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#### Myauk

- Jan 1, 1970

- 0

As I am working on Slope ADC configuration as described by AN863

(http://ww1.microchip.com/downloads/en/AppNotes/00863a.pdf)

Microchip application note AN929 http://ww1.microchip.com/downloads/en/AppNotes/00929a.pdf

, page 3 figure 5 shows Ramp Rate Method where by using a single

Capacitor and a known ref: resistor, the resistance of any unknown

resistor Rx can be measured. The method goes as follows:

1. Connect the supply voltage through the ref: resistor Rref and let

the capacitor chraged until it reaches threshold voltage Vth, the

charge time T1 is recorded.

2. Discharge the capacitor and then connect the supply voltage again

through unknown resistor Rx, and let the capacitor charged until it

reaches threshould voltage Vth, charge time T2 is then recorded.

And theoractically, Rx = (T2/T1) * Rref

I used C = 0.47 uF, Rref = 200 ohms, Vth = 2.02V, Vsupply = 3.08V (I

use PIC18F45k20 port RA1 and RA2 to supply voltage to RC ckt, and RA0

as the comparator input).

Results are not as expected.

When Rref = 200 ohms with C=0.47 uF , time T1 is measured to be 138 us

to reach 2.02V. Again, when Rx = 360,540 ohms, with the same capacitor

0.47uF, time T2 is measured to be 200,292 us. The time Ratios T2/T1

does not match the time ratio in this experiment.

I understand that the source characteristics of the I/O port in the

MCU affects the charge time behaviour but I am not sure how exactly it

relates and I do not know how to model the circuit so as for me to

calculate predictable results.

Any suggestions?

Regards