3.1 Voltage Divider

The acquisition time is a fixed 2 μs on the PIC16F15244 microcontroller family. For best results, the capacitor must charge to 0.5 Least Significant Bits (LSb) within the 2 μs time. 1 LSb is defined as:

V_{L S B} = \frac{V_{R E F}}{2^{n}}

Where V_REF is the voltage reference and n is the number of bits in the ADC result.

Note: n = 10 on the PIC16F15244 family.

Additionally, the impedance into the ADC is recommended to be 10 kΩ or less. For a voltage divider, the equivalent external impedance seen by the ADC is the top and bottom resistors in parallel:

R_{E Q} = \frac{1}{\frac{1}{R_{T O P}} + \frac{1}{R_{B O T T O M}}}

Note: This does not consider the microcontroller’s internal resistances. See the ADC chapter of the device data sheet for more information.

If it is not possible to reduce the impedance of the resistor ladder, adding capacitance (≤ 33 nF) across the bottom resistor may help by providing a low-impedance source of charge for the ADC at the cost of response time. This is discussed in detail in Maximizing the Signal: Tips and Tricks to Properly Acquiring Analog Signals (DS00004225A).

Tip: Another solution for high voltages is to switch to another measurement topology (see the Isolated Sensing chapter).