# Common mode rejection range for INA122

I am fairly new to electronics and I am encountering some issues in using the INA122 instrumental amplifier. I am using it as in Figure 3 in the spec sheet for the device https://www.ti.com/product/INA122

Now, I am seeking to measure the difference between Vin+ and Vin-, which will have slightly differing values around 13V. Originally, I was powering the inamp with a single supply at ~13V, but found that the output voltage was ~3V even when the difference in the outputs was 0V. After some digging, I gathered that my input voltages were higher than the allowed "Common Mode Voltage Range", and so I was saturating the outputs. In the spec sheet, it only lists this range (0-3.4V) for when the inamp is powered with 5V.

I tried using 19V to power the inamp, but the output is still 0.6V when there is no difference. My question is, am I still outside the common mode range? Any ideas how much voltage is needed to power the inamp that will allow my setup to work? Or am I missing something entirely?

• Never used INA122. If I see the lower left picture page 4 of datasheet, common mode is -15V to + ~10 V when Vs=+/-15V. But in first page "COMMON-MODE RANGE TO (V–)–0.1V" ? Seems to be an internally shifting by 0.5 V to allow unipolar supply. Must simulate if I found its model. Commented Jul 12, 2021 at 14:06

"am I still outside the common mode range?"

Simulation show that if INA122 is supplied with +/- 15V, common mode (in the circuit tested) goes from -12V to +11V with 0.1 V differential input. So it is ok. Two others configurations added lower.

Don't forget that for this amp, a return path is needed for the inputs, see datasheet, page 8 :

... "Without a bias current path, the inputs will float to a potential which exceeds the common-mode range of the INA122 and the input amplifiers will saturate. ! ... "

Provide at least two resistors 1 or 10 MegOhm ... to ground. This input bias current is approximately –10nA (current flows out of the input terminals).

Do you see the function of the two 10 MegOhm resistors ?

Simulation made with microcap12 from http://www.spectrum-soft.com/index.shtm

• I'm not sure that I follow... I only have a single power supply at ~13V (so V- = 0V), which I can step up to ~19V. My differential input "must" be able to measure ~12-13V. When you mention return path, what are these resistors connected to and where? I don't understand why they are necessary. Commented Jul 12, 2021 at 15:00
• Not a problem, I think. Unipolar power supply can be higher. Add only the two resistors to ground, and I think all will be ok. I try simulating this configuration. Commented Jul 12, 2021 at 15:03
• As in, put a high resistance resistor between each input (the two of which I am measuring the differential voltage) and ground? Why will this help? Commented Jul 12, 2021 at 15:09
• Inputs must be biaised. See the remark and new simulations in my updated answer. I will add another simulation without resistors. If model is ok, it will saturate. Commented Jul 12, 2021 at 15:16
• Thanks for your help. I tried to implement it but even with no differential input, I am still saturating at 600mV. I downloaded your software and tried my own simulations, and my setup "should" work - 19V supply should result in 13V being within the appropriate range. But maybe the range is still slightly too low in practice? Commented Jul 13, 2021 at 1:21