>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>On Behalf Of PicDude
>Sent: 21 April 2007 01:12
>To: Microcontroller discussion list - Public.
>Subject: Re: [PIC] Current limiting resistor effects
>
>
>On Friday 20 April 2007 11:26, Michael Rigby-Jones wrote:
>> I think you may be over thinking this :D
>
>If only I had a penny for every time I was told this. :-)
>
>
>> 1) The datasheet recommends a maximum source impedance of 
>10k, you are 
>> significantly below this, even with the 1k series resistor 
>(worst case 
>> 1.27k). Providing your aquisition times are set correctly, 
>you should 
>> not experience any degradation in accuracy.
>
>Here, I was not so worried about exceeding that max 
>impedance, but how the series resistor would affect
>the ratio of the voltage divider. I guess what I'm 
> trying to verify is that Zad is large enough that adding 
>RL would make a very insignificant change in this ratio.
>I have since done some math and figured out that the max
>effect would be at the highest value of Rsen. Assuming 
> that the internal impedance of the A/D is just Rss 
>(7kohms) (and we know it's much more than that), then
>adding RL=1Kohm causes a 1.1% change in the ratio of the
>voltage divider.  Significant, but again I know that the
>internal impedance of the A/D circuit would me much more 
>than 7K.

If you look at the block diagram, Rss is in series with the sample/hold capacitor, so at DC the impedance is governed only by the leakage current which is quite small at +-500nA.  Your 1k input resistor would give a maximum error of 1000*500E-9 = 0.5mV which is 1/10th of an LSB with a 5volt reference.  

>From that you can see that a 10k input impedance would give an error of around 1LSB, which is why Microchip suggest to keep under this value to achieve the best accuracy.

Regards

Mike

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