Removing offset from ACS710 current sensor output

[mojo-chan]

Hi. I am using an ACS710 hall effect sensor to measure current in a device for fully automated testing. The problem is that the ACS710 produces a 0-5V signal with 0A biased to 2.5V. My microcontroller can only measure 0-2V and I am only interested in positive currents, so what I need to do is shift the whole thing down by 2.5V so that 0A gives 0V into my ADC.

Keep in mind that the signal from the ACS710 won't vary much, so although it is AC the frequency could be 0.001Hz or less. Essentially it is DC. Ideally I would also like to amplify the signal at the same time to give 1A = 2V.

I suppose the best way to do this would be to use an op-amp to remove the 2.5V offset, but I can't find a circuit for doing that. Obviously capacitator coupling won't work because the signal is not AC. I could use a resistor divider to scale the output to my ADC's 0-2V requirement but then I would only get half the resolution.

Any ideas?

 

[crutschow]

You can use an op amp for this.

What supply voltages do you have available?

 

[mojo-chan]

Thanks for the response. I have 3.3V, 5V and 12V available but can generate pretty much anything I need.

Oh, and I have a precision 2.048V reference (LM4040) that I used as the analogue reference for the ADC, with a 1.024V bias voltage generated by a resistor divider. That is why the ADC can only measure up to 2.048V - the XMEGA ADCs only go as high as VCC-0.6V anyway, and VCC is 3.3V.

 

[mojo-chan]

Okay, I have a circuit that works in the simulator. This both scales (10.8x gain) and offsets the output to 1.89V, which I can work with. The output is inverted so voltage falls as current goes up.

I am having trouble making it work with an LM321 but in theory it should be fine.