# Electrical measurement questions

#### Shagas

Joined May 13, 2013
804
Hello ,

I've got some relatively simple measurement lab assessments coming up on tuesday and I found some previous year questions in our repository. I've translated some of the assignment questions and wrote them up in paint, adding some of my own questions in grey color. Could someone take a look and fill in the things I'm not sure about / correct any mistakes/ assumptions that I made?

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#### Shagas

Joined May 13, 2013
804
More questions:

What is used nowadays to measure Im/Re parts of an output voltage? Are Vectorvoltmeters still used?

#### WBahn

Joined Mar 31, 2012
24,967
Think about the output voltage of an simple inverting op-amp in terms of the two resistor values and which resistor, if it is unknown, gives you a convenient, meaningful relationship between the output voltage and the resistance value.

#### Shagas

Joined May 13, 2013
804
If you are talking about measuring an unkown resistor then I'd put a known 100ohm resistor as input and the unknown as the feedback and with feeding a voltage of say..10mV to the input and the output voltage will be proportional to the gain from which I can calculate resistance.

#### WBahn

Joined Mar 31, 2012
24,967
If you are talking about measuring an unkown resistor then I'd put a known 100ohm resistor as input and the unknown as the feedback and with feeding a voltage of say..10mV to the input and the output voltage will be proportional to the gain from which I can calculate resistance.
Using 10mV sounds pretty problematic. What are the opamp rails? Since you are assuming an ideal opamp, I'd design it so that the output just touches the rail at its highest value. Don't forget that you need to utilize your 5V reference voltage in your design.