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INA321 Input Common-Mode Range


I'm currently struggling with an INA321 that seems to be showing symptoms that somewhat resemble instability (the output is drifting in a sinusoidal manner, though only slightly [around 300uV pk-pk at a gain on 10V/V]). Though I can't seem to identify any cause for the instability.

The device is configured as follows. A Wheatstone bridge is connected to the same supply as the the INA321, which is 2.7V. The Common-Mode Input sits around 1.35V with a maximum differential voltage of 9.45mV maximum.

In trying to figure out what could be causing the issues with the instrumentation amplifier I have managed to confuse myself with regards to the acceptable input voltage range for the device. Could someone please help me understand the section of the datasheet explaining the common-mode input voltage range?

Here is what I understand and can figure out:

1) "The upper limit of the common-mode input range is set by the common-mode input range of the second amplifier, A2, to 1.2V below positive supply."

1) This would mean the upper limit would be 1.5V (would this be correct?)

2) "The lower limit of the input range is bounded by the output swing of amplifier A1, and is a function of the reference voltage according to the following equation: VOA1 = 5/4(Vcm)-1/4(Vref)"

2) VOA1 = 1.6875V-1/4(Vref)

If I have a low Vref, say 15mV this would mean VOA1 = 1.68375V. How does this help me to work out if my input is within the common-mode input range?

3) If I look at the graph on page 6 everything seems to be fine, but I'm unsure if I'm interpreting it correctly.

Thanks for your help in advance,


  • Hello Gavin,

    Yes, given a 2.7V supply voltage the upper limit of the common-mode range is 2.7V-1.2V=1.5V.  This is also listed in the Electrical Characteristics table.

    The equation for VOA1 that you reference is for the output of the first amplifier, A1. The purpose is really to show that the common-mode range (which depends on VOA1) is further dependent on the reference voltage.

    The figure you refer to is with respect to the output of the device, which is more useful than the aforementioned equation.  Given a gain >=10V/V, the output should be rail-to-rail.  Such a small reference voltage (15mV) should have little affect on the lower limit of the common-mode range.  Finally, your common-mode (1.35V) is well above the minimum and still below the maximum Vcm of 1.5V.  Therefore I do not see any issue with operating the device in the given configuration.


    Pete Semig

    General Purpose Amplifier Applications

  • In reply to Pete%20Semig:


    I have a similar question about the common-mode input range. With the INA2331, is it possible to have REF=0V with Vs=3.3V and a differential input of 0 to 13.2mV. 

    I'm confuse with the minimum value of input common-mode range. Would it be preferable to have Vref = Vs/2?