ADC Frequency

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Hello. I'm not sure this post is in the right forum ... I use ATMega8-16 rated at 16 MHz and I'm surprises to see that ADC frequency is not conform to spec. Error is probably of my own ... Frequency is theorically 16 MHz / (2^^n) with in normal running 13 clocks per data numerized. In fact, according to n = 1 ... 7,  I found on scope (and also in simulation with Avr Studio) that total time used for one digitalization is 2 ^^ n multiply by

26, 20, 17, 15, 14, 14, 14 with n = 1 ... 7. Where is the error ? 

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Note the adc has a upper clock spec of around 153kHz so the conversion rate is only around 15kHz

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Kartman wrote:
Note the adc has a upper clock spec of around 153kHz
Well technically it's 200kHz (to maintain 10 bits).

 

Problem is that 16MHz is a bad choice for ADC usage. 16MHz / 64 is 250kHz. Which is too fast while if you use /128 that only gives 125kHz.

 

A better choice of crystal might be something like 12.8MHz because 12.8MHz / 64 = 200kHz which will get the maximum 15.38kHz conversion rate. So a conversion time of 65us.

 

I have no idea why one would choose the ancient old dinosaur "mega8" if ADC performance is important. The Xmega range are far better suited for ADC use.

Last Edited: Wed. Feb 7, 2018 - 10:17 AM
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Many thanks for replies.