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### Topic: How to measure a small bipolar voltage? (Read 3208 times)previous topic - next topic

#### renesis

##### Oct 08, 2012, 12:37 am
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?

#1
##### Oct 08, 2012, 01:40 am
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.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### HX2600

#2
##### Oct 08, 2012, 01:54 am
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

#3
##### Oct 08, 2012, 02:05 am
it's like a 14 minute delayed echo ....
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### HX2600

#4
##### Oct 08, 2012, 02:18 am

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.

#### renesis

#5
##### Oct 08, 2012, 07:56 pm
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?

#6
##### Oct 08, 2012, 09:53 pm
Use a single supply op amp, like the LM358 on the Arduino boards.

Want a 0-5V output when done to be compliant with Arduino ADC input.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### retrolefty

#7
##### Oct 09, 2012, 12:59 am

Use a single supply op amp, like the LM358 on the Arduino boards.

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

#8
##### Oct 09, 2012, 01:17 am
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.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

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