INA121 Bad noise performance

Hello Vanush,

Your basic INA121/TLE2426 circuit looks correct; however, the poor 50 Hz rejection is more likely due to the physical/electrical application of your circuit.

The INA121 has high CMRR at 50Hz, and any true common-mode signal applied to the inputs equally should be effectively rejected. The fact that the 50 Hz signal is prominent in the output indicates the 50 Hz being applied is entering differentially and is being amplified like any other differential signal by the INA121 gain setting. Here are some suggestions of things to check and circuit techniques that may help:

1. Make sure the differential signal being applied to the INA121 is applied using a balanced line such as twisted pair cable. An unbalanced input line will convert a common-mode signal into a differential mode signal which will be amplified by the INA121 differential gain setting. Coaxial cable is not a balanced line, but most twin wire cables having a shield are balanced. Since any signal applied to the INA121 inputs differentially will be gained up by the INA121 gain setting this part of the circuit is sensitive to unwanted signals as well.
2. Be sure you are using a good clean PC board layout with short, low-impedance ground returns and power supply bypass capacitors at the INA121 power supply pin.  Use a 100 nF ceramic right at the V+ pin, and a large electrolytic or tantalum capacitor reasonably close to the V+ pin.
3. A place where 50 Hz differential mode ac could be getting into is the REF pin being driven by the TLE2426 1/2 V+ voltage level. The TLE2426 has a noise reduction pin so take advantage of it. Add two 10 uF electrolytic capacitors to that pin; one from the positive supply, and one to ground. Their connection is shown in the schematic provided in the attached file. Also, add a capacitor at the INA121 REF pin. The file schematic shows a 10 nF connected to the pin, but this can be increased to a larger value. Just make sure that you observe the stability information shown in Figure 17 of the TLE2426 data sheet.

A good, clean application circuit should result in high rejection of the unwanted 50 Hz signal.

I hope this helps!

Regards, Thomas

PA - Linear Applications Engineering