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I have a signal that will swing from -5v to 5v. I am interested in the 0 to 5v part only. I will set a reference voltage and give it and the signal into a comparator. The comparator's rails are 0, and 5v.

Is the less than 0 part of the signal that is being input into the comparator's non-inverting input going to damage the comparator op amp?

Will it have some strange behavior?

Will it work ok but just take some time to recover? How much time?

I haven't said if it is rail-to-rail...does it matter?

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  • \$\begingroup\$ What does your chosen comparitor's datasheet say? You haven't posted a link or given a part number so we can't answer. \$\endgroup\$
    – Transistor
    Feb 5, 2018 at 23:33
  • \$\begingroup\$ Oh, I mentioned I haven't said if it is rail to rail. What I should have said is I haven't really chosen one yet because I wanted some insight into this issue. \$\endgroup\$
    – thejun
    Feb 6, 2018 at 7:13

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You can choose a device that can withstand the input signal range you supply or use an external network to clamp the unwanted input to the negative rail (gnd) ( more likely)

Otherwise it will behave strange and possibly invert logic for Rail to Rail types. It can also damage input junctions, if it exceeds the Vcm input range.

Yes specs matter.

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This can work, however there are a few things to watch out for. Watch the common mode voltage range, a rail to rail input op amp would be a good way to make sure the range goes from 0V to 5V ( not 0.5V to 4.5V for an amp with common mode range for example)

The other thing to watch out for is input protection, if you go below the 0V rail, if the opamp has protection it could possibly burn out the protection diode. Most opamps cannot go to far below the negative rail (this will most likely be specified in the absolute maximum ratings section)

Either way you will probably need to employ external input protection as shown below. In your case, you would only need to protect the negative rail, and use a current limiting resistor with a diode on the input.

If your using a summing configuration, you may already be current limited. The voltage source impedance must also be taken into consideration (a high impedance source won't be able to burn anything out)

enter image description here

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  • \$\begingroup\$ Some comparator datasheets specify "Inputs Can Exceed Supplies Without Phase Reversal" without going into too many details. The clamping diodes (1N5177) as answered by laptop2d are Schottky diodes. You want to use these because their forward voltage is approximately 0.4 V and is less than the forward voltage of the of the op amp's input protection diodes and will conduct (and clamp) first. \$\endgroup\$
    – D Duck
    Feb 6, 2018 at 2:13
  • \$\begingroup\$ Yeah, linear also does have their "over the top" series of op amps, I don't know if that also applies to "under the bottom" \$\endgroup\$
    – Voltage Spike ♦
    Feb 6, 2018 at 2:26
  • \$\begingroup\$ Hey this is a cool way to protect it, but it doesn't really achieve the clipping that I wanted below zero. The comparator's rail's are 0 and 5v. \$\endgroup\$
    – thejun
    Feb 6, 2018 at 7:37
  • \$\begingroup\$ This is only to protect the inputs, if you get a rail to rail op amp, it will clip at zero, but you need to ensure you protect the inputs \$\endgroup\$
    – Voltage Spike ♦
    Feb 6, 2018 at 16:36

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