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ADS1292R: What are the max/min capacitive loads for the RLD amplifier

Laurent Schijns
Prodigy 10 points
Part Number: ADS1292R

Dear sir/madam,

For my current design i opted for the ADS1292R to be able to measure resp. and ECG. We are using the 2 electrodes setup and that works perfect. We have made a filter setup for safety and for filtering. Now this is working perfect but after adding a big current consuming switching device also  to the test setup i am starting to see the on/off switching current peaks (of that device)  in my respiratory signal. Adding a capacitor of appr. 68nF to the output of the RLD amplifier to GND (because this amplifier is used to get 1/2Vdd) and this cleared this problem in stationary situation. But when the other divice is driven harder (meaning higher turn on currents) the switching peakes can again be seen on the RR signal.

These peaks on the RR signal are very high and short peaks and could easily be fitlered in software, but i could also get a better capacitive buffering on the RLD amplifier output part of the ADS1292R. But i want to make shure that i'm not loading the RLD ampl to mutch with capacitive load. what would be the safe maximum capacitive load for this internal amplifier?

Greetings,

Laurent

  • 0
    TI__Mastermind 47625 points

    Hello Laurent,

    Thanks for your post and welcome to the forum!

    Unfortunately I do not believe this information is readily available. 

    What is the purpose of the "big current consuming switching device"? Frequency and magnitude of the resulting transient?

    Can you provide a schematic of your current setup?

    The RC combination between RLDOUT and RLDINV (1.5nF, 1MOhm) on the ADS1292EVM has a cutoff frequency of ~106Hz. I'm not sure what your RC combination is, but adjusting values here may help as well. Keep in mind that the cutoff should not be less than 50/60Hz so that the power lines interference passes correctly.