ADS1298: RC filtering question.

Pablo Sebastian Parodi

Part Number: ADS1298

Hello. I'm working in a project with the ADS1298. My question is related to the RC filtering in each electrode input.

In the datasheet of the ADS1298 (page 99, Layout Example) you show differential and common mode filters (screenshot 1), however in the ADS1298ECG-FE User's Guide (SBAU171D, page 52) there are simple RC filters, two filters in cascade.

In the past, I implemented the datasheet scheme, with the differential and common mode filters with an aprox. 150hz cut off frequency to limit the signal at the bandwidth of interest for an ECG.

Now I'm working in a new implementation and saw the filtering used in the evaluation module, and came across with the question of why that filters have a higher cut off frequency (153 kHz with the 22.1k and 47pf, and 338 kHz with the 10k and 47pf) and, why you used that filters and not the ones in the datasheet?.

Thank you.

over 4 years ago

0 Ryan Andrews over 4 years ago

TI__Guru 57296 points

Hola Pablo,

Thank you for your post!

The RC filters in the signal path are primarily used for antialiasing purposes. It's not necessary to set the cutoff frequency exactly at the end of your bandwidth of interest. The goal should be to attenuate signals near the modulator sampling frequency for delta-sigma ADCs. I've written a blog series on the subject to help explain this concept, part 1 of 3 is linked below:

https://e2e.ti.com/blogs_/archives/b/precisionhub/archive/2015/09/04/aliasing-in-adcs-not-all-signals-are-what-they-appear-to-be

This EVM released some time ago and through experience with other customers, we've learned that some use both common-mode and differential RC filters in their designs. A differential filter will help maintain good CMRR, which is a key spec in ECG applications. We recommend setting the differential filter cutoff frequency at least one decade less than the common-mode filter cutoff frequency.

Best regards,

0 Pablo Sebastian Parodi over 4 years ago in reply to Ryan Andrews

Prodigy 140 points

Thank you Ryan!, that answers my question really clear.

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ADS1298: RC filtering question.