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DC offset

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efsnick

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Just so I dont screw up my circuit any worse than it already is....I am trying to measure the DC offset on common dual op amps with a volt meter. I was told just to short the non and inverting inputs together, then measure the outputs on each. Is this correct? If so, what am I looking for??? If not, any Ideas? I do not have an oscilloscope...
 

crutschow

Well-Known Member
Most Helpful Member
Just so I dont screw up my circuit any worse than it already is....I am trying to measure the DC offset on common dual op amps with a volt meter. I was told just to short the non and inverting inputs together, then measure the outputs on each. Is this correct?
Not even close. First, if you just short them together there's not a path for the input bias current so the amp won't operate properly and likely saturate. But even if you connected both inputs to ground to provide a path for the current, the open loop gain of the op amp (which typically is 100k or more) would amplify the offset and the output will still saturate at close to one of the supply rails.

You don't need an oscilloscope. But you need to configure the op amps as non-inverting amps with a known gain. A gain of 1000 will amplify 1mV of offset to give 1V at the output with the + input grounded. So you can just divide the output voltage by the gain to get the input offset voltage.

To get a gain of 1001 connect a 10kΩ resistor between the negative (-) op amp input and the op amp output. Connect a 10Ω resistor between the - input and ground. Also connect the plus (+) input to ground. Apply op amp power and measure the offset multiplied by a gain of 1001.

If the offset is too large or small to give a good measurable output for a gain of 1000 then adjust the 10KΩ resistor value accordingly.
 
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RadioRon

Well-Known Member
That (your orignal post. I overlapped with crutschow) doesn't sound right. The way I use to measure offset voltage is echoed in this link:
http://tuttle.merc.iastate.edu/ee331/lab/ee331_lab2.pdf
You will see that a circuit is shown where the op amp is wired with a feedback resistor to the inverting input and an input resistor to the same input that is connected to ground. The ratio of Rf/Ri is the gain of this inverting configuration. The gain should be quite high to make this practical, and in the example it is 470. A typical opamp might have bias current of less than 10mV and often less than 1mV and your average voltmeter can't measure these low voltages accuratly. So applying a lot of gain means that the input offset voltage is multiplied by the gain, making it easier for you to measure. Be sure to divide your measurment by the gain to get the actual offset voltage, and as the paper shows, pay attention to the resistor on the plus input too.
 
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