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Seemingly Simple Circuits, Part 2: Diff-Amp Calibration & Measurement

With one-op-amp diff-amp parameters defined in the first article in this series, the following calibration procedure can be applied manually or by a μC-based system. Each step follows a set, measure, calculate sequence. It also moves us closer to a bench-oriented experience with the diff-amp.

1. Short the diff-amp input: Set vI+ = vI– . Measure vO . Then vOS = vO , and the offset voltage is determined.

2. Set shorted inputs to a reference voltage VR : vI+ = vI– = VR . Measure vO . Then

Solve for

and ACM has been determined. VR must be within the CM range of the amplifier under test, and for the next two steps, it should also be within the input voltage range of the amplifier. If not, attenuate VR to VR /T , where T is the attenuation. Then use VR /T in the following steps instead of VR .

3. Set vI+ = VR and vI– = 0 V: The differential input voltage is now VR . Measure vO . Then

Solve for differential gain,

Now that we have Av , we can also find

4. Set vI+ = 0 V and vI- = VR : The differential input voltage changes to –VR . Measure vO . Then

Solving for

Now we have a second derived value of the gain, Av , which can be averaged with the value from step 3. We can also find

We have now determined all the basic diff-amp parameters: vOS , Av , ACM , Av+ , and Av- . Some redundancy in the measurements results in two values for Av and

If the op-amp + input voltage, v+ , is also acquired along with vO , then it is possible to measure both the differential and common-mode input voltages. The calibration procedure is essentially the same as given previously, though it is constructed so that the calibration parameters can be acquired quantities, rather than calculated amplifier parameters. They also result from the procedure. The externally measured input quantities to the calibration procedure are VOS and VR . The acquired voltages of the amplifier are vO and v+ . The amplifier model equation, based on the one-op-amp diff-amp and repeated from Part 1, is

where

Then calibration proceeds down the rows of the following table.

The calibration parameters can be either of two sets:

In processing amplifier outputs using these parameters to correct error, the inputs can be found by solving the previous equations for Av and ACM :

Then solve for the inputs and use the following equations to process the raw amplifier output values.

An alternative calculation of vI– is

1 comment on “Seemingly Simple Circuits, Part 2: Diff-Amp Calibration & Measurement

  1. JamesBryant
    October 30, 2014

    The universal op-amp test circuit described in Analog Devices' RAQ 67 and in Analog Dialogue Vol 45 April 2011 can readily be adapted to making op-amp, in-amp and subtractor measurements. I cannot post the URL but a quick search of Analog Devices' website will find it quickly enough.

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