Differential Op-Amp

I have this circuit where I have to measure three things.

#1.Measure the dc output voltage with zero differential voltage.
#2.Measure the differential gain by using V1 and V2 as the inputs. Verify the differential gain at 1KHz.
#3.Measure the common mode gain with a 10 volt peak voltage at 1 KHz.

For #1 I don't really understand what the instructor means by zero differential voltage. What I get is that both V1 and V2 must be of same voltage and with that setup I should measure DC voltage at vout. Is this correct?

For #2 I would set V1 to 100mVpk and V2 to 50mVpk. Since the gain is set for 10 the output should be (V1-V2)*10. Is this also correct?

For #3 I would use only one input at 10Vpk and measure the gain?

You are correct for #1 and #2. #1 is measuring the output offset with zero differential input. #2 is measuring the differential gain.

For #3 you are measuring the common-mode gain. Common-mode is a signal common to both inputs so you apply the input signal to both tied together and measure the output voltage.

for #1, that common voltage must be zero, although since most of the measurements are AC, I'm wondering if he means 2 signals, 180 deg out of phase..

KeepItSimpleStupid said:

for #1, that common voltage must be zero, although since most of the measurements are AC, I'm wondering if he means 2 signals, 180 deg out of phase..

Click to expand...

2 signals, 180 deg out of phase are a differential signal and will give an output signal. I don't understand your thought. #1 is just measuring the output DC offset with no input.

Your 100% correct. Oops. Thanks for the safety net.

a-(-a) = 2a; b=-a

a-a would = 0; signal applied to both inputs.