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I'm trying to detect small current pulses (~1.5mA). Currently I'm generating a ~20mV pulse on a sense resistor and feeding that to the input of an opAmp with some hysteresis. I'm having a bit of trouble where the output of the OpAmp will sometimes latch low and not return after the pulse passes and would like to try to use a comparator for the circuit. The problem I'm having with that is that my supply and my input are both at 6V.

Can someone recommend a good comparator that has full common mode input range with no more than 10mV of hysteresis? Or instead a better design overall?

I've found this part that seems to come close but the hysteresis might be too much.

*Based on some of the replies so far, I should mention I have a 3V3 rail available as well on the board if that helps.

enter image description here

EDIT: Redesigned circuit based on below suggestions- yet to evaluate

enter image description here enter image description here

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  • \$\begingroup\$ If you're literally only looking at pulses. Why aren't you level shifting or using a capacitor to move the input signal away from Vdd? \$\endgroup\$
    – horta
    Commented Apr 8, 2014 at 17:43
  • \$\begingroup\$ I think that a better drawn schematic could help too. The capacitors' capacitances are missing, and you can't even tell the pulse current direction... \$\endgroup\$
    – Vladimir Cravero
    Commented Apr 8, 2014 at 17:48
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    \$\begingroup\$ Fixed schematic- it's not only pulses, the pulses are on top of fairly constant current that can vary between 1 and 20mA \$\endgroup\$
    – spizzak
    Commented Apr 8, 2014 at 17:57
  • \$\begingroup\$ What speed are the pulses? How long do they last? Are they always producing a pulse of about 20mV? \$\endgroup\$
    – Andy aka
    Commented Apr 8, 2014 at 18:58
  • \$\begingroup\$ The pulses are around 250us and roughly 50% duty cycle. The pulses are always 1.4mA so they should always be around 20mV \$\endgroup\$
    – spizzak
    Commented Apr 8, 2014 at 19:36

2 Answers 2

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Try this circuit with a dual op-amp. It uses the suggestion by @horta to move the voltage away from the rails.

U1A produces a stable voltage at +3V. The input is capacitively coupled to that voltage. Since your pulses are -22.5mV, the threshold is set at 3V - 11.25 mV approximately (set by the ratio of R9 to R8 approximately) +/- 2mV of hysteresis (set by the ratio of R7 to R8||R9 ~= R8)

schematic

simulate this circuit – Schematic created using CircuitLab

If you want to continue to use a R-R op-amp, you could adjust the circuit values in your schematic to give similar hysteresis and threshold, as in my diagram. If the input offset voltage of the op-amp is less than 1mV, similar values for the voltages could be used. In that case,

330 (your schematic)-> 100 ohms

100K(your schematic)->68K

390K(your schematic)->150K

However, the failure of the circuit you have now to work properly indicates something else may be wrong- the bias current of the amplifier or the voltage offset is preventing the non-inverting input from rising high enough to reset the hysteresis. There also will be very little useful hysteresis for a 250usec pulse with that 47nF capacitor on there- the positive feedback will take a long time to build up (time constant 3.7msec).

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  • \$\begingroup\$ Makes sense, but how's it better than the original circuit? \$\endgroup\$
    – spizzak
    Commented Apr 8, 2014 at 17:59
  • \$\begingroup\$ "The problem I'm having with that is that my supply and my input are both at 6V." <- He's fixing this problem by making the input voltage to U1B operate around 3V rather than 6V. \$\endgroup\$
    – horta
    Commented Apr 8, 2014 at 18:01
  • \$\begingroup\$ It allows a non-RR op-amp. It isn't really operating right at 6V, the input pulses are 6V - 22.5mV and his threshold is set at 6V - 19.8mV with hysteresis of 0/-5mV. That's too much on the low side. The op amp might also have a few mV of offset. I'll add a comment based on spizzak's remark. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Apr 8, 2014 at 18:06
  • \$\begingroup\$ What kind of part should I be looking for for U1? \$\endgroup\$
    – spizzak
    Commented Apr 8, 2014 at 18:53
  • \$\begingroup\$ @spizzak It depends a lot on the pulse length. If it's audio-type frequencies, most op-amps with low Vos, rail-to-rail output and common mode range that includes 4V on a 6V supply will be good enough. Low Vos or you might run into problems with the small pulse. If your pulse is MHz you'll need something more specific. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Apr 8, 2014 at 20:04
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It would be wiser to amplify the signal far above the supply noise then use a Schmitt trigger or comparator with positive feedback as required.

One solution uses a current sense chip on either 3.3 V rail or 6V rail, whichever is cleaner.

http://www.maximintegrated.com/app-notes/index.mvp/id/1180 using a gain of 50 on a MAX4372.

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  • \$\begingroup\$ I like the current sense IC idea. You get a nice linear output based on current over the shunt resistor. And most of these work nicely in the 0-100mV range for detection. You can easily work with that IC's output for the OP's comparator with schmitt trigger output \$\endgroup\$
    – KyranF
    Commented Apr 8, 2014 at 23:24

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