Hello,
I have a design that measures FECG with the ADS1294 and we have been experiencing CMRR issues with 50 Hz interference. Initially, the issue was pinned down to large tolerances in the front end components. After lowering tolerances to 0.1% for resistors and 1% for capacitors (C0G/NP0), the CMRR did not improve. Right now I have been doing some testing in order to identify the cause of low CMRR, by injecting common mode interference at 50 Hz directly into the inputs of the ADS1294 with a signal generator. The purpose of these measurements has been to identify the component or components in my analog filters that were degrading CMRR. I have however some results that i cannot explain. The results I want to ask about are listed below:
- Signal Generator directly to CH1 inputs, 4Vpp @ 50Hz common mode signal: CMRR = <-120 dB
- Signal Generator coupled to 100k series resistors on CH1 inputs, 4Vpp @ 50Hz common mode signal: CMRR = -73.1 dB
The series resistors were measured wih Agilent 1272A DMM: 99.74k, 99.77k. I would consider them well matched.
For information the ADS1294 is connected to a +/- 2.5V supply. The common mode input signal was referenced to GND. I am using AC leadoff, but no other functions such as RLD/WCT or pace detection. How come the CMRR degrades this much by just adding series resistors?
Best regards,
Niclas Gustafsson
