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ADS131E08: Can we multiplex each channel?

Howard Zou
Genius 12365 points
Part Number: ADS131E08
Other Parts Discussed in Thread: ADS131M08, ADS131A04, ADS1278, ADS8588S

Hi,

my customer is multiplexing one channel of the ADC with analog switch to sample signal 1 and signal 2, is it okay to do that?

When we switch from signal 1 from signal 2, how long should we wait to start the conversion?

Any other consideration needs to be taken when multiplexing the signal?

  • 0
    TI__Guru* 97835 points

    Howard,


    Using a multiplexer on the front end of an ADC should be fine. In this device we have multiple independent ADCs, but in many others, there are integrated multiplexers.

    For the ADS131E08, the device uses a sinc3 filter. This is basically a 3 data period moving average. That means that if there is a change in the input, it will take at least three data periods to settle the ADC data. Generally, the input change isn't completely synchronized with the data output, so I would really wait four /DRDY indications before using the output data. There is a write-up in the datasheet that describes this in detail on page 28 of the datasheet.

    When using a multiplexer you must be concerned with any capacitive elements between the multiplexer and the ADC. If there is capacitance (and you'll need to check the output capacitance of the multiplexer also) charge may be retained from one channel to the next. This adds to the delay time for switching channels, and may appear as a small amount of cross talk.


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Genius 12365 points

    Joseph,

    thanks. I've read page 28.

    Then I wonder if it's okay to pull START pin low to stop the conversion, and switch from signal 1 to signal 2, then pull START pin high to start the conversion.

    Then we only have to wait the settling time, and can read out the result at the 1st DRDY high to low edge, right?

  • 0 in reply to Howard Zou
    TI__Guru* 97835 points

    Howard,


    You can use the START pin, but you'll still need to wait three /DRDY periods to get settled data. Again, the digital filter in the ADS131E08 is basically a moving average filter that takes the three /DRDY periods to settle. If you're starting from 0V, and measure 1V, the first /DRDY would give about 1/3V, the second /DRDY would give about 2/3V, and on the third /DRDY the ADC would give 1V.


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Genius 12365 points

    Thank you.

    My customer is multiplexing each channel for 2 signals. Now it seems ADS131E08 could not meet their sample speed requirement.

    Then my customer would like to us to recommend ADC with below requirement:

    16 channel input, each channel need to be sampled at least 8kSPS with 20bit resolution.

    I haven't found any part that could meet the requirement, could you please recommend?

  • 0 in reply to Howard Zou
    Prodigy 100 points

    Hi Howard,

    Why don't you use two ADCs like the ADS131M08, it's cheap, has the resolution you need and you can even synchronize them.

    Thanks,

  • 0 in reply to Howard Zou
    TI__Guru* 97835 points

    Howard,


    Mauricio does have good point in using 2 ADS131M08 devices. The device is fast enough for the application and has eight channels. It is also targeted at less expensive applications. However, I would be concerned that the noise is a bit high for 20-bit applications. Note that the ADS131M08 is not yet released. However, I believe it should release in the middle of December and is sampling now.

    The ADS131A04 may have been a candidate, and while it is 2x faster, it has half as many channels. Another possibility is the ADS1278. However this device has a high-latency filter which probably removes it as a candidate.

    There may be some SAR type devices that may also work. However, I would need to know more about the signals that would be measured. Are the inputs single ended or differential? How large are the input voltages? What will the system measure?


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Genius 12365 points

    Single ended. Input voltages has been adjusted to 0-4.096V by OPA. The orignal signal is the single cell battery's charge current and voltage.

  • 0 in reply to Howard Zou
    TI__Guru* 97835 points

    Howard,

    With an OPA on the front end, the inputs could be scaled a bit smaller to fit the range of the ADS131M08. Again, this device is much less expensive than the ADS131E08 and two ADS131M08s would be a good alternative to using one device with multiplexing.

    There may be some similar simultaneous sampling SAR devices, like the ADS8588S. However, the resolution typically goes up to 16-bits (with some single ADC devices that go up to 20-bits).

    Joseph Wu