• State
  • Replies 3 replies
  • Subscribers 46 subscribers
  • Views 32 views
  • Users 0 members are here
Support feedback
Options
Options
Related

ADS1243: Longer Start-up When Cold

Simon Wilkinson
Prodigy 80 points
Part Number: ADS1243


Hi. I've been looking at an occasional confguration issue with one of our designs that uses ADS1243 (HT version in ceramic DIL package).

In this application the power to the ADC is controlled. I previously determined that it is necessary to wait about 64ms after power-up before trying to configure the ADC using the SPI bus. This is to allow time for the (4.9152MHz) crystal oscillator to start up + ADC reset & start-up. The setting in the normal software uses twice this value.

Sometimes, however, this 125ms did not work. After extensive testing I discovered that the start-up behaviour of the ADC when cold is unexpected. Sometimes the start-up time is much longer, up to nearly 200ms, and this it not because the crystal oscillator takes longer to start-up.

What the test software does is, after waiting for 25ms, it repeatedly tries to access the ADC - to read the chip i/d - until this succeeds (expected value is received) and then it configures the ADC for continuous samples mode.

The 2 'scope screenshots show SPI clk. in green, the power supply in blue and the ADC crystal clock in red. The first one is at room temperature; the 2nd after cooling the ADC region of the PCB using freezer spray. In the second one you can see that the SPI retries period lasts a lot longer (148ms in this case), but the crystal oscillator start-up time is about the same.

This behaviour was very much a surprise to me. When the product using the ADS1243 is deployed it will not be in cold conditions, so this issue only occurred during testing in the factory and so it is not a real problem - the delay in the normal software will be increased to 500ms.

Yours sincerely,

Simon W.

20251117-0001-room_temp_w_SPI.png

20251117-0002-cold_148ms_SU_time_w_SPI.png

  • 0
    TI__Guru 53696 points

    Hi Simon Wilkinson

    Thanks for bringing this to our attention. I am not aware of any startup issues with the ADS1243-HT at low temperature

    • Can you clarify what temperature you are bringing the board to? Have you been able to recreate this behavior in a thermal chamber where the temperature is more controlled, as opposed to using freeze spray?
    • If you control the temperature, does the power on time decrease approximately linearly as the temperature increases (but still longer than normal)? Or is this more like a "step" change where all of the sudden this behavior goes away completely? If it's the latter, at what temperature does this occur?
    • Have you seen this behavior with multiple boards / devices?
    • Is this a new problem you are seeing i.e. you have other designs that use ADS1243-HT without issue, and now this problem is happening? Is this an issue with a product / system in production? Or is this your first design with this ADC?

    -Bryan

  • 0
    Prodigy 80 points

    Hi Bryan,

    thanks a lot for replying.

    (1) I have not used a temperature chamber but the thought had occurred to me. We are seeing this issue now in the field. The PCB with the ADC is inside a metal tube and also on the PCB there is a CPU that is reading 14 deg C. This temperature makes sense to me as it didn't show the issue after I first applied the freezer spray, but only when it had been warming up for a few (10 - 30) seconds.

    (2) I did elevated temperature testing and it was fine. There is a perceptible increase in the start-up time - see image below.
    What is more evident is the start-up time reducing again as the PCB cools down again, afterwards.
    The longer start-up times (not in these results) are rogue values - e.g., it can be 60ms, 57ms, 61ms, 185ms, 62ms etc.

    (3) Yes we have seen this behavior on other boards: 2 in my equipment for testing and (now) 3 in the field.

    (4) Yes it is a new problem. We have been using the ADS1243HT for a several years but it is being used differently now, in a new design, in that previously we powered on the board and then ADC initialisation occurred after a few seconds (no issue with this). Now the ADC power is being directly controlled and so the time between turning on the power and talking to the ADC is more visible.

    We do not expect to see this problem in production as the product will be installed where the temperature is 30 deg C or hotter and this issue has not been seen at room temperature (20 deg c) or above.

    Simon

    ADC start-up time vs. temperature


  • 0 in reply to Simon Wilkinson
    TI__Guru 53696 points

    Hi Simon Wilkinson

    What crystal are you using for this design? Can you provide the part number

    Are you able to try using a discrete clock oscillator instead of a crystal to see if the situation improves? There is a suspicion that there might be an issue with the crystal + ADC clock generator circuit + load caps that is causing this issue, so we'd like to at least rule that out

    Finally, so I understand, are you just pointing out this behavior? Your last post said you do not expect to see this problem in production, which implies that you don't necessarily need a resolution. So it wasn't clear to me if you were just letting us know about this behavior in case we were unaware or if you actually wanted us to investigate this further (likely requiring FA)?

    -Bryan