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TMS320F28379D: ADC analog inputs current sinking

Jozef Šedo
Prodigy 200 points
Part Number: TMS320F28379D

Hi,

I would like to ask, how many microamperes is current sinking on ADC input pins of TMS320F28379D?

When I measured sinking current, some analog input pins (e.g. ADCINA3, ADCINB3) have zero sink current and some ones (e.g. ADCINA1, ADCINB1) have sink current of tens of microampers.
This current sinking affects previous circuits and then measurement is inaccurate.

There is the sink current on the ADC input pin even when ADC module is disabled (ADC power down and ADC module clock is gated-off).

Regards,

Jozef

  • 0
    TI__Guru 60865 points

    Hi Jozef,

    The static leakage is given in the datasheet in the electrical characteristics table:

    Note that the pins in question do indeed have DACs multiplexed on them, hence the additional leakage current seen.

    The other component to consider, while not strictly leakage, is that you will see an average effective in-rush or out-rush current potentially as high as Ch*sample rate.  This is due to the internal ADC S+H capacitor, Ch, equalizing with the external driver at the beginning of each sample.  Each sample pulls (or pushes) some charge from the pin and when repeated periodically this can appear like a sample-rate dependent leakage current:

  • 0 in reply to Devin Cottier
    Prodigy 200 points

    Hi Devin,

    This information was exactly what I needed. Thank you.

    I have to use a Fully-Differential Amplifier to drive ADC in Differential Mode.

    Regards,

    Jozef

  • 0 in reply to Jozef Šedo
    TI__Guru 60865 points

    Hi Jozef,

    For driving fully differential ADC, you need:

    • A signal source that is naturally fully differential with compatible output levels (some sensors are this way)
    • Or, conversion to fully differential signal using a fully differential amplifier.  Topology may differ depending on whether you are doing
      • Differential-to-differential conversion
      • Or, single-ended to differential conversion
    • Or, conversion to fully differential using two (or more) single-ended op-amps.  There are a variety of topologies for this as well, although using a fully differential amplifier will have the best performance.  

    Note also that F2838x family devices are very similar to F2837x family devices, but they also support using 16-bit ADC in single-ended mode (naturally the performance is still better in 16B differential mode).