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MSP432P401R: Input current required for ADC14 in single-ended mode?

Scott Whitney
Expert 1120 points

Part Number: MSP432P401R

Hi folks,

I'm trying to get some information to resolve a discussion with my electrical engineering counterpart.

We have a battery voltage that is monitored through one of the ADC14 inputs (running in single-ended mode).  However, to "prevent the battery from being drained by measuring it", our EE has put in a circuit that essentially disconnects the battery from the ADC14 whenever we are not actively measuring it (i.e., performing ADC conversions).  Further, he has asked me to put in a delay (currently 1 minute) between bursts of ADC conversions for the battery.  After a minute, I connect the battery signal to the ADC14 input, then do a burst of measurements and average them.  Currently I'm taking 500 samples, but that number can easily be changed.

My question is:  How much input current does the ADC14 input actually require?  I'm not convinced that the extra circuit nor the complexity of waiting for 1 minute before taking another burst of readings actually saves us any power, but it does slow down our ability to "see" the actually battery voltage in "real time".  I'd rather update the actual battery voltage after every burst of averaged samples, but can't simply ignore his concern about draining the battery.

I apologize if this is obvious, but I could not find the required ADC14 input current from my perusal of the MSP432 data sheet, nor the Family Reference Guide.

I sincerely appreciate any help you can provide, and if other information is needed, please let me know.

Thanks in advance!

Scott

  • 0
    TI__Guru 59221 points
    Scott,
    The current consumed by the ADC input is that required to charge the sample-and-hold capacitor in the ADC. This is typically 12pF (Table 5-26 in the datasheet). If the battery is 3V and the sample capacitor is 12pF then you would consume about 36pC (not considering other parasitics). You will find that charge is not thrown away and that subsequent measurements of the same channel do not ‘consume’ as much current – but you still need to make sure that the sample-hold time is sufficient to get to the present voltage and not some intermediate voltage (based upon RC) between the present and previous voltages.

    To your point, I cannot find any information about the ADC impedance/leakage when the IO is not being sampled. The digital IO specification for leakage is 20nA (max) and the Roff impedance of the comparator (analog input) is on the order of 50Mohms typical (comparator, table 5-34). I will ask for clarification, but I would not expect the value to be very different from the comparator. I will ask that this be updated in the datasheet.

    Let me know if you have additional questions.

    Regards,
    Chris
  • 0 in reply to Chris Sterzik
    Expert 1120 points

    Thank you, Chris,

    I understand how you arrived at 36pC, assuming the sample capacitor is charged up to steady state, since Q = V * C.  However, that doesn't quite tell me what the current is, since it will be dependent on the input impedance (or resistance) of the ADC input before the capacitor.

    Also, the same capacitor is used to hold the voltage for each channel that is in use, so if we are swapping between multiple ADC14 channels, it's a bit hard to predict how much charge would already be on the preceding channel.

    If you can find any more info about the input current or input impedance for the ADC14, I'd appreciate it.  I guess the key for my EE counterpart would be the input impedance and maybe the equivalent circuit, from which we could calculate the input current.

    Many thanks again,

    Scott

  • 0 in reply to Scott Whitney
    TI__Guru 59221 points

    Scott,

    The impedance and equivalent circuit can be found in the datasheet and the TRM, respectively.  In the datasheet you can also find in the notes an example where the sample hold time is calculated based upon example values for Cpext and Cpint.  I will continue to work to get you more information about the off state.  Additionally, you always have the option of turning the Analog input back into a high-impedance digital input to ensure that you get the leakage provided in the datasheet.

    http://www.ti.com/lit/ds/symlink/msp432p401r.pdf#page=64

    http://www.ti.com/lit/ug/slau356f/slau356f.pdf#page=670

    Regards,

    Chris

  • 0 in reply to Chris Sterzik
    Expert 1120 points

    Hi Chris,

    Thanks very much for your excellent and detailed reply.  I have provided this info to my EE counterpart, and hopefully this is enough information for him to estimate the current needed to drive the ADC14 input.  I have a strong suspicion that most of the potential current drain on the battery will be a result of a FET-controlled resistive divider network that is used to scale the battery voltage down into the 0.0 to +3.0VDC range that is allowed by our external voltage reference.

    If you get further details, please pass them along.  In the meantime, please accept my sincere thanks for your outstanding support on this topic.  I hope that it helps others, too!

    Best regards,

    Scott

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