Graph voltage not matching input

[glskinner84]

I'm connected to a NI9215 with a 1Mohm resistor between my com and ai-. I just recently got my offset corrected but when I put a voltmeter to the screw terminal I read 1.0v and my graph reads 1.4. What am I missing?

[James.M]

Have you tried different input terminal configurations? You have it set to default, but maybe you want to explicitly tell it Differential.

[glskinner84]

Hey James, I changed it to differential and it stays the same...does it look like I'm performing a correct offset subtraction?

[James.M]

Your voltmeter is going to read the average value, so it's not going to see the 1.4V sine wave, just the average. I'm guessing you're seeing exactly what you're supposed to be seeing.

[glskinner84]

isn't my offset routine giving me an average as well?

[James.M]

I'm not sure exactly what you're asking. You do get an average value of about 1V and then you subtract that value so now you're centered around zero.

[glskinner84]

I guess I'm just a bit wary using the offset method I'm using. I just wanted to know if it's correct. I'm not sure where you're seeing 1v? My DC is around .500.

[James.M]

Why are you doing the offset subtraction? If you're trying to subtract the DC value from the data values, then yes this is correct. If it's the correct thing to do for your needs, that's something for you to decide.

Are you seeing exactly 1V on your voltmeter? or is it wavering?

[glskinner84]

I'm just getting my signal to be clean and centered with the offset. That's all.

Reading the screw terminals with a volt meter gives me 1.07v/+-0.003v. My Fluke multimeter going through an AB switch tells me 1.54v. They're both steady.

This is my graph with these voltages applied.

The red is without the offset applied.

[glskinner84]

Correction, the measured voltage across the 1Mohm resistor is 1.07, measuring the voltage directly from ai+ and ai- (not including the resistor) is 1.57v.