
  • \$\begingroup\$ @7E10FC9A The input bias current of LT1720 can be as low as -6uA. This may cause the voltages of dividers to shift. Maybe you should decrease the divider resistors. I personally suggest you build the circuit, perform test and measurements, and fine tune if necessary. \$\endgroup\$ Oct 22, 2021 at 10:09
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You can remove R6 and R7 that slow down and alter ADC conversions.

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  • \$\begingroup\$ Thanks for the comment ! LTC1720 has CMOS output, I'm thinking about replacing ADC with digital I/O, is an AND gate necessary ? \$\endgroup\$
    – 7E10FC9A
    Oct 22, 2021 at 11:20
  • \$\begingroup\$ Exactly. Connect that digital output to an interrupt input pin of your microcontroller so that the microcontroller can be woken up on event. \$\endgroup\$ Oct 22, 2021 at 18:33

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