I want to measure the input voltage (range from 4V-10V) with AD converter on my PIC18F97J94. I need to use resistor divider to scale the voltage between 0V-3.3V to work with my ADC.

I used voltage calculator and came up with values below:

Vin = 10V (max voltage) R1 = 7.5k R1 = 3.7k Vout = 3.3V

Will this schematics work and how do I calculate capacitors value to filter the supply noise? Please find picture attached.

How do I make sure the impedance is within datasheet value? If Im right, maximum impedance on ADC would be 2.5kohm?

enter image description here


2 Answers 2


Optimism can be good but in this instance you need to be more realistic. The ADC input of your PIC may have a theoretical range of 0 to 3V3 but, in practise, this will be more like 50mV to 3.25 volts. Read the data sheet on the ADC and it will tell you what the zero and full-scale errors might be on any particular channel of the device. These errors are equivalent to the digital end-stops of the conversion process and as a rule of thumb I ensure that my biggest and smallest signals are within 50 mV of the specified front-page-of-the-datasheet range.

Other than that your calculation looks accurate enough but why not give a little bit more headroom on the 10V end of the signal? Also, you only need the capacitor to ground to cut-down on noise i.e. C1 is not needed.

  • 1
    \$\begingroup\$ C1 is not wanted. \$\endgroup\$
    – Autistic
    Nov 14, 2015 at 10:58

The impedance is less than 2.5K if the input is always connected to a low impedance source. If it is left open, the impedance is 3.7K looking out from the ADC input.

The -3dB cutoff freqency of the single pole filter is

\$f_c= \frac{ (R_1 || R_2) C_2}{2\pi}\$

So the input will be attenuated by about 30% at fc- you can calculate C2 from that. You don't need or want C1 generally.

As Andy points out, you should attenuate a bit more because your 3.3V reference will have a tolerance as will the resistors and the ADC will have a maximum error spec, and you should accommodate worst-case conditions for all simultaneously to be sure of being able to measure 10V without the ADC saturating. ~10% lower value for R2 might be reasonable, but you should run the numbers.


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