Chris McSweeny wrote:
> First of all I can use high value resistors - however according to
> datasheets the max input impedance is 10k, which would suggest that is
> the maximum value of the lower resistance. Given I only need to
> measure every second or so though and the voltage isn't varying
> rapidly, can I use much higher value resistors and rely on not needing
> to charge the input capacitor up very much each time, and so ignore
> the datasheet?

You may be correct that this would reduce the error induced by 
charging/discharging the internal sampling capacitance. Another question worth 
asking is does this capacitance charge/discharge each time an A/D conversion 
takes place or only each time the ADC input selection is changed?

In such situations I have no problems with using very much higher resistances 
(100's of K or several M's) but also add external capacitance directly to the ADC 
pin, say 100nF. Overall response time is slow due to the RC constant, but is not a 
problem as you're just measuring battery voltage which doesn't change quickly. The 
AC impedance to the ADC is now low, and reduces the effect of the sampling 
capacitance on the reading. The high resistances will then mostly be a factor to be 
considered in conjunction with input bias current and the offset error it may produce.

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Brent Brown, Electronic Design Solutions
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Hamilton 3200, New Zealand
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