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ADC Conversion Clock Select bit setting for PIC16F87XA, 18F2x2, 18F4x microcontroller

There is a fact to mention about the resolution of the A/D converter. Since the converter converts the signals, it takes a certain time. The conversion time of the low resolution A/D takes less time than the conversion time of the high resolution A/D. You have to consider carefully : if you select an analog to digital converter with high resolution – system will not be as fast; but if you select an analog to digital converter with high-speed response – you will loose the resolution of the system.

Acquisition Time: When an specific channel is selected the voltage from that input channel is stored in an internal holding capacitor. It takes some time for the capacitor to get fully charged and become equal to the applied voltage. This time is called acquisition time. According to datasheet 16F87XA family: The minimum acquisition time is 19.75micro second. And 18F2X2/4X2 have 12.86micro Seconds(depends upon many things. See datasheet).Once acquisition time is over the input channel is disconnected from the source and the conversion begins.

ADC Clock: ADC Requires a clock source to do its conversion, this is called ADC Clock. The time period of the ADC Clock is called TAD. It is also the time required to generate 1 bit of conversion.

The ADC requires 12 TAD [16F87XA datasheet] to do a 10 bit conversion. It can be derived from the CPU clock (called TOSC) by dividing it by a suitable division.The source of the A/D conversion clock is software selected as a multiple of the clock frequency TOSC.For correct A/D conversions, the A/D conversion clock (TAD) must be selected to ensure a minimum TAD time of 1.6 micro second(according to datasheet).

Possible options for a Crystal are:
• 2 TOSC
• 4 TOSC
• 8 TOSC
• 16 TOSC
• 32 TOSC
• 64 TOSC

A 10MHz crystal is attached to a pic16F877A.

Our FOSC = 10MHz. Therefore our TOSC = 1/10MHz =100nS

16 TOSC = 16 x 100 nS = 1600nS = 1.6uS

which is same as the the minimum requirement.

See Use of Vref+ pin of PIC16F87xA ADC [applicable to all micro] at


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