I am trying to build the input stage for an EMG acquisition system based on the INA121 in amp.
I'm using single supply, so I'm using a split supply as seen in the schematic.
However, when I use this circuit, I get lots of 50hz interference, so much that my EMG signal is produced 'on top' of a sine wave, as if the common mode rejection of the amp is failing. The FFT on my scope shows a large spike at 50Hz which is larger than all other frequency components.
Any idea what I'm doing wrong?
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:
A good, clean application circuit should result in high rejection of the unwanted 50 Hz signal.
I hope this helps!
PA - Linear Applications Engineering
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