AVR-Chat

On Apr 26, 2006, at 12:03 PM, Peter Harrison 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.
>
The 7usec is not out of line, considering that the sample/hold is  
sampling until 1.5 sample clocks after asserting ADSC (which is, at  
best, 7.5usec given that 200KHz is the max clock for 10 bit  
conversions).

> 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 you considered "free-running" mode?

> Have I got it right and this is just how it works, or have I missed
> something? Is the switch resistance really that variable?

-- 
Ned Konz
MetaMagix embedded consulting
MetaMagix@gmail.com