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| op amp offset |
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| Simon:
I just spotted an older blog I had not seen yet about "chopper amps" very informative and an eyeopener (thanks dave). It was an eye opener on what I might have been getting wrong ! now I have this current regulating circuit and I am using a 0.001 ohm resistor as the sensing resitor, I know VERY low, well it passes 20 amps and works in a confined space so power dissipation is an issue, so I have up to a 20 mV signal. i used one part of a TL084 opamp to make a 100 gain differential amp and feed the output to a pic, things went a bit haywire although the circuit did work but it was reading less than it should have. having seen the blog it jumped to mind (as I am currently working on an upgrade to the project), now the TL084 has a 15 mV offset, so does this mean that potentially I was getting a 5 mV signal instead of 20 mV ? how do I counteract this ? use an op amp with a small offset voltage ? high precision is not required but repeatability is. |
| migsantiago:
There's a new low offset ampop from Microchip, the MCP651. http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en540381 It has an auto-calibration pin which trims the offset up to 200uV. |
| Simon:
right but is my theory correct ? to be honest I am limited to what RS and farnell supply, they usually have a part that meets my needs but I don't get to choose the specific one |
| jahonen:
If you want repeatibility, then look also for the offset drift. And remember that in a differential amplifier, your offset will be amplified by the amplifier, so if you have 15 mV input offset, it will produce a 1.5 volts offset with 100x gain at the output. Regards, Janne |
| Simon:
yea well that looks like the cause of the odd behaviour, I was using a 100 gain so the 15 mV became 1.5 V. its not the effect of the gain that is the issue it's that I start with a 0-20 mV signal on my input so a 15 mV offset is like an overwelming part of that. so is the offset normally positive or negative ? in my case the output was lower than expected so I'd say negative but I'm new to all this so probably overlooked something. |
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