ADS1291: How to bias electrode inputs with AC Coupling?

Ferhat Bayir

Part Number: ADS1291
Other Parts Discussed in Thread: ADS1293, ADS1298

Hellow. Im using ADC1291 for getting ECG data. Can you share document, how to make AC Coupling. What is the circuit needed.

Below picture if i use 10M to AVSS and AVCC

This picture if i use RL (RLDOUT) connected to IN1P and IN1N with 10M

Below picture shows my shematic

over 6 years ago

0 Ryan Andrews over 6 years ago

TI__Guru 66346 points

Hello Ferhat,

Thank you for your post!

Where are the AC-coupling capacitors in your schematic? A series capacitor is needed to AC couple each electrode input. A typical value is around 0.1 uF. With 10-M pull-up/pull-down resistors (or a single 10-M pull-up to RLDOUT), 0.1 uF sets the high-pass corner frequency to either 0.16 Hz or 0.32 Hz.

I'm not sure how many customers have used the technique of a single 10-M pull-up to RLDOUT, but it should work, in theory. A couple considerations to make are:

The RLD amplifier should have a closed-loop bandwidth greater than 60 Hz in order to reject power line common-mode noise. Currently, R19 and C23 create a pole at 40.6 Hz. Try changing these values to 1M and 1.5 nF (fc = 106 Hz).
The RLD amplifier must sense the common-mode noise on the electrode inputs in order to create a cancellation signal at the output. This is done by enabling the RLD_SENS bits for a given channel. Try enabling RLD_SENS[1:0] = 11b to select Channel 1 (IN1P and IN1N) for common-mode derivation.

Best Regards,