# maximum recommended impedance - pic microcontroller

Discussion in 'Homework Help' started by ecka333, Oct 11, 2011.

1. ### ecka333 Thread Starter Active Member

Oct 1, 2009
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I need to measure voltage up to 30V with PIC18F1320 microcontroller. I am going to use divider with to resistors R1 and R2. So maximum input voltage to microcontroller wil be 5V. But microcontroller datasheet says that: The maximum recommended impedance for analog sources is 2.5 kOhm. So how to calculate resistor chain impedance; maybe it will be 2,5k o maybe sum of R1 and R2, that is 15kOhm?

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2. ### SgtWookie Expert

Jul 17, 2007
22,182
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Use a 10nF or larger cap to analog ground at the ADC input sample pin. That will provide a low impedance source for your ADC to sample. Make certain that you wait for a sufficient amount of time between samples so that the input cap will be very close to representing what the voltage would be without the cap.

Another way would be to use a low-offset RRIO opamp to provide a low impedance input from the high-impedance junction of the voltage divider.

3. ### ecka333 Thread Starter Active Member

Oct 1, 2009
76
0
Ok, thank you. If i will use opamp, can i directly connect opamp output to microcontroller input?

4. ### crutschow Expert

Mar 14, 2008
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3,234
From the PIC's input point-of-view (for AC), the two resistors are in parallel. So the impedance it sees with your resistor divider values is thus (2.5k*12.5K)/(2.5k+12.5k) = 2.08k ohms.

Last edited: Oct 13, 2011