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ADS127L11: Modulator saturation

Fredda
Intellectual 625 points
Part Number: ADS127L11

Tool/software:

Hi,

The first sample after an ADC reset seems to always result in a modulator saturation (MOD_FLAG bit of the STATUS register). All other samples are fine. What can be the cause of this?

BR

/F

  • 0
    TI__Mastermind 46670 points

    Hello Fredda,

    I checked the first conversion result after reset on an EVM and the MOD_FLAG bit is not set, which is the expected correct operation. 

    The only other possible explanation is that the ADC input voltage has exceeded the full-scale input by 10% or more.  This could occur if an input amplifier output went all the way to supply rail (5V), as an example.  The EVM uses a 2.5V reference; forcing the input to 5V (this is within the absolute input voltage range of the ADC but exceeds the full scale 2.5V by 100%) results in the MOD_FLAG getting set.

    I would check the ADC inputs with a scope during reset to see if there are any voltage glitches.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Intellectual 625 points

    I have the input configured as:

    This will give:

    • VREF = 2.5 - 0 = 2.5V
    • Dynamic VIN range: -2.5 V to 2.5 V

    and:

    • VINMIN = 0 – 2.5 = -2.5 V
    • VINMAX = 5.2 – 2.5 = 2.7 V

    So in theory the input voltage will never exceed the full-scale input by more than 8%.

    Range is set to 1x and the precharge buffers in the ADC are active and thus the absolute input voltage of VAINP and VAINN is limited to:

    • Min: AVSS +0.1 = 0 + 0.1 = 100 mV
    • Max: AVDD1 - 0.1 = 5.2 - 0.1 = 5.1 V

    However, that may not have an impact on the situation.

    Just started investigating this so I will definitely check with a scope.

    Thanks.

    /F 

  • 0 in reply to Fredda
    TI__Mastermind 46670 points

    Hello Fredda,

    You are right at the edge of saturation if the TIA goes to the 5.2V rail.  Also, if the reference is not low impedance, there may be a settling time error of the reference due to a load change on the REFP pin immediately after reset.  I would also check the reference with a scope during reset.

    Regards,
    Keith