Setting up the AtoD Sampling

Look up the following in the device datasheet:

Tad = ADC Clock period ADC Clock Period

Sample Time

Conversion Time (value not needed to setup but this tells you how long it will take to convert)

ADCS – ADC Conversion Clock Select bits

ADC_CONV_CLK_#Tcy 

This value sets Tad:

Tpb (Peripheral bus speed) x Value Used = Tad

SAMC – Auto-Sample Time bits

ADC_SAMPLE_TIME_# (A/D Auto Sample Time x Tad)

This value sets the sample time

Example – PIC32MX120F032B

Tad = ADC Clock period ADC Clock Period = 65nS min

Conversion Time  = 12Tad typ

Sample Time = 1Tad min

With a 40MHz osc and 40MHz peripheral bus these values would provide the required times:

ADC_CONV_CLK_3 provides Tad = 75nS

ADC_SAMPLE_TIME_3 provides 3Tad so more than the 1Tad minimum but enuring our input impedance etc doesn't affect the sampling capacitor being fully charged each time.