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MSP430FR5959: ADC input impedance / max ADC input current

Part Number: MSP430FR5959

Hi team,

 

I am having a hard time finding the input impedance for the ADC on the MSP430FR5959 after a customer had asked. The only hint I can find is in the electrical specifications, ADC timing parameters on page 45:

http://www.ti.com/lit/ds/symlink/msp430fr5959.pdf

 

There is T-sample says that Ri = 4K. Is this the ADC series input resistance?

 

They were looking to confirm the max ADC input current?

 

Thanks in advance,

Billy

  • Hi Billy,

    Based on the table from the datasheet, the highlighted values represent the range of input impedances for the ADC MUX across temperature and voltage.

    This means is there is not a fixed resistance value. It will vary with temperature and voltage.  Your customer will have to take this range into account when calculating their power budget.

  • That is not the input resistance, that is the series resistance of the MUX. The on resistance of the ADC is much higher than this!
  • Hi Dennis,

    My customer also pushed back asking to double check this answer.

    Also asking about that 10/15pf Ci shown in the same table, is this capacitance the sample/hold capacitance or is this the total capacitance of the input?

    Thanks for your help to confirm this,
    Billy
  • Yes, excellent point Keith!

    This is true, however, what is tested in manufacturing is the input resistance and capacitance of the MUX.  ADC input is not, therefore there is no specification for the datasheet.

    I assume your customer is interested in the input resistance because they want to determine the correct sample and hold period.

    As described in SLAA624A, it is the input resistance and capacitance of MUX channels, along with impedance of the source that is driving that pin that determines the appropriate sample and hold time.  There is also information in the device's user guide, ADC chapter, 34.2.6.3.

    If they want the input resistance for some other reason, let me know.  I would be curious to know why.

  • He might also have a relatively high source resistance and wants to know what the voltage drop might be.
  • "They were looking to confirm the max ADC input current"

    The problem is that the ADC has a complex and loosely specified input characteristic.


    In my experience the SH capacitor is precharged to 1/2 ADC ref voltage at the beginning of the sampling period.
    Therefore any input current is a function of input voltage, sampling frequency and leakage current.

    Peter

  • Hi Billy,

    I spoke with one of our ADC designers for MSP430. He explained that during the "sampling" phase, the ADC input not connected to the input impedance of the MUX or external voltage source. During the "hold" phase, that input network is disconnected and the ADC input is then connected holding capacitor and associated circuit. Because of this, any voltage source applied to an analog pin is not directly connected to, or affected by the input of the ADC, only the MUX and input holding circuit.

    Hopefully this satisfies your customers inquiry.
  • Thanks for the help Dennis, and double checking with design.

    I appreciate it!

    -Billy

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