# Why am I having a saturation in the output of INA122?

Even with no voltage difference on the inverting and non-inverting input, my output signal is at approximately 2.8V. I am powering the amplifier with a voltage of 3.3V, provided by my arduino. At the inputs, the signals are coming from a balanced Wheatstone Bridge, with 4 resistors of 100k ohms. I am not using a gain resistor, because the minimum gain of this component is 5. I have tried to power the circuit with 5V, symmetrical source +/-12V, but I had the same saturation problem. I have already tested 4 different INA122's and the result was the same. Does anyone have any idea what the problem could be?

• Can you put node voltages on a schematic? Mar 24, 2022 at 1:13
• Could please help us with the schematic Mar 24, 2022 at 2:28
• The resistors are 5% tolerance, so the voltages at the inputs won’t be quite the same. Not quite enough to explain a 2.8V output with G=5, but as a test, you may as well remove this effect by connecting two inputs to the same voltage divider and see what your output is. Mar 24, 2022 at 2:36
• I tried to connect the two inputs to the same node and even then it is still saturated at 2.8V Mar 24, 2022 at 2:54
• Input voltages may be outside the input common mode voltage range. Try using voltages closer to 0V. e.g. around 0.5V. The Common Mode Range includes the lower supply rail, but is around 1.6V lower than the positive supply. You may be just outside the upper limit. Mar 24, 2022 at 3:46

## 1 Answer

All all connections verified and all node voltages?

Please list

Verify all pin voltages 1 to 8 It is clearly my bad eyes.

Update with conflicting values.

Comments p6 2.8V
Schematic p6 0.30... Av=5 Vio= 0.06 which from 3.3V is 0.06/3.3 = 1.8 %

Expected error with 4x 1% parts = 4%

Conclusion

• Results on schematic in spec.

• other observations missing context or errors

• According to the 1NA122 data sheet, it is valid not to have a gain resistor which results in a gain of 5 as stated by the OP. Mar 24, 2022 at 2:18
• It is not necessary to use the Rg. With this open line, the resistance is infinite and I still have 5 gain. Even knowing this, I have already placed several resistor values between pin 1 and 8. Even so, the result was the same. Mar 24, 2022 at 2:32
• Pin Voltages: 1 - 0V / 2 - 1.65V / 3 - 1.65V / 4 - 0V / 5 - 0V / 6 - 2.8V / 7 - 3.3V / 8 - 0V / Mar 24, 2022 at 2:48
• Thanks but that with a high gain Rg? Mar 24, 2022 at 4:54
• Minimum gain... Mar 24, 2022 at 23:28