ADS1293: How to choose discrete components for RLD amplifier loop?

Hi Philip,

Thanks for your post!

Under figure 84. in the ADS1293 datasheet you can see the disclaimer below:

"(1) The ideal values of R1, R2 and C1 will vary per system / application; typical values for these components are: R1 = 100 kΩ, R2 = 1 MΩ
and C1 = 1.5 nF."

Hello Philip,

Thank you for your post.

The purpose of C1 and R2 is to set the closed-loop bandwidth of the RLD amplifier. The User Guide shows 1 nF and 10M, which sets the -3 dB cutoff to 15.9 Hz. Meanwhile, the data sheet values of 1.5 nF and 1M set the -3 dB cutoff to 106 Hz.

Meanwhile, the ratio of R2 / R1 sets the gain of the RLD loop at DC. In the User Guide, the DC gain is set to 1000 V/V (60 dB), while the data sheet values set the gain to 10 V/V (20 dB). This gain will reduce over frequency since the R||C feedback components create a low-pass filter at the cutoff frequency described above.

The important detail is to ensure that the gain of the RLD loop is sufficiently high for the common-mode frequencies that you're trying to cancel (i.e. 50 Hz and 60 Hz). Here's a comparison of the response for the  two filter types below. You can see that you should have better rejection of common-mode noise with the values in the User Guide. However, care must be taken to ensure that the signals at lower frequencies are not too large such that the amplifier output saturates.

Best Regards,