Editing Analog-to-digital converter (section)

==== Measuring analog resistance or capacitance ====
If the analog value to measure is represented by a resistance or capacitance, then by including that element in an [[RC circuit]] (with other resistances or capacitances fixed) and measuring the time to charge the capacitance from a known starting voltage to another known ending voltage through the resistance from a known voltage supply, the value of the unknown resistance or capacitance can be determined using the capacitor charging equation:

<math display="block">V_\text{capacitor}(t) = V_\text{supply}\left(1 - e^{-\frac{t}{RC}}\right)</math>

and solving for the unknown resistance or capacitance using those starting and ending datapoints. This is similar but contrasts to the Wilkinson ADC which measures an unknown voltage with a known resistance and capacitance, by instead measuring an unknown resistance or capacitance with a known voltage.

For example, the positive (and/or negative) pulse width from a [[555 timer IC#Modes|555 Timer IC in monostable or astable mode]] represents the time it takes to charge (and/or discharge) its capacitor from {{Frac|1|3}}&nbsp;''V''<sub>supply</sub> to {{Frac|2|3}}&nbsp;''V''<sub>supply</sub>. By sending this pulse into a microcontroller with an accurate clock, the duration of the pulse can be measured and converted using the capacitor charging equation to produce the value of the unknown resistance or capacitance.

Larger resistances and capacitances will take a longer time to measure than smaller one. And the accuracy is limited by the accuracy of the microcontroller clock and the amount of time available to measure the value, which potentially might even change during measurement or be affected by external [[Parasitic element (electrical networks)|parasitics]].