# Homework Help: I need some help about opamps?

1. Jul 20, 2009

### smuscat

Hi!

Please tell me if understand wrong.

Common mode ,is when the two same signals are applied to differential amplifier?

Thanks

2. Jul 20, 2009

### Staff: Mentor

That's pretty much correct. A common-mode signal is applied to both conductors together. A differential mode signal is applied between the two conductors, in a opposing fashion.

So if you have a transformer with a center-tapped secondary, you drive a differential signal through to the secondary by driving a signal into the primary, and you can drive a common-mode signal out the secondar with respect to ground by driving the center tap of the secondary with respect to ground.

3. Jul 20, 2009

### smuscat

I have the following situation.

An opamp is used to filter 50 hz supply from a patient to read a perfect ecg.

Attached you can find the opamp configuration.

The AC interference is connected to the opamp in the inverting input and non inverting as common mode and an ecg signal is applied between the inputs of the opamp.

My questions are the following.

Is the AC interfernce signal is eleminated by the opamp? I think Because it has the property to amplfie the difference between the two inputs in other worlds cancel out because they are of the same magnitude and phase difference.

Will the opamp finally feature the ecg signal only on the output? I think yes.

Thanks

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4. Jul 20, 2009

### Staff: Mentor

The CM rejection you get will depend on the capabilities of the opamp. Look in the opamp datasheet to see what kind of CM rejection it advertises.

But in general, yes, that is what you are using the differential amplifier characteristics of the Opamp to do.

However, a couple comments about the sketches. First, you do not use an Opamp in open loop configuration, so the diagram on the right should show the full amp circuit with feedback. The CM rejection you get also depends on the feedback configuration, and how balanced an input impedance you present to the input signals. Look up instrumentation amplifier configurations of Opamps to see how to best reject CM interference.

Finally, the drawing on the right is not drawn well. It should show capacitive coupling of the CM interfering signal to both inputs on the opamp (via two separate capacitances), not just a coupling to one side (with and implied low-impedance tie via the differential signal source, which is not always the case).

5. Jul 20, 2009

### smuscat

You are right what you wrote.

Sorry for my luck of theory but I only need a confermation what I wrote.
Thanks Berkeman

6. Jul 20, 2009

### Staff: Mentor

No worries. BTW, I forgot to mention one other thing. You should look for the "input common mode range" specification of the opamp that you are intending to use in this project (or in this problem). Can you tell us why it might be important not to exceed that input voltage range?