# Building an Ammeter w/ High CMV and High Bandwidth

#### MR_SidePart

Joined Mar 19, 2007
1
Hey folks,

I've been viewing the site for a couple of years now as an extra resource for my courses. I haven't visited the forum though until now. I have a pretty decent problem (in my opinion) and I'm just not sure that I can come up with the answer on my own.

I'm building an ammeter for a project that I'm doing for one of my labs. Any other day of the week, I'd just use a transresistance amplifier to convert the current into a proportional voltage. That doesn't quite work here, I need to develop a more complex front end for this.

Here are the big requirements for the ammeter:
Be able to measure a high CMV (40V)
Voltage across the leads cannot exceed 100mV
Bandwidth has to be around 1MHz

The idea here is that some circuits use a fast pulse instead of DC to prevent excessive heating of a load (BJT, resistor, etc). So basically this ammeter will have to measure the current of that pulse and produce an output voltage that can be read on an oscilloscope. Then the current can be calculated since it'll be proportional to the peak voltage on the scope.

The catch is that I of course can't use a current sensor that's all built into a single IC. I've thought about starting off with a high CMV amplifier and then a precision instrumentation amplifier to boost the CMRR. The trouble though is that every time I try to simulate the circuit (I'm trying out an AD626 and AD620) I'm getting confused about how to hook the two up! (FYI: The circuit involving both of these amplifiers will only provide a bandwidth up to 500kHz, so if anyone has any better ideas to up the bandwidth, I'm all ears).

If anyone's curious, here's the project outline: http://www.ece.umn.edu/class/ee3102/project.pdf

If anyone has any suggestions or reading material or whatever (I do have access to libraries and things) I'd be glad to hear them.

Thank you,
Dave

#### beenthere

Joined Apr 20, 2004
15,819
This is apparently an assignment, so I am moving it to a different location.

As soon as you have some work to show, we can start commenting on it. We won't do your work.