I was testing arduino analog input accuracy for that i used a voltage devider with R1=18k and R2=3k ,the result was good (1bit error)
but when changed the resistor value to R1=R2=460k i get a big fluctuation around (10 bit).
I searched about that and i found that high impedance will make the internal capacitor(14pF) take more time to charge.
SO what is the solution with high impedance i realy need solution cause my preject circuit have output impednace > 100K.


High impedance:

LOW impedance:

 

Hello,

Have a look at this page:
https://electronics.stackexchange.com/questions/67171/input-impedance-of-arduino-uno-analog-pins

To avoid high impedances on the input of the ADC, you could use a buffer between the voltage divider and ADC input.

Bertus

 

A good buffer for this purpose is a low noise, low offset op amp (not a 741) connected as a gain of 1 follower.

Alternately can you slow down the sample rate to allow more time for the internal capacitance to charge?

 

thank you,
actually i tried buffer using LM358N and it was the same fluctuation ,i measured the output impedance of LM358N and it was about 1k .
the thing that i didn't understand in those fluctuation is :when the time of charge is long , the first N reading will be wrong ,after that the capacitor will charge and we will get the right value ?? but the fluctuations (values) is going up and down why? is there a discharge of the capa ??

 

hi HA,
I did tests sometime ago for a PIC's ADC acquisition time.
How often are you reading the ADC input.?

Note: Zsrc 1R thru 12500R in 2500R steps.

A PIC d/s recommends no greater that 10K source impedance.
E