AVR-Chat

My recollection is (without checking data sheet - BAD!) is
that the S/H switch is triggered about 1.5 ADC clocks after
the ADC starts. I would guess that this could play a
significant role in your problem. The data sheet also
inferred, if I remember (again, unchecked) that the S/H
switch was closed for one half an ADC clock. There is an
ADC timing diagram for many of the devices that show you
this basic operation.

Jim


On Wed, 26 Apr 2006 20:03:34 +0100
 Peter Harrison <peter.harrison@helicron.net> wrote:
> Hi
> 
> I have an optical sensor. It triggers an emitter and
> reads a response 
> from a photodiode and amplifier. after firing the
> emitter, I wait until 
> the amplifier has had time to reach a stable output - say
> 5us - then I 
> initiate an ADC conversion.
> 
> Now, If I turn off the emitter too soon, the ADC result
> is decreased so 
> my question is: How long should I keep the signal present
> at the ADC 
> input to allow the internal sample and hold capacitor to
> charge?
> 
> According to the (ATMEGA32) datasheet, the S/H capacitor
> has a value of 
> 14pF but the series resistance of the multiplexer is
> shown as 
> 1..100kOhm. This seems a bit of a large range. Time
> constants could be 
> up to 1.4us requiring that I wait as long as 7us or so to
> charge the 
> capacitor. The source impedance is an op-amp output so it
> should not 
> have any effect to speak of. The sheet also describes the
> sampling time 
> as negligible. Seems like a new, optimistic definition of
> the word to me.
> 
> My code seems to need to wait at least 1us before turning
> off the 
> emitter. As I need to perform 5k conversions per second,
> I could do 
> without wasting all that time in the timer interrupt
> waiting for the 
> signal to settle. I already have to arrange that on a
> timer tick, I 
> collect the previous result and then fire off another
> conversion.
> 
> Have I got it right and this is just how it works, or
> have I missed 
> something? Is the switch resistance really that variable?
> 
> Pete Harrison.
> 
> 
>  
> Yahoo! Groups Links
> 
> 
> 
>  
> 
> 

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