38.3.7 Selecting the Analog-to-Digital Conversion Clock Source and Prescaler
The ADC module can use the internal Fast RC (FRC) oscillator output, system clock (SYSCLK), reference clock (REFCLK3) or peripheral bus clock (PBCLK) as the conversion clock source (TQ). See ADCCON3 register from Related Links.
When the ADCSEL[1:0] bits (ADCCON2[31:30]) = ‘01’, the internal FRC oscillator is used as the ADC clock source. When using the internal FRC oscillator, the ADC module can continue to function in Sleep and Idle modes.
For correct analog-to-digital conversions, the conversion clock limits must not be exceeded. Clock frequencies from 1 MHz to 28 MHz are supported by the ADC module.
The maximum rate that analog-to-digital conversions may be completed by the ADC module (effective conversion throughput) is 2 Msps. However, the maximum rate that a single input can be converted is dependent on the sampling time requirements. In addition, the sampling time depends on the output impedance of the analog signal source. For more information on sampling time, see ADC Sampling Requirements from Related Links.
The input clock source for the ADC is selected using the ADCSEL[1:0] bits (ADCCON3[31:30]). The input clock is further divided by the control clock divider CONCLKDIV[5:0] bits (ADCCON3[29:24]). The output clock is called the “ADC control clock” with a time period of TQ.
The ADC control clock is divided by the ADCDIV[6:0] bits (ADCxTIME[22:16]). This acts as the clock source for the respective dedicated ADC modules with a time period of TADX.
The ADC control clock is divided before it is used for the shared ADC by the ADCDIV[6:0] bits (ADCCON2[6:0]). The time period for this clock is denoted as TAD7.
