Microcontroller ADC reading?

[eem2am]

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

 

[BobK]

It should be in the spec sheet somewhere. But anyway, use a capacitor across the lower leg of the divider (about 100n) this will make the impedance very low.

Bob

 

[(*steve*)]

It should be in the spec sheet somewhere. But anyway, use a capacitor across the lower leg of the divider (about 100n) this will make the impedance very low.

Only for transient events. Doesn't help much for constant current draw.

A simple rule of thumb is to have 10 times the current flowing through the voltage divider than the current you wish to draw. That gives you an output voltage within about 10% of what you calculate. Is 10% ok?

If the current is constant (or nearly constant), you can factor it in to your calculations.

 

[KrisBlueNZ]

I agree with BobK. If the ADC uses a switched capacitor architecture, the actual DC current flowing into the input will be fairly low. (But as Bob said, check the specs.) What's important is that the input has a low impedance at AC, so that the spikes of current caused by the capacitor switching will not make it jump around. As long as the voltage isn't changing quickly, I would use a 100nF capacitor across it. But don't use a MLCC! They're not too bad for decoupling supply rails, but when performance is important, they're crap.

 

[BobK]

Yes, the capacitor is there to provide the burst of current needed when the ADC is sampling. This is standard practice with microcontroller ADCs. There is effecitively no current draw at other times.

Bob

 

[gorgon]

It's still on page 719 in the datasheet, and you need to make some calculations to define the max resistance you may use.