Author Topic: How to check opamp?  (Read 1638 times)

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Offline giosif

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How to check opamp?
« on: December 29, 2016, 03:21:41 am »
I am trying to understand whether some opamps I've got (different makes and types) are good or faulty.
Thing is, if I test them on the breadboard in a buffer or non-inverting amplifier configuration, the output is as expected.
However, if I simply supply power to the opamp but leave all other pins disconnected, there is voltage on the output pin of the opamp (e.g. 5.9V).
This seems to be the case for all the opamps I have doubts about their health.
Furthermore, for one of these opamps (which is a dual opamp package), one of the output pins does put out voltage with all input and output pins disconnected, while the other output doesn't (put out voltage).

Are these opamps good or not?

Thanks!
 

Offline rstofer

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Re: How to check opamp?
« Reply #1 on: December 29, 2016, 03:34:48 am »
There are offset voltages at work even with the pins disconnected.  My guess is that the op amps are just fine.
 
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Offline MK14

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Re: How to check opamp?
« Reply #2 on: December 29, 2016, 06:09:57 am »
General purpose Op-amps have open-loop gains of something like x250,000
They also have offset errors (between the inputs), of about a millivolt or so.

So in all likelihood, if not connected to anything except power, the output will saturate towards one of the power rails.

Also their input resistance/impedance (if you call it that) will be so high that it will easily pick up stray signals, such as mains interference.
 
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Offline JoeN

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Re: How to check opamp?
« Reply #3 on: December 29, 2016, 09:41:58 am »
Try that same testing methodology on logic circuits and you will see the same thing.  Unconnected inputs cause outputs that will drive you insane.  The value of the outputs will contain no useful information.
Have You Been Triggered Today?
 
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Offline giosif

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Re: How to check opamp?
« Reply #4 on: December 29, 2016, 02:11:36 pm »
Ok, some of the explanations you mention did cross my mind and suspected the op amps might be fine, but I'm a newbie and, also, the apparently different behaviour threw me off course there.

Thanks to all three of you for the responses!
 

Offline tggzzz

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Re: How to check opamp?
« Reply #5 on: December 29, 2016, 02:32:38 pm »
If someone (beginner or otherwise) posts a question where they neatly characterise their problem's condition, the desired/unexpected behaviour, what they do/don't know, and indicate they aren't asking other people to do their thinking for them, then they will find many people willing to help. You did all that :) ; many don't :(

Keep exploring, testing, thinking, and having fun safely!
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline giosif

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Re: How to check opamp?
« Reply #6 on: December 29, 2016, 02:48:34 pm »
If someone (beginner or otherwise) posts a question where they neatly characterise their problem's condition, the desired/unexpected behaviour, what they do/don't know, and indicate they aren't asking other people to do their thinking for them, then they will find many people willing to help. You did all that :) ; many don't :(

Keep exploring, testing, thinking, and having fun safely!

Thanks, tggzzz!
Yes, trying to do all those things. Not sure it allways works out that way, though.  :)
Cheers!
 

Offline Lee Leduc

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Re: How to check opamp?
« Reply #7 on: December 29, 2016, 03:14:45 pm »
You can use a simple Square generator circuit to flash an LED as a simple go/no go tester for op amp. If the LED flashes, the op amp is functioning.

https://startingelectronics.org/beginners/circuits/op-amp-oscillator/
 

Offline giosif

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Re: How to check opamp?
« Reply #8 on: December 29, 2016, 09:21:24 pm »
You can use a simple Square generator circuit to flash an LED as a simple go/no go tester for op amp. If the LED flashes, the op amp is functioning.

https://startingelectronics.org/beginners/circuits/op-amp-oscillator/

Thanks, Lee!
Just tried that on a couple of the op amps and I confirm it works "as advertised" (and the op amps are good).  :)
I have trouble understanding that circuit, though (yeah, that basic is my knowledge)...

 


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