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TLA2024: Can the PGA gain be programmable dynamically for different channel?

HongRen Chang
Intellectual 390 points
Part Number: TLA2024
Other Parts Discussed in Thread: TLA2022

Dear sir,

I plan to use the TLA2022/4 to monitor the voltage and current of USB A interface. the voltage and current input range is different. Can I set different PGA gain for different channel? And can it be set dynamically during continuous read?

Thanks

BR

Ren 

  • 0
    TI__Guru* 93485 points
    Ren,


    It is possible to change the configuration register at any time when using continuous conversion mode. However, the results may not be as you expect them.

    If the device is in continuous conversion mode, it is difficult to determine when the conversion starts and ends (especially because this is an I2C device without a DRDY line). When the device has the configuration register set with a new I2C write, the configuration register is written, but the change in the register does not go into effect until the conversion in progress is completed.

    For example, if the device is making a conversion to get AIN0-AIN1 with FSR=±2.048V. Then you change the channel to AIN2-AIN3 and set a new FSR to ±0.256. This change in channel an PGA does not occur until first conversion is complete. Because it's hard to determine when the conversion started and ended, there is an uncertainty when the new channel configuration occurred of about 1 data period. Then there is another data period that you'll need to wait for the conversion to complete.

    If you are changing from channel to channel, it's easier to go with single-shot conversion mode so that you know when the conversion started (with about a 10% uncertainty and a 20us delay to power up the device) because of the internal oscillator. Then you would wait 1/data period + 20us to read the device, set the configuration register, and then start a conversion.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 390 points
    Dear Wu,
    Thanks for your quick reply.
    Can I get the stable result if the chip works under single-shot mode? Regarding the same example, I set the FSR=±2.048V for AIN0/1, then wait 1/data period + 20us to read the result(here I can read several times for a stable result). And then wait for several to tens ms, I set the FSR=±0.256 for AIN2/3, also wait 1/data period + 20us to read the result. Repeat the first step again for channel AIN0/1...These results would be the expected results, right?

    BR
    Ren
  • 0 in reply to HongRen Chang
    TI__Guru* 93485 points
    Ren,


    You should get a stable result with single-shot mode. The ADC is single-cycle settling, so starting a conversion the data should be good with each conversion without having to wait for settling.

    Note that with ±10% variation in the internal oscillator you would need to wait (1/data period)*1.1 + 20us.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 390 points
    Dear Wu,
    Thanks.
    The wait time is OK for our application.

    BR
    Ren