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ADS1921: ECG data output issue

Jacinta Sartor
Prodigy 40 points
Other Parts Discussed in Thread: ADS1291

I am having trouble getting the correct output data from the ADS1291.

The following is the electrode lead set up

R3, R4, R10 and R11 = 200 kOhm

C17 and C18 = 0.82 uF

The register configurations are:

I'd like some clarification on BIT5 of CONFIG2, the wording is ambiguous to me. The data sheet says:

Does this mean that 0 will disable the internal reference and 1 enables the internal reference? or that 0 disables the external reference and 1 enables the external reference?
I want to be using the internal reference buffer. To do this I would set BIT5 to be 1, correct?

The following function reads the ECG data from the SPI.

From my understanding the device data output is 24 status bits + 24 bits x 2 channels.

Since I only want the data from channel 1 i have taken out bytes 3, 4 and 5 to then be combined into an array (myarray)

The indexer is only there to get a small sample for debugging purposes.

I have been using an ECG signal simulator, however can not get the output to match the input.

Here is an example of the output I'm getting

Any ideas what the problem may be?

  • 0
    TI__Mastermind 38745 points
    Hello Jacinta,

    I believe the issue is related to the RLD amplifier. You are using the RLD to bias the inputs, but it is turned off so the input common-mode voltage is floating. Please set but 5 of the RLD_SENS register and clear bit 1 i.e. RLD_SENS = 0x20.

    The PDB_REFBUF bit enables the internal reference buffer. You should set this bit when you plan on using the internal reference. In your case, since you are using the internal reference, set the bit.

    Brian
  • 0 in reply to Brian Pisani
    Prodigy 40 points

    Hi Brian,

    The main issue was the incorrect connection of the electrode USB pads to the ADS.

    Your suggestion to configure the RLD_SENS register to 0x20 also helped greatly to clear up some noise.

    However I've now encountered another issue.

    The function to read the data from the SPI has changed slightly

    Using a moving average filter I'm now receiving the following output

    The issue is that I expected the bias to be at 0 but it appears to be somewhere around the -5000 mark.

    Do you have any idea why this may be?

    Thank you

    Jacinta

     

  • 0 in reply to Jacinta Sartor
    TI__Mastermind 38745 points
    Hello Jacinta,

    What you are seeing is electrode offset. The DC voltage between the electrodes may be a few hundred mV in some cases and this is normal and it is to be expected. Generally patient monitors have some kind of DC removal filter so the data van be vertically aligned on the screen.

    Brian