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ADS1292R: ADS1292R

Bob Ozawa
Prodigy 10 points
Part Number: ADS1292R
Other Parts Discussed in Thread: ADS1292

Tool/software:

Hello,  I am trying to use the ADS1292R to measure EEG signals.  Below is the schematic.  I am trying to use the RLDOUT using the internal AVDD/2 as the common mode reference for the INP/N input channels.  For testing, I am shorting the RLD signal to the REF_ELECT_IN signal to set the common mode for the INP/N signals.  Shouldn't this work?

I am finding that the PGA amplifiers appear to get saturated above a common mode voltage of ~0.6V.  If I use a common mode voltage lower than 0.6V (by using an external reference for the RLD buffer) it seems to work fine.  Is there something wrong with the schematic?

I thought maybe it was because I am tying all of the INN pins together, so I isolated channel 1, but it didn't help.

Some other odd behavior that might be related:

  • The reference voltage measured at the VREFP pin is ~ 2.3V instead of 2.42V
  • Trying to set the RLDOUT to AVDD/2 results in 3.0V instead of 1.5V
  • When trying to read VDD through the MUX in registers 4 and 5 gives ~0V on channel 1 and 2.42V on channel 2.

I would really appreciate help with this.

Thanks

Bob Ozawa 

  • 0
    TI__Guru 66466 points

    Hello Bob,

    Thank you for your post. 

    Shorting RLDOUT to "REF_ELCT_IN" effectively ties the RLD output voltage to the negative side of each channel; however, the positive inputs still do not have a deterministic common-mode voltage. Instead, the positive inputs are left floating. Is anything applied externally to the positive inputs? What is the goal of the test?

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 10 points

    Hi Ryan,  Thanks for the quick response.

    I am trying use the ADS1292 in a single ended configuration as shown below.  If I increase the common mode voltage above ~0.6V, it looks like the output of the PGA amplifier gets saturated, and eventually goes to the rail (3V) when the common mode voltage reaches ~ 1V.

    I think this configuration is discussed in the datasheet.

    Bob