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ADS52J90: Auto offset mode: how is computed the dc value value substracted

Pierre Lieutaud
Prodigy 230 points
Part Number: ADS52J90


Tool/software:

Hello

I'm willing to use the auto offset removal feature (chap 8.3.5.1.2) .

The DS doesn't specify how the DC is computed.

Is it computed in background all the time and might change over the time?

Or is it computed only once, after the TX_TRIG or the MANUAL_START has occured, using the number of samples specified in reg AUTO_OFFSET_REMOVAL_ACC_CYCLES?

In that case the DC value will stay the same for all the time of operation.

Regard,

Pierre

  • 0
    TI__Expert 7725 points

    There are two options 

    1) If you know the offset already you can write that offset

    2) If you dont know offset , once tx_trig is given it will accumulate based on start sample (TXTrig or OFFSET_REMOVAL_START_MANUAL) and number of samples to accumulate (AUTO_OFFSET_REMOVAL_ACC_CYCLES). This happens on every tx_trig.

    Does this answer your query ?

  • 0 in reply to Sachin H K
    Prodigy 230 points

    Hi,

    Method 1) give good results but requires a manual action. Kind of calibration.

    Method 2) does not give good result in my application. This is the reason of my question. In my application, the system only generate one TX_TRIG for all the system operation time. It is generated at startup time.

    So if I clearly understand your answer, in my application, the DC offset will only be computed once and probably not at the best moment. Right?

  • 0 in reply to Pierre Lieutaud
    TI__Expert 7725 points

    Yes correct . You can readout the compute offset value and check whether it matches your offset expectation . 

    How do you know offset is from device or input ?Are you ac coupling the input ? 

  • 0 in reply to Sachin H K
    Prodigy 230 points

    Yes, the ADC is AC coupled.
    The worst offset value I have manually measured is 38 LSB offset. I guess , that matches the datasheet data of +/-7mV.

    I am looking at the OFFSET_REMOVAL_START_MANUAL of the auto offset correction mode. Maybe that's a better way to get the auto mode doing the job in my application.

  • 0 in reply to Pierre Lieutaud
    TI__Expert 7725 points

    Ok. You can use that .Hopefully with this you can get the required offset .