What is current draw of an ADC reading in micro?

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treez

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Hello,

We are reading a voltage rail with the on-chip ADC of the SAM3N00B microcontroller via a potential divider.
We are trying to assess what is the maximum allowable Ohmic value of the upper divider resistor.
The ADC reading will draw some current through this upper divider resistor, so we must not make this resistor too big, otherwise it will affect the potential divider voltage reading.

Do you know what is the current drawn by the ADC when an ADC reading is taken?


SAM3N00B MICROCONTROLLER DATASHEET:
http://www.atmel.com/Images/doc11011.pdf
 

Page 719 of data sheet:

The user can drive ADC input with impedance of Zsource up to:
• In 8-bit mode: TRACKTIM = 0.1 x Zsource + 470
• In 10-bit mode: TRACKTIM = 0.13 x Zsource + 589
with TRACKTIM (Track and Hold Time register) expressed in ns and Zsource expressed in
ohms.
Note: Csample and Ron are taken into account in the formulas
 
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i saw that, but with our 15K upper divider resistor, it means we can take an ADC reading every 2.5us........this cannot surely be correct......?
-if this is correct, then the ADC reading draws mere picoamps of current, and that cannot be the case.......some of the PICs demand no more than 10K upper divider resistor.

How can the 2.5us value be correct?
 

Table 35-34. Analog Inputs
Parameter Min Typ Max Units
Input Voltage Range 0 VADVREF
Input Leakage Current ±1 μA
Input Capacitance 12 14 pF

With an input leakage current of 1uA, there will be a 0.015 volt drop across the 15K resistor.
 
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