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TMS320F28069: Analog input max current

Max. Sonnaillon
Prodigy 215 points
Part Number: TMS320F28069

Hello,

I have clamping diodes to protect the analog inputs (ADC), but I cannot find exactly how much voltage/current they will take.

I know that they "shouldn't" have more than VDDA and less than GND, but my clamping diodes will allow a little more than than (400mV). I added a resistor in series (215) so the current would be limited to something like +/-2mA.

The only spec I find in the datasheet is in the Absolute Maximum Ratings, note (3) Continuous clamp current per pin is ± 2mA.

If that limit applies to the analog inputs, I would be right at the limit, but that's a worst case that assumes that the internal clamping of the ADC has no voltage drop. In reality I may be much better than that.

I need a reliable design so I want to be sure I'm understanding the real limit.

Could somebody tell me a little more about the internal circuits and how much these inputs can take?

Thanks,

Max

The part number is: TMS320F28069

  • 0
    TI__Guru 62975 points
    Max,

    The +/-2mA applies to the ADC pins as well -- this value is meant to prevent permanent damage to the internal ESD-protection diodes that are placed on each pin. In the scenario where the ADC input protection diodes are activated, the extra energy is redirected to the VDDA/VSSA supplies -- the accuracy of ADC conversions may be affected while the supplies are absorbing the energy.

    You should be ok as long as your system is designed to operate within the VSSA to VDDA range, and the current-limiting components are only there for unforeseen, spurious events.

    Do you have bandwidth requirements for the ADC inputs? The series resistor will act in combination with the internal ADC S/H cap as an RC filter. It is not very significant, but you should be aware of the effects.

    -Tommy
  • 0 in reply to tlee
    Prodigy 215 points

    Tommy,

    Thanks for your answer.

    I'm clamping the opamp signals to 0 and VDDA with very-low Vdrop schottky diodes in a current product. The issue is that these diodes are becoming hard to find and the closest I find have a bigger voltage drop.

    Are the ESD diodes normal PN diodes with ~0.7V drop? If that's the case then I should be good, because my clamping diodes allow VDD+0.4V and VSS-0.4V worst case (and then the 215ohm in series that would limit the current).

    Regarding the series resistor, I have a little RC (215ohm+220pF) right before the ADC input to intentionally limit the bandwidth.

    Regards,

    Max

  • 0 in reply to Max. Sonnaillon
    TI__Guru 62975 points
    Max,

    I think that the diodes do have a knee around 0.7V.

    -Tommy