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ADS1191/1192 Signal Conditioning Question

John Raynes
Prodigy 30 points
Other Parts Discussed in Thread: ADS1191

I have studied TI application note sbaa160a.pdf, 'Analog Front End Design for ECG Systems Using Delta-Sigma ADCs'.  It explains very well the key difference in using 16 bit vs 24 bit ADCs for ECG signals - that difference being with 24 bit conversion, there is enough resolution to eliminate the need higher amplifier gain needed with 16 bit converters.  The reduced gain required thereby eliminates the need for a two-stage analog pre-conversion gain, with a blocking HPF filter in between, to avoid running out of headroom.  So far so good.

I then did a detailed datasheet comparison of the 16-bit 1191/92 vs the 24-bit 1291/92 analog front ends for ECG.  They are very similar overall, but I expected to see some differences in their front-end set up with regards to accommodation of blocking HPF filters.  I didn't find any references at all to the HPF function within the documents.  Is there a configuration option that I missed, or instead, are the 16-bit 1191/92 products routinely used without any analog HPF in the signal chain?  My own analysis of the required resolution for the ECG application agrees with the ap note, in that there isn't enough resolution in the worst case when using a 16-bit conversion to look at a signal that may be as low as a few hundred microvolts but riding a 300mV offset.  Any insight you can provide would be appreciated.


Thanks,

John Raynes

  • 0
    TI__Mastermind 38745 points
    Hi John,

    Thank you for your detailed question. Your understanding of differences in the devices is correct. There is no analog high-pass filter on the ADS1191/2 which accounts for a large DC offset. The question of whether or not to use a 24-bit converter is really a question of need. If you anticipate the signal amplitude to be too small for the 16-bit converter to accommodate with a sizable DC offset which would need to be maintained within the full-scale range of the converter, then that particular solution won't work. You can either keep a similar signal conditioning scheme and use the 24-bit converter to obtain the desired dynamic range, or you can block the DC and adjust the gain and common mode on the signal to the 16-bit ADC just like you described. Certain 24-bit applications may also eliminate the DC offset through signal conditioning.

    Let us know what sort of system are you building and what we can do to help.

    Regards,
    Brian Pisani