Analog input (ADC) - input resistance

[michap]

Can you please give us a value for the input resistance of ADC0 .. ADC3 ?

I have "defined" about 47kOhm - it seems for me very low value for an ADC input - is this true?
(value wil be needed for voltage divider calculation - for voltages >3V ...)

Thanks
Michael

[PoltoS]

Looking in the spec gives even less: Rin = 7kOhm, Carray = 29pF. Let me re-check it with dev team

[michap]

Found out the "problem":
(my voltage divider for testing is 100kOhm+100kOhm)

- Setting PIN 3...6 to pinmode INPUT - they will be in a high-impedance state - all is fine.
- Make an analogRead() the right value will be given back.
- after analogRead the PIN of the ADC is in pinmode OUTPUT
-> that's why the voltage will be changed at the reference point (input).

Changing back pinmode in code to INPUT - all will be ok again.

Is this by design - that pinmode will be after analogRead set to OUTPUT ?

Michael

[Rogga10000]

Hello,
Have you figured out the appropriate values ​​of the voltage divider?

I need to convert 5V to about 3.3V for analogRead on A2 and A3. I planning to use two resistors, 330K and 180K. At 5V, I get 3.23 V over 330K to earth, then I have a little margin.

I'll get the right value in the controller using the straight-line equation.

Grateful for your comments

/Rogga

[michap]

Hi,

you should not have higher voltage as VCC at analog input - this is about 3.05V.
So 3.0V is a good max value.

If you calculate the resistors (including the rolerance range) and check the voltage before with a good multimeter I think it will work fine. If possible you can add a capacitor...
With 100k + 100k I see no difference at multimeter.

Michael

[Rogga10000]

Thanks Michael!
I will measure the voltage of a control circuit that I want to impact as little as possible. My input voltage is <5V. I might choose 200K and 300K. I can not understand the capacitor in a voltage divider?

Thanks for your help

Rogga

[michap]

can not understand the capacitor in a voltage divider?

Only to reduce the noise - for a high Ohm input.

Michael

[Rogga10000]

OK, should I, in my case, connect the capacitor in parallel with the 300K?

/ Rogga

[michap]

yes, parallel to ADC input, 100nF should be ok.

Michael