Lpc2000

Sutton Mehaffey wrote:

>Does anyone (or Philips Techs) know the max input current leakage of
>the ADC channel input pins? 
>
>The LPC2148 datasheet says that the input leakage current is max 4uA,
>but I am thinking that this MAY apply just to the digital signal pins.
>We are trying to use one of the ADC channels in conjunction with a
>very accurate voltage divider for determining battery voltage for a
>battery powered application.
>
>We need the actual SPECIFICATION max of the input leakage current
>(and the polarity if it can be depended upon) so as to be able to
>calculate a worst case error.
>  
>
Just some observations from the peanut gallery, FWIW.  I, too, have 
noticed, what appears to be, a definate avoidance of giving complete 
spec's regarding pin loading / source / sink current for the LPC2000 
processors.  I generally see DC specs which contain a lot more detail 
than that of the LPC2000s.  IMHO, this is deliberate?


If they are using an approximation technique for measuring the voltage, 
you may not have a consistant load presented by the ADC input.  Assuming 
that they are using a fixed resistance ladder with FET switches gating 
the samples within the ladder may not be the case.  Therefore, the act 
of performing a conversion may present you with a varying load?

I mean, if you are *that* sensitive to changes in load, then why not 
eliminate them (Philips) from the unknowns and use a unity gain buffer?  
Place a simple op amp buffer between your voltage divider and the input 
to the ADC.  Something with a FET input.  Done.  BUT, you still have the 
Common Mode spec of the op amp to contend with...

Otherwise, you may have to lower the "source" resistance (increase the 
current through the divider) such that it is 10..20 times the max load 
presented by the ADC input?  Look at it this way, if the voltage divider 
is drawing 1ma and the input to the ADC is @ 0.004ma (source vs load 
ratio of 250), how much shift in the voltage would you see if  the ADC 
input varied by 4ua (connect / disconnect)?  Having a load 1/250th of 
the source is pretty darn negligable!  Kirchoff's Law...

If you are *that* sensitive to the 4ua load, perhaps your units would 
need to be individually calibrated to match the processor's ADC input 
load, op amp common mode, etc..  With the degree of accuracy you 
suggest, this is probably the case in any event.

Regards,

TomW


-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
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