• State
  • Locked Locked
  • Replies 4 replies
  • Subscribers 78 subscribers
  • Views 2167 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

MSP430F5529 ADC required input current range

tml
Expert 1140 points
Other Parts Discussed in Thread: MSP430F5529

Hi,

I'm searching the MSP430F5529 datasheet (http://www.ti.com/lit/ds/symlink/msp430f5529.pdf) trying to find the information on how much current MSP430's ADC module need to be applied to analog input to be probed correctly. This is to properly calculate the voltage divider. 
Input impedance of the analog input would allow me to calculate the typical current range (Ia = Uin / input impedance, where 0 < Uin < Vcc ) but cannot find it either.

How can I obtain this info from the datasheet?

Best Regards,

tml

  • 0
    Guru 46710 points

    tml said:
    How can I obtain this info from the datasheet?

    You would want to read 5xx series Users Guide (SLAU208), ADC chapter "Sample Timing Considerations". There's formula to determine required voltage source impedance for given sample timing

  • 0 in reply to Ilmars
    Expert 1140 points

    Ilmars said:
    How can I obtain this info from the datasheet?

    You would want to read 5xx series Users Guide (SLAU208), ADC chapter "Sample Timing Considerations". There's formula to determine required voltage source impedance for given sample timing[/quote]

    This is not really what I am looking for, or I cannot interpret it the way that I need :)

    These formulas state for minimum sampling times and not the minimum input signal current that is needed to be provided for analog input.

  • 0 in reply to tml
    Guru 46710 points

    tml said:
    These formulas state for minimum sampling times and not the minimum input signal current that is needed to be provided for analog input.

    ADC input is RC circuit which cannot be considered as constant current sink so asking to specify current of ADC input is... unproductive.

    What's your problem you need to solve? Consider that the higher signal source impedance, the slower shall be sampling which in result impact sample rate.

  • 0 in reply to tml
    Guru 227245 points

    The ADC needs a certain charge (in As) rather than a current. The higher the source impedance and the smaller the maximum current, the more time you need in the sampling stage to fill the sampling capacitor.

    There is a hint for the required minimum current: the minimum ADC clock speed tells you that for 13 clock cycles of this sped, the stored charge will dissipate to more than ½ LSB. This can be used to calculate the maximum source impedance to have the charge capacitor ever filled: the minimum input current needs to be larger than the leakage current.

**Attention** This is a public forum