Lpc2000

Hi Sutton,

Just one small problem, 4uA on 100K is 400mV, not 40mV, which when
reflected up the divider likely means an error of 2V.

Mike Anton


> -----Original Message-----
> From: lpc2000@yahoogroups.com 
> [mailto:lpc2000@yahoogroups.com]On Behalf
> Of Sutton Mehaffey
> Sent: Sunday, March 05, 2006 4:24 PM
> To: lpc2000@yahoogroups.com
> Subject: [lpc2000] Re: ADC input leakage current
> 
> 
> Tom,
> 
> I expect to put a tantalum cap of perhaps 1uf or so on the voltage
> divider junction and sample very infrequently, perhaps once per
> second. Perhaps it is "no worries" about what the analog input leakage
> current  is  in such circumstances.  If it is 4ua then the offset on a
> 100k  source  Z  would be only about 40mv and reflected up the divider
> for  a  15  volt  max reading would equal a measurement error of about
> 200mv  which  is NOT inclusive of resistor tolerance errors.  However,
> if  the leakage were actually specified and I knew the polarity of the
> analog  input  leakage  current (and it did not change direction under
> any  circumstances)  I  would  be able to design for a typical setting
> with knowledge of the worst case parameters.
> 
> Without  such  information,  It is a "shot in the dark" as to what the
> unit-to-unit error spread in production will be.  I do not like to use
> pots  or  select-at-test  resistors.  I would much rather have a fully
> characterized part.
> 
> As to the option of a unity gain amp in front of the input.. Added
> cost, plus the added power drain make that option unattractive unless
> I cannot get it to work otherwise.
> 
> Sutton
> 
> 
> 
> --- In lpc2000@yahoogroups.com, Tom Walsh <tom@...> wrote:
> >
> > 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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