Inverting opamp..how to check with a scope?????

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Inverting opamp..how to check with a scope?????

Postby chicago_mike » 01 Jun 2012, 22:28

So I am trying to invert a signal. No biggie. I am feeding the signal to pin 6 of a NJM2068. 5 to ground. 7 is output..check.

I matched the gain to the non-inverted signal, but on the scope, it doesn't look like my signal is inverted.

I'm using a digital scope and I wonder if it's not showing the invert by default?? Is that normal? I dont have a second probe to go between. :(

How else can I test polarity?

I am inverting the signal to send to a second amp in a 2 channel amplifier to bridge the outputs.



Heres the schematics. I know there are differences. I know the gain is different too. One is 6db and the other is 12db.

My amp is the hartke HA5000, I am referencing that and the 5500 as a mono amp source. you will notice..be default I cannot bridge the amp as it doesn't invert a signal to one of the power modules..

I pulled out the crossover section...I don't use that.

Do I need to get another opamp and invert again?
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Re: Inverting opamp..how to check with a scope?????

Postby DrNomis » 02 Jun 2012, 01:01

How many input channels does your Oscilloscope have?, if it has two channels, use one to monitor the input signal and the other channel to monitor the output, then set the triggering so that it's getting it's signal from the channel that's monitoring the input, if the op-amp is inverting the signal you should be able to see that the output signal is 180 degrees with respect to the input, if your Scope only has one input then it'll be difficult... :thumbsup

What kind of signal are you feeding into the op-amp?.... :hmmm:
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Re: Inverting opamp..how to check with a scope?????

Postby chicago_mike » 02 Jun 2012, 19:23

1Khz sine and square waves.

It's a 2 channel scope, but I only have one probe. I think I can Save the signal on either channel on my scope though. you know, to compare, like you say. :)


HEres my question: Lets say I do have the circuit wired right...for inverting the signal. However, its NOT inverting....

Could the opamp be bad?
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Re: Inverting opamp..how to check with a scope?????

Postby DrNomis » 04 Jun 2012, 09:16

chicago_mike wrote:1Khz sine and square waves.

It's a 2 channel scope, but I only have one probe. I think I can Save the signal on either channel on my scope though. you know, to compare, like you say. :)


HEres my question: Lets say I do have the circuit wired right...for inverting the signal. However, its NOT inverting....

Could the opamp be bad?



It's possible, do you have power going to it?, also, are you using a single, or dual-tracking power supply?, if it''s single then the output of the op-amp should be sitting at 1/2 the supply voltage... :thumbsup

Another thought is that you might not have the + input of the op-amp referenced to circuit-ground, if that's the case then the op-amp wouldn't be amplifying the difference between the + and - inputs, and because the - input is tied directly to the output via a resistor (if it's providing amplification), or via a wire-link (acting as a buffer), then you'll be seeing the signal on the output of the op-amp in phase with the input.... :thumbsup

My other thought is that because you've only got one scope-probe, when you touch the tip to the output the scope's triggering puts the trace in phase with the input, you should try and buy a second scope probe as this will enable you to use both channels of your scope to compare the input and output signals, the phase difference between the two signal will then be clearly displayed on the scope... :thumbsup

In the meantime, you could build a differential amp with a spare op-amp IC, feed one signal into the + input and feed the other signal into the - input, the op-amp ic will then amplify the difference between the two signals, if both signals are in-phase and equal in amplitude/level, then the output should be zero, meaning no-difference, if on the otherhand one signal is 180 degrees out-of-phase with the other, and of same amplitude/level, then you should get an output equal to the difference between the two signals.... :thumbsup
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Re: Inverting opamp..how to check with a scope?????

Postby Dirk_Hendrik » 04 Jun 2012, 12:46

Could you draw the circuit you're trying to measure?

From what I make of your description it's like this:
not.gif


And that's not going to work
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Re: Inverting opamp..how to check with a scope?????

Postby Fender3D » 04 Jun 2012, 18:17

You can't see any difference since each applied signal will sync with the scope trigger...
You need to check the 2 signals normal and inverted at the same time, using 2 scope channels or using x/y input to display the Lissajous pattern.
BTW
you will need 2 probes...
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