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ads131e06: tBG charge time when using external reference

Dave Knight
Intellectual 590 points

Part Number: ADS131E06
Other Parts Discussed in Thread: ADS131E06

I'm trying to figure out what worst case tBG is to resolve a timing issue where tBG > tPOR, as described on page 61 of datasheet. From figure 28 on page 26 of datasheet, tBG is related to R1*22uF.

Which value is appropriate for R1 to use to calculate tBG when we're using an external voltage reference(REF5050)?

Is there any advantage to using 5RC as tBG? It seems that only 2.5 time constants is needed for the bandgap to reach 1.1V. I could see longer time constant being good for dealing with tolerance on the 22uF capacitor.

  • 0
    TI__Guru 67796 points
    Hi Dave,

    Thanks for your post.

    Even though you are providing an external reference, the device will still use that precision bandgap voltage for other internal analog circuitry. Therefore, the value of R1 will either be 12.5k or 10.5k, depending on the register setting for the internal reference magnitude and regardless of whether the internal reference buffer is powered-down or not. Setting CONFIG3[5] = 1 would result in a smaller RC time constant by setting R1 = 10.5k.

    Other than dealing with component tolerances, like you mentioned, I don't see an added advantage to waiting as long as 5*RC. 3*RC is probably a safe bet as that gets you close to 95% of the final value (typically 1.2 V).

    Best Regards,
  • 0
    Intellectual 590 points

    yellow is 24V

    green is 5.3V, which powers the ADS131E06.

    Blue is 3.3V digital

    pink is VCAP1.

    It seems based on page 61, that we should have to wait about 1 second before issuing the reset pulse. We've had about 100 PCBs work with a reset pulse around 120ms. Why is this?

  • 0 in reply to Dave Knight
    TI__Guru 67796 points
    Hi Dave,

    I see what you're saying - it takes about 1 second for VCAP1 to reach 1.1V according to your scope capture, while the tPOR counter only takes 2^18 / 2.048MHz = 128ms.

    Honestly, this may be a little conservative, but we have to specify the tPOR timing this way to guarantee that the device powers up in a known state. If the internal bandgap does not reach at least 1.1V before the /RESET pulse, it's possible that the internal one-time programmable registers are not read correctly. This can result in strange behavior such as a misread of the gain/offset trim values or a misread of the Device ID, causing an 8-channel device to think it's a 4-channel device, etc.

    tPOR = 128ms is the absolute minimum you must wait. However, if it seems as though this works consistently, I suggest that you at least read the Device ID register before determining if the device is ready to go. If the Device ID is incorrect, then you must issue a second /RESET pulse.

    Best Regards,
  • 0 in reply to Ryan Andrews
    Intellectual 590 points

    Ryan,
    We're using an external reference. My understanding is that VCAP1 is used to derive the 2.4V or 4V internal reference. Do we  need a 22uF capacitor on the output of VCAP1 when we're using an external reference?  Can we use a 2uF cap for VCAP1? What other important voltages in the ADS131E06 is VCAP1 used to derive?

  • 0 in reply to Dave Knight
    TI__Guru 67796 points
    Hi Dave,

    You are correct that it is used to derive the internal reference voltage. I believe reducing the internal reference noise was the main motivation for sizing the capacitor so large.

    Other than powering the one-time programmable (OTP) bits and generating the main analog bias current (and subsequent current mirrors), I'm not sure what else the internal bandgap is used for, but I don't believe it's anything critical to ADC performance. 2.2uF should be fine on VCAP1. Again - the primary trade-off is mainly a noisier internal reference, which won't affect you.

    Best Regards,