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Multiple ADS1248 for high channel count interfacing

Tom Russell
Prodigy 175 points
Other Parts Discussed in Thread: ADS1248

I am designing a board with 32 single-ended thermocouples and plan to use 5 ADS1248.  While I could use an external MUX, the sample rate will be too low.  At a minimum I need DIN, DOUT, SCLK 5 CS bits and possibly others.  I was thinking I could have wire all 5 ADS1248 to one START bit and start them all at once.  I see you can also have individual bits for DATARDY.  Any suggestions for interfacing and things to watch out for?


Also, any advice on the use of digital isolators for the interface?  Clearly they can protect the rest of the circuit, but is there a big benefit in noise reduction vs. having a separate ground plane tied to system digital ground at one point?

Thanks,

Tom

  • 0
    TI__Guru* 97435 points

    Tom,


    In taking data with that many devices, you could also issue a SYNC command to functionally do the same thing. However, using the START pin should be much simpler. Once the START pin is pulse high (or the SYNC command is issued), the ADCs should start a conversion. If they are at the same data rate, they should all complete the conversion at the same time as well.

    I would note that if you are using the internal oscillator, there is a certain variance in the data rate. If you look at the Electrical Characteristics, the internal oscillator typically runs at 4.096MHz, with a minimum of 3.89 and a max of 4.3MHz. From the mean, this is a 5% difference, so there might be a difference of 10% between the fastest and slowest conversions. You could track all of the /DRDY pins, but I suspect it would just be easier to wait the maximum expected amount of time and just grab the data from each device afterwards.

    Note that this timing becomes more difficult at higher data rates. Once you start the conversion, and the conversion completes, you'll still need to get the conversion data out before the next conversion comes along.

    Additionally, there may be some analog settling that may add some time to your measurements. If you have large filters on the inputs or reference, there may be time needed to settle the measurement, if you have to turn on any references or excitation currents. With thermocouples, it's not as likely you'll have to deal with this issue, but I thought I'd raise this point.

    As for digital isolators, I don't have much experience with them. I know there are plenty of applications where they are used for protecting the micro side of the circuit, but I've never had to work with them. As for doing separate analog and digital ground planes, I think we've had better experiences with doing a single plane for both the analog and digital supplies.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 175 points

    Thanks Joseph


    Just to make sure I understand.  Is issuing the SYNC command the same as bringing START high?  If I was only going to use SYNC command, can I just tie the START line low?  I don't need to have synchronous conversion.  I only mentioned connecting all the START pins together to save bits. Please confirm that I really only need the DIN, DOUT, CLOCK and 5 CS lines in order to select and read one channel after another on each unit.

    The clock frequency variation will not matter, except for its affect on 60Hz rejection, which is of some concern.

    Tom

  • 0 in reply to Tom Russell
    TI__Guru* 97435 points
    Tom,


    Using the SYNC command is functionally similar to using the START pin to initiate a conversion. If you want to use do this, you could tie the START pin HIGH (not low).

    As you've mentioned, you could use the DIN, DOUT, SCLK and the five /CS lines to communicate with the devices.


    Joseph Wu