AVR-Chat

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.