what's the use of this passive RC filter

Question

I was looking through the design of this board, which uses ADS1299 for measuring EEG signals.

EEG signals have a freq band of usually about 5-100Hz and they are low in their SNR as well.

Before the signals from the electrodes (EEG) are received at the pins of the ADS1299, they first have TVS diodes connected to ground from them, and they go though this RC low pass filter (R = 2.2K, C = 1nF). So this would make a cutoff frequency of 72 KHz.

I have 2 questions about this design:

1. Am I correct in assuming that the TVS diode is there to clamp any voltage spikes (e.g. for static charges being discharged)?

2. What is the main purpose of the RC filter?

Answer 1

Am I correct in assuming that the TVS diode is there to clamp any voltage spikes (e.g. for static charges being discharged)?

ESD is the primary reason why a designer would put a TVS on a conductor that a human could come into contact with.

What is the main purpose of the RC filter?

Filter out noise at 72kHz, but the resistance also provides current limiting for any inputs downstream attached to the filter. Most ADC's have input protection diodes, but these are limited to 100mW's (maybe ~100mW) so large currents could defeat the power rating of these diodes, and if they fail so does the ADC. A resistor ensures that these diodes are protected.

Answer 2

1. Yes, TVS diode is for ESD protection

2. The RC filter is an anti-alias low pass filter for the ADC input. Also the allowed range for the resistance value is set by standards for medical electronics.