Is it okay if I leave an opamp or comparator input open?

Op amps have such a high gain that leaving an input open will probably drive the output into saturation, which isn't ideal. Better to configure unused op amps as unity-gain (tie in- to out) and use a pair of resistors to set the input voltage to half the supply voltage. That keeps all the pins in spec and happy.

Okay. I think I get it. Thanks! Hmm.. I do remember reading somewhere that the opamp input terminal has to have a path back to ground to make it happy.. or something like that. I don't quite remember, but I guess that would make sense if we tie the inputs to a voltage source or ground.

Would a circuit like this work?

GND | | V - | GND | |\\| VIN2 -- ----|-\\ | >-VO VIN3-|+/ |/| VCC

(created by AACircuit v1.28.6 beta 04/19/05

VIN3 in the diagram is a fixed voltage source, while VIN2 is either there or not (like plugged in or plugged out). Thanks!

It probably won't damage it but it certainly won't work in any meaningful way.

Graham

Some opamps will draw a lot of supply power if their outputs rail, as they will if they are not operating closed-loop. And in some dual or quad-section opamps, running one section open-loop will mess up other sections. In general, keep all opamps running closed-loop, like this:

or

or some other way to keep it happy when your input is disconnected.

John

Thanks, Graham! I'd like to make it work in a meaningful way and not damage it. I think I understand now.

Thanks, John!

dear freinds,

there are many application notes to this effect from almost all big manufacturers. i think comparator pins can be left open, coz they are one way or the other in saturation. saturation state for op-amp is not good, one coz they take a lot of current and second it will put in a lot of noise on the supplies.

the best idea is to keep op-amp in a known state and for that u should tie both minus and positive pins such that u ahve a output close to mid point of supply voltages applied. do read application notes of national for this application.

with regards

manoj