Hey guys back at it again, sorry.

I am using a standard MCP6548 Comparator to finish the first stage of my project.


simulate this circuit – Schematic created using CircuitLab

OP's schematic.


simulate this circuit

A little tidy-up to separate the rectifier and comparator functions, run R4 downwards to GND and clean up the power supply. Section labels don't hurt either. [Transistor]

The probem is the comparator isn't working. When - > +, it is not staying at GND. When the input signal CH1 is Higher than CH2 it should stay at GND. Yes the output of the OPA350 is solid and working as it should.

  • 2
    \$\begingroup\$ The first two words in the datasheet say "open drain", are you familiar with the term? \$\endgroup\$
    – Mike
    Jul 5, 2018 at 20:49
  • \$\begingroup\$ I am not sorry:/, honestly first time using a comparator so I am pretty sure its operator error \$\endgroup\$
    – Leoc
    Jul 5, 2018 at 20:50
  • \$\begingroup\$ When the input signal is positive with respect to ground, where does the current go (if any is present?) How does any current sink? \$\endgroup\$
    – jonk
    Jul 5, 2018 at 20:52
  • 1
    \$\begingroup\$ AHHH, I see I remember now it requires a pull up resistor from output to VCC to output high. \$\endgroup\$
    – Leoc
    Jul 5, 2018 at 21:00
  • 1
    \$\begingroup\$ I got everything working, the pull up fixed everything, and I somehow changed my voltage divider branch and that's why it didnt go to ground. @jonk Thank you for everything ! \$\endgroup\$
    – Leoc
    Jul 5, 2018 at 21:49

1 Answer 1


I’m glad to see you got your first circuit working.

Just a little note.

First rail to rail Op Amp is a negative input wave “inverting precision peak detector” that produces a positive output at x1 gain.

Since the comparator only detects inputs greater than the reference voltage so the negative peak detector is somewhat unnecessary if you expect positive input peaks to be symmetrical.

Thus you can go directly into the comparator.


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