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eCigarette Forum => Modding => Simplified Electronics => Topic started by: doobedoobedo on January 21, 2015, 07:23:15 PM

Title: op amp without offset?
Post by: doobedoobedo on January 21, 2015, 07:23:15 PM
I'm looking for an op amp where I don't need to worry about offset voltage. It's to boost the signal from a current sensor which outputs 28mV/A. I need it in the 0-5V range so I can read it more accurately with an arduino.

I was looking at the 741 until I noticed the datasheet mentioned up to 15mV offset. I could take a reading at no output at start-up and set that as my zero, but then I could lose resolution.

Can anyone recommend me a decent one where I don't need to worry about the offset? Or am I worrying unnecessarily?
Title: Re: op amp without offset?
Post by: CraigHB on January 22, 2015, 02:51:24 PM
Any standard op-amp will require calibration of some kind.  They all have some offset, temperature drift, and gain skewing.  You can avoid that with a "precision" op-amp, but they cost more.  Offset is a concern in measuring applications because you amplify the offset by the gain.  So for example if you are using a gain of 20 and op-amp offset is 15mV, your offset on the output side would be 300mv which 10% of the ADC count for a reference zero to 3 Volts.

Take a look at the parametric search on TI's web site.  They have some real nice precision op-amps.  Also, Microchip and Linear Technology have some good precision op-amps.  Just narrow the offset parameter in the parametric search on those manufacturer web sites and you'll see them right away.  They're actually kind of common since they're often used where high accuracy is required.

A suggestion would be the Microchip MCP6V26.
Title: Re: op amp without offset?
Post by: doobedoobedo on January 22, 2015, 04:59:12 PM
Thanks Craig. The gain isn't that high, only about 7x and I've done a better search looking more closely at the offset, now I know what to look for, and the TI LMV321 http://www.farnell.com/datasheets/1854736.pdf is my current favourite.

The MCP6V26 looks good but farnell (where I get most of my stuff from) only have 5 left and aren't getting any more.