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I'm doing my first electronics project where a signal that varies between -1 to 1 V is going into an analog pin on the Arduino. The amplified signal has to be read as fast as possible by Arduino. The source signal is at 60 Hz.

Will this LM324 be a suitable 4-channel opamp for my needs? How concerned should I be about the 2 mV input offset and 20 nA input current bias?

To make full use of the 10bit ADC, would you recommend using an opamp with gain of 2 and biasing the signal by +2.5V? If so, will it be better to create the bias using a potential divider across +5V and GND, or using a 2.5V voltage regulator?

Or would it be better to set AREF to 2V and bias the signal by +1V? In this case, will it be better to use a 2V voltage regulator, or a voltage divider across Arduino's 3V3 and GND using 20M and 33M resistors?

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A 2mV offset voltage will become a 4mV error after amplification. You haven't shown the schematic for your amplification, but the current bias will also become error after amplification.

That LM324 variant is surface mount, but there are other LM324 variants in a DIP package, which will work fine in a breadboard.

  • Thanks, would this 14-PDIP version of LM324 be a better choice? (digikey.com/product-detail/en/LM324N/296-1391-5-ND/277627) – Athena Wisdom Mar 23 '14 at 3:38
  • It will have the same specs, it will just be better for breadboarding. – Ignacio Vazquez-Abrams Mar 23 '14 at 3:39
  • Is it possible to use the opamp to bias the amplified signal by 2.5V? Or will using a potential divider across the ground and +5V pins of the Arduino be the easier route? – Athena Wisdom Mar 23 '14 at 3:41
  • It is definitely possibly to bias the signal. You won't want it at exactly 2.5V though, because of the current bias. And in fact, biasing it will remove it from the rails. This is a good thing. – Ignacio Vazquez-Abrams Mar 23 '14 at 3:42
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Just an alternate idea I saw: If you dont need high accuracy, you can use a voltage divider to bias -1 to +1 into 0 to 1, and set the analog reference to 1.2, and get almost the same result but skip the op amp.

A voltage divider with 2 equal resistors will give you the voltage halfway between its two inputs. If the bottom of the voltage divider is at 1v, and the top input is at -1, you get 0 out.

If the bottom is at +1, you get 1 out.

I saw a really clever design using 2 1 megaohm resistors, and actually using the aref pin as an output to provide the bias for the divider, setting the reference to the 1.2v setting, putting the signal in the top of the divider and ground and connecting the adc to the output of the divider.

Aref wasnt meant as a current source, but they use 1meg resistors so the current draw will be at most a few microamps.

A filter cap on aref probably draws more than that if vcc is noisy(if there wasnt current to filter we wouldnt need the cap!) So a few uA should be fine.

They put a 1nf capacitor betcan the input and ground to fix the issues with the ADC input current causing voltage drop. It worked up to 100hz. They never tested accuracy because it wasnt needed though. I'd guess that the adc internal reference was the major error source, probably a few percent.

You can also actually bias with a voltage divider to the 5V supply, and get a 0 to ~1.6V output, it's just a little harder to understand.

This simulation explains the concept(5v version) http://www.falstad.com/circuit/#%24+1+5.0E-6+81.92041607667615+50+5.0+50%0Ar+304+160+304+224+0+1000.0%0Ar+304+224+304+304+0+5000.0%0AR+304+304+256+304+0+0+40.0+5.0+0.0+0.0+0.5%0AR+304+160+208+160+0+1+40.0+1.0+0.0+0.0+0.5%0Aw+304+224+400+224+0%0Ax+368+208+456+214+0+24+To+ADC%0Ao+3+64+0+35+2.5+0.00625+0+-1%0Ao+4+64+0+35+2.5+9.765625E-5+1+-1%0A

Left click the output, use view in scope.

  • That's a really interesting approach that's simplier! Any chance that you still have the schematics for the 2nd idea you mentioned? I can't figure out what it means to put the signal in the top of the divider and ground. – Athena Wisdom Mar 23 '14 at 16:04
  • To make full use of the 10bit ADC, would you recommend using an opamp with gain of 2 and biasing the signal by +2.5V using a potential divider across +5V and GND, or bias using a 2.5V voltage regulator? – Athena Wisdom Mar 23 '14 at 16:34
  • Or would it be better to set AREF to 2V and bias the signal by +1V? In this case, will it be better to use a 2V voltage regulator, or a voltage divider across Arduino's 3V3 and GND using 20M and 33M resistors? – Athena Wisdom Mar 23 '14 at 16:37
  • Edited to include the 5v bias way of doing it, with a link to a web based simulation. – EternityForest Mar 24 '14 at 4:00

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