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ADC sampling rate PIC18F2550

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expertengr

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I just read Chapter 21 of the data sheet of PIC18F2550 but I have some questions. From the Table 21-1 it look like that TAD (A/D conversion time per bit) depends on the A/D conversion clock which can be set by ADCS2:ADCS0.

https://ww1.microchip.com/downloads/en/DeviceDoc/39632e.pdf

Let's suppose FOSC is constant, consider the following two settings

1- Writing 000 to ADCS2:ADCS0 make A/D conversion clock FOSC/2 and according to Table 21-1 TAD is 2 TOSC.
2- Writing 100 to ADCS2:ADCS0 make A/D conversion clock FOSC/4 and according to Table 21-1 TAD is 4 TOSC.

The sampling rate in Setting 2 is two times slower then setting 1 because the bit conversion time TAD in Setting 2 is 4 TOSC which is two times slower than bit conversion time TAD in Setting 1 as TAD is 2 TOSC. This is understandable. This means if we change FOSC it can also change TAD. Is there any minimum value for TAD ?

As the minimum acquisition time is 2.45 u sec according to Equation 21-3. The design parameter here is acquisition time which can be set by ACQT2:ACQT0 in terms of TAD and it should be more then 2.45 u sec. Is that correct ? For faster conversion we should have TAD as low as possible but greater then which value, any minimum value of TAD for stability ? and acquisition time little above 2.45 u sec. Please let me know if this is correct ?

In Table 21-1 what Maximum Device Frequency represents ? As writing 110 to ADCS2:ADCS0 configure 64 TOSC which represents slower A/D bit clock TAD and more acquisition time but why the Maximum Device Frequency is higher i.e. 48 MHz ?
 

Hi,

Is there any minimum value for TAD ?
I assume there is a "max Fosc" specification in the datasheet.

I even assume there is a "max sampling rate" specification in the datasheet.

Klaus
 

Minimum Tad - see Table 28-29, Parameter #130.
As for the maximum device frequency, look at Figure 21-1 and note the 'Chold' on the right-hand side. You need time for that to charge to the input voltage and stabilise. The longer the sampling period (which is measured in terms of the Fosc) the faster the Fosc can be to give the same settling time.
Susan
 

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