I am trying to apply gain for a 0.0 - 16.7 mV signal with an INA122P, with a single 10 V supply for all the circuits, with Vref driven at 0.0 mV.
Here is the circuit that I am using, no Rg applied, for which the datasheet said that the default gain of the INA122P is 5.
I have simulated the sensor with resistors to create a Wheatstone bridge, with a differential voltage output of around 0.0 - 16 mV, which I have confirmed by measuring with a multimeter.
Here is the measurement of Vdiff and the output voltage:
|Voutput (Gain = 5)
My question is, why is the output of 0.0 mV and 1.4 mV Vdiff not resulting in a 5x gain?
In the example, 1.4 mV * 5, so it is supposed to be 7 mV. It seems to have an offset of around 1-2 mV Vdiff. Looking at the INA122P's datasheet, it might have to do with the following graph, because I believe my Vcm is sitting at around 5.005 V and it might not provide linear output, but I'm not quite sure how to refer to this graph; that's one speculation.
Things I have tried:
- I have tried to connect both the INA122P's positive and negative inputs from the same voltage divider of the Wheatstone bridge to provide 0.0 Vdiff, and it still provides 9.8 mV.
- I was thinking to lower the Vcm by plugging both positive and negative input supplied from the ground of my circuit (maybe bad practice), which provides 2.7 mV, which is lower.
- The TLC271 single-supply buffer creates a 5.9 mV increase to Vref, achieving 0.0 mV when I removed the buffer.
Some similar questions I have observed:
INA122 output provides saturation around 0,3V It seems that my circuit provides better performance, but I still need to confirm: why is there an offset of around 9.8 mV when I apply 0.0 Vdiff?
Is there something I overlooked? Should I apply some decoupling capacitors?
Another similar question I have checked, made me think to apply 1 MΩ resistors: Common mode rejection range for INA122
Also, here is my breadboard, after I removed the op-amp buffer: