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ADS1231: ADC kinda falls asleep. May it be internal oscillator's fault?

Iurii Vlasenko
Expert 1050 points
Part Number: ADS1231

Hi, everybody!

Preamble.
Know this excellent chip for about 7-8 years now. At least 200 chips were used. No problems, if the printed board is duly cleaned after the population :) 
But sometimes, very rarely, I meet a situation: the chip ceases to respond. I used to change the chip, that's all. One or two times, not a big deal.

But now the problem happened to 2 boards - and the situation needs some solution. 

Use case.

The schematic is standard:

VDDD is 3.3V (in other cases 5.0V), VDDA is 4.7V (or 5.0V, but separated from VDDD). Control signals come from msp430 FRAM MCU, no ISOlators in most cases.

MCU waits for #READY signal at the DOUT and starts to read the result. The problem is: the line DOUT is constant "1". 

What puzzles me: sometimes after the power is ON - ADC begins to work. But a single power cycle - and it falls asleep again.

I have no possibility to drive the CLKIN port now.

What may be a cause?

Cheers,
Yury 

  • 0
    TI__Guru** 113765 points
    Hi Yury,

    Make sure the clock connection of the CLKIN pin has a good connection to ground. Also make sure that the PDWN pin stays low for at least 10us after both analog and digital supplies have reached nominal voltage. If the analog supply is released after the digital, then the RC circuit for the PDWN most likely will be invalid. If I were to guess I would say there is a timing issue between the analog supply and/or the RC timing on the PDWN pin. See Figure 22 of the ADS1231 datasheet.

    Best regards,
    Bob B
  • 0 in reply to Bob Benjamin
    Expert 1050 points

    Hi Bob,
    Thank you very much for a quick answer!
    The inadequate reset was among my assumptions. I just shortened the PDWN pin to GND several times (power ON, of course). Should it resolve the issue - and initiate the control circuitry of the chip? Alas, the shortening did not help.
    But I check the timing (and, maybe, repeat the shortening) tomorrow. It could be, e.g. some wrong capacitor value.
    I'll report.

    UPD: The PDWN circuit is OK. Here are the VDDA (yellow) and PDWN signals. Everything as expected. Manual short of PDWN to the GND is tested once again.
    Well, I'm afraid the replacement of the chip is inevitable... Or?



    Cheers,
    Yury

  • 0 in reply to Iurii Vlasenko
    Expert 1050 points
    No. The chip is out of suspicion. I've installed another one - the same misfunction. Maybe, something is wrong in the printed board (it's a new design, though 4 boards are working in the field for a several months). Maybe, some capacitor or resistor is mischoosen during the last assembly. It's a wonder! So simple board and...
    Well, I have to double check. Thanks for the attempt, Bob!

    Cheers,
    Yury
  • 0 in reply to Iurii Vlasenko
    TI__Guru** 113765 points
    Hi Yury,

    As you have said, it is a simple device and there is not much that can go wrong. Pulsing the PDWN pin should reset the part. Make sure that the digital and analog grounds are connected together and that both supplies are available at the ADS1231 input pins.

    Let me know if I can be of more assistance.

    Best regards,
    Bob B
  • 0 in reply to Bob Benjamin
    Expert 1050 points
    Shame on my bald head!
    The cause of the ADC "sleeping" is the low level of the CLOCK signal. It's a long story to tell, why it becomes low after the power ON. But the fact is, it does become, in the last firmware version. And the ADS1231 keeps its DOUT signal high. So simple...
    Yes, I knew it. Somewhere in 2010 or 2011 :)

    Once more: The SCLK must be low to enable the ADS1231 to show #READY on the DOUT pin.

    Cheers,
    Yury