0
\$\begingroup\$

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.

\$\endgroup\$

2 Answers 2

Reset to default
0
\$\begingroup\$

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.

\$\endgroup\$
2
  • \$\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\$
    – Rohat Kılıç
    Oct 22, 2021 at 10:09
0
\$\begingroup\$

You can remove R6 and R7 that slow down and alter ADC conversions.

\$\endgroup\$
2
  • \$\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\$
    – Enrico Migliore
    Oct 22, 2021 at 18:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.