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ADS8598H: ADS8598H isolated interface to microcontroller

SARA GALLINI
Prodigy 20 points

Part Number: ADS8598H
Other Parts Discussed in Thread: ISO7763, TCA9534, ISO1640, ISO1641, ISO7762

Tool/software:

Dear technical support,

in our custom data acquisition project, we are designing an isolated interface between a microcontroller (STM32) and the ADS8598H ADC.

In order to limit the number of signals crossing the isolation barrier, serial communication between ADC and uC is preferrable.

Therefore, we thought about isolating the main signal for the communication with an ISO7763, i.e. the DOUTA, DOUTB, FRSTDATA, CSn, SCLK and, eventually RESET signals.

From my understandings, also the two CONVST signals and the BUSY must be monitored, while the other configuration pins can be implemented via hardware.

Now, in the event of changing the oversampling ration and the range dinamically without acting on hardware, we want to have access on these pins too.

For clarification, this is the (unfinished) schematic:

Now my question: is there any advantage of using a supportive microcontroller (e.g. MSP430) over an I2C expander (e.g. TCA9534) to control the eight signals that are now disconnected in the schematic? To optimize PCB space, we want to minimize the isolation ICs, hence we discourage the direct connections of STM32 pins to a total of 14 pins of the ADS8598H.

Could I find any issue by controlling CONVSTA, CONVSTB and BUSY through a GPIO expander?

Thanks in advance for your support,

Andrea

  • 0
    TI__Guru**** 183361 points

    Hi Sara,

    Welcome to our e2e community! 

    The two CONVST signals could be tied together, that just depends on how you plan to operate the ADS8598.  You could also use DOUTA only to get data from both A and B channels.  With the GPIO expander, I don't think you would be able to get to full speed operation of the device so I'd prefer to see you keep CONVSTx, and BUSY and perhaps FRSTDATA connected directly through the ISO7763.  The OSR, RANGE, RESET and STBY pins are normally static I/O resources, so I don't see a problem putting them through the IO expander.

  • 0 in reply to Tom Hendrick
    Prodigy 20 points

    Tom,

    kindly thanks for your answer!

    To maximize the throughput of the ADC, we ought to keep DOUTA and DOUTB separated. 

    We could tie CONVSTx together as we don't really need to convert only the signals from one set or another. Moreover, we would like to adjust the OS ratio dynamically, so i think this would be a good idea. My only concern about this is that sometimes we need to collect the signal only from one channel: in this case, I suppose we would need to sample all channels anyway and then discard the unwanted data.

    Now we are left with 7 lines that should be isolated... can FRSTDATA be left floating? I guess that the first channel data can be retrieved by counting 18 CLK falling edges on DOUTA after a CSn falling edge?

    Kind regards,

    Andrea

  • 0 in reply to SARA GALLINI
    TI__Guru**** 183361 points

    Hi Andrea and Sara,

    I see by the note in your schematic that you have considered tying the /CS input low.  You could perhaps then use the ISO7762 - bring one CONVST over (for both A and B) and the SCLK as inputs then have DOUTA, B, BUSY and FRSTDATA as outputs.  Tne FRSTDATA output is almost redundant since you could use the falling edge of BUSY as an interrupt.  You could also stick with the ISO7763 and send CONVST, SCLK and RESET over, bringing DOUTx and BUSY back.  You would still need to have the GPIO expander isolated as well, are you considering something like the ISO1640 or ISO1641?

  • 0 in reply to Tom Hendrick
    Prodigy 20 points

    Tom,

    I figured out i'd rather have control over /CS than over FRSTDATA, which in fact feels redundant in my application. If I remember correctly, by controlling /CS we can choose whether to read the data during or after the conversion, for example?

    This is the new modified schematic: if everything looks good, I will go on this route.

    p.s.: don't mind the double REFSEL signal, it's just there for a future decision.

    Kind regards,

    Andrea

  • 0 in reply to SARA GALLINI
    TI__Guru**** 183361 points

    This looks good to me!  Please do let us know how everything works out once you have your hardware together.