Lpc2000

--- In lpc2000@yahoogroups.com, "Sutton Mehaffey" <sutton@...> wrote:
>
> 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

Other manufacturers (notably Microchip) specify that the source
impedance of analog signals must be below 10k ohms.  I certainly
wouldn't be driving the converter with 100k.

Richard