AVR-Chat

--- In AVR-Chat@yahoogroups.com, "Dave Mucha" <dave_mucha@y...> wrote:

> OK, so if you wanted to monitor
> an analog value, an external ADC 
> would be best if the speed of
> the samples were high.

If you need a sample rate higher than the AVR can provide, the only 
way is to use an external converter. Remember (from earlier posts in 
this thread) there is a speed/accuracy tradeoff available in the AVR. 
If you select the higher ADC speed, you loose some accuracy.

> for the successive approximation,
> would the accuracy be higher if 
> the process were relatively steady
> and the change between readings 
> were rather small ?

No. Successive approximation wipes the slate clean and starts again 
for every conversion.

> Temperature is a very slow moving
> process so one would not expect to 
> see a process swing of more than a
> few percent over a few minutes.  
> So, would that mean internal ADC
> readings would be pretty accurate ?

This is an ideal application of the internal ADC. Use the lower 
sample rate, clocking at 200kHz, for best accuracy. If you are using 
a thermistor, you don't even need an external reference voltage. Just 
work "ratiometrically" by connecting the fixed resistor / thermistor 
divider to whatever you use as a reference. Just don't use VCC 
without cleaning it up, ideally with an inductor and capacitor.

Graham.