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ADS1292: Difference with ADS1194 - MISO signal

Part Number: ADS1292
Other Parts Discussed in Thread: ADS1194, , ADS1298, ISOW7841

Hello,

I have a board with ADS1194 chip connected to a controller using a 4-channel isolator. Those 4 channels are used for the SPI communication so there is no access to DRDY pin. The way that is currently used to know when data is ready is polling the MISO signal, CS is kept low all the time and the ADS1194 chip behaves as follows:

- If MISO is currently low, when data is ready, it pulls it high and after ~2 us MISO is pulled low again (#1 on the image)

- If MISO is currently high, when data is ready, it pulls MISO down (#2 on the image)

In the end, controlling the falling edge of MISO signal allows to know when ECG data is ready.

I was planning to use ADS1292 chip for another board which would also use a digital isolator. While testing the device on a prototyping board, I noticed that ADS1292 does not have this kind of behaviour on MISO pin even though I thought it would. (#3 on the image).

Is this the expected behaviour on this chip?

  • Hello Alejandro,

    Thanks for your post!

    As you found with the ADS1194, DOUT can be used to indicate when new data is available in RDATAC mode. When /CS is held low, the falling edge of /DRDY will be synchronized to the first falling edge of DOUT.

    The description in the ADS1292 datasheet mentions the same feature, but I have not had the chance the verify it myself. Let me confirm with a designer that this feature should behave the same way.

    Have you verified that you are using RDATAC mode in both devices?


    Best Regards,

  • Hello Ryan,

    I have verified that both devices are on RDATAC.

    In fact, for the ADS1292 in the prototyping board, I have been able to acquire continuous data polling DRDY (which I can poll now but I won't be able to when using the isolator) and I do not get to see any pulse on MISO signal before the SPI transaction starts.

    (Channel4 is /DRDY) On the ADS1194, MISO signal has a pulse (low-high-low) when the data is ready.

    Hope that designers shed some light on this topic.

    Thanks,

    Alejandro

  • Hello Alejandro,

    Thanks for your patience on this.

    Our designers confirmed that this feature does not exist in the ADS1292. The description in the "Data Output (DOUT)" section about this feature was likely a copy/paste error from the ADS1298 datasheet. I've made a note to remove this in the next datasheet revision.

    We're sorry for the confusion, please let us know if you have any further questions.

    Best Regards,
  • Hello Ryan,

    I had already spent some time on a hardware design that is using ADS1292 as the ECG frontend combined with the ISOW7841 4-ch isolator from TI. I noticed this issue when I bought a sample of ADS1292 chip.

    I was expecting to use MISO input from the chip to receive the DRDY signal as explained on the datasheet.

    Which kind of solution would you suggest to solve the problem of not getting that information via MISO pin?

    Best,

    Alejandro

  • Alejandro,

    Which four signals are you connecting to the isolator: /CS, DOUT, DIN, and SCLK? My suggestion would be to tie /CS low on the ADC side of the isolation barrier and isolate /DRDY instead. Otherwise, you will need a 5th isolation channel, which I understand adds considerable cost. Is that something you could do?

    Will you need to spin another PCB in order to try this approach?

    Best Regards,
  • Hello Ryan, 

    Adding a 5th channel to the isolator is not possible because of space requirements and most isolators with 4+ channels do not have isolated power.

    I am isolating /CS, DOUT, DIN and SCLK. I will tie /CS low and connect /DRDY as you suggested. I thought that /CS was needed for the power-up sequence, can you please confirm that this is not the case?

    Thanks,

    Alejandro

  • Hi Alejandro,

    You can tie /CS low permanently according to Figure 63 in the datasheet.

    Best Regards,
  • Thanks Ryan, that's what I will do.