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Author Topic: How to measure a small bipolar voltage?  (Read 1646 times)
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Hi all,

New to arduino, and not amazing with circuits- but I've seemed to get by until now...

After searching around, I've not been able to come up with a solution.

I'm looking to measure a small bipolar voltage from an HPLC output, and essentially graph the output.

The voltage is in the range of +/- 100mv.

I've figured I cannot use a voltage divider as the incoming voltage range is too low. Would there be a way to amplify the negative and positive signals, and then apply to a voltage divider to read to an analog input of the arduino?

Is there a better way?

Thanks in advance,
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Use a single supply op amp, add some DC offset to your signal, and some overall gain, get your signal up into the range that the ADC is looking to work with.


* adding_offset_gain.jpg (30.07 KB, 960x720 - viewed 89 times.)
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You could use a summing amplifier:



Where V1 would be your sensor voltage and V2 is a 100mv source. Set Vout to 5V and calculate appropriate resistors.

At -100mV in Vout=0V
At 0V in Vout=2.5V
At 100mV in Vout=5V
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it's like a 14 minute delayed echo ....
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it's like a 14 minute delayed echo ....


LOL, yeah sorry I was looking for pictures to add to my post and you beat me to it.
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Amazing!

Thanks a lot to you both!

I've calculated out the resistors, off to build the test circuit now.

Is there a particular op-amp I need? Or will a general one do? I take it I will need a negative voltage as well?
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Use a single supply op amp, like the LM358 on the Arduino boards.
http://www.ti.com/lit/ds/symlink/lm158-n.pdf

Want a 0-5V output when done to be compliant with Arduino ADC input.
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Use a single supply op amp, like the LM358 on the Arduino boards.
http://www.ti.com/lit/ds/symlink/lm158-n.pdf

Want a 0-5V output when done to be compliant with Arduino ADC input.


Is the LM358 a rail to rail op-amp? I don't think it is and that would limit it's ability to get to 100% measurement range of the ADC, if the chip is powered with ground and +5vdc. And if powered with a higher Vcc then there is always a risk of overvolting the input pin?

Lefty
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LM358 was an example. I think it'd be fine with some tweaking of the offset and checking for non-distortion of the output.
Or find a wider output single supply op-amp.
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