Re: Probing circuits with op-amps

Plus beware of op-amps where input pins go to ground without a series
resistor. Older op-amps can self-destruct if the negative power supply
dies and goes slightly positive.
 On Aug 30, 2013 2:07 PM, "Ken Sumrall" <k_lists@scrapheap.net> wrote:

> Your understanding is correct. When an opamp is connected in a negative
> feedback configuration (the most common case) the difference between the
> two input pins is 0 volts, and as one is tied to ground, they will both
> measure 0 volts with respect to ground. If you measure voltage across the
> input resistor (or equivalently the voltage at the input of the input
> resistor
> with respect to ground) you will see the signal you are looking for.
>
> However, my guess would be the problem is with electrolytic caps on the
> sound board. They dry out with age and heat, and start to become less
> effective. Try replacing all the electrolytic caps on the sound board,
> and see if the problem goes away.
>
> ___
> Ken
>
>
> On 08/27/2013 07:26 AM, Altan Pinball wrote:
>
>> Hello all. I'll admit up front i'm using you guys for some basic
>> electronic questions, it's not directly vector related. Hope that's
>> not a problem.
>>
>> I'm trying to diagnose a sound circuit that has started having a hum.
>> It's not a grounding problem and swapping the sound board with
>> another causes the problem to go away. The voltage is correct and
>> there is little, if any, AC ripple.
>>
>> What I'd like to do is use my scope and see the sound waves that are
>> generated before the op-amp. My working assumption is the op-amp has
>> gone bad, so I'd like to see the input to the op amp is and what the
>> output is.
>>
>> After playing around with this a bit, I realized it's not as simple as
>> connecting a probe (with proper ground) to the input pin and then to
>> the output pin. The circuit, by design, feeds the output back into
>> the input, striving to make the IN+ and IN- equal. Because of this,
>> I never see anything interesting on the scope when probing the input.
>>
>> Now... is my reasoning correct?
>>
>> FWIW, the circuit is a typical inverting amp with IN+ going to ground
>> and IN- having the source. There are the expected resistors in there
>> also.
>>
>> What is the correct way to see the input to the op-amp? I'm thinking
>> I would need to pull the leg on a resistor that is inline with the
>> op-amp's input (to break the connection before the feedback and before
>> the op-amp) and measure. Is there a better way?
>>
>> Or am I all wet and I've got it all wrong?
>>
>> Thanks...
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>>
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Received on Fri Aug 30 15:15:17 2013