Selecting & Using Op Amp for Correct Gain Amount

I want to utilize a data logger to monitor the current on an AC circuit using a CT. I’m looking at a Dent clamp on CT (link) which outputs 2.2 mV at 0A and 333 mV at 150A. The data logger’s sensitivity is within -100 mV - 2500mV. My goal was to amplify the 2.2-333 mV to around +/- 1000mV.

When I look at op amps (example) that I think are the correct supply voltage (around 12V) and offset (~2-3mV) the voltage gain is always around 85 dB which would blow the output way past the 2500mV max on the data logger.

Should I be looking for lower gain op amps? Or adding a voltage divider on the output to step this back down?

Full disclosure: I’m a mechanical engineer who is just wading in to data acquisition and sensing systems.


This is not quite correct as stated; the 2.2mV/amp figure represents a sensitivity figure/coefficient, hence 0A*2.2mV/A=0mV. The expression of your data logger’s input character doesn’t seem quite right to me, so I might suggest a careful reading of those specs also.

The “gain” figure you’re referencing would be “open loop” gain, e.g. gain before any feedback is applied. Ideally this figure would be infinite, but it’s not, so it gets characterized because when negative feedback is used to achieve more useful gain figures, the difference between 85dB of open loop gain and 100dB makes a small, but sometimes non-negligible difference.

Analog Devices’ op amp applications handbook is a comprehensive resource on all things op-ampy. The below comes form page 8 of section 1, and is the basic form of the circuit you’ll want.

This thread contains similar information in more condensed form. It needs updating, but might be found helpful. Something like a TLV271CS-13 might be an inexpensive, flexible, and fairly well-rounded choice for many purposes.

Did that too, but then I turned to the dark side…

Thanks for the clarification on the 2.2 mV/A stepping - this was a great catch. The 85 dB open-loop gain makes a lot of sense, thanks for explaining that - lots of Googling did not turn up such a concise answer!

Adding a load to the output is what I figured would happen - basically a voltage divider to arrive at the voltage magnitude I want for my input.

As for the datalogger I’m using a Campbell Scientific CR310. The analog input specs are the -100 to 2500 mV I stated in the post. I plan to test this circuit for reading AC amps followed by using a hall effect sensor for reading DC amps.

This thread comes to memory and may be something you’d care to read.

Depending on the character of your sensor(s), amplification may not be required. I’d seek to avoid the added complexity if doing so is possible.