[sdiy] Pots read by microcontroller and sourcing question

[Jean-Pierre Desrochers]

Hi Jack,

This depends on the type of uProcessor you use.
But 'most' of them will have high impedance inputs
on their specific pins while used in digital inputs or A/D modes.
For example looking at Microchip PIC16F88 specs
http://ww1.microchip.com/downloads/en/DeviceDoc/30487c.pdf
Table 18-13 states:

Recommended Impedance of Analog Voltage Source: 2 kohms MAX.
Note 4: Maximum allowed impedance for analog voltage source is 10 kΩ.
         This requires higher acquisition time.

So if you use a 100kB pot, the worst case (max impedance)
of it will be at center position which is
2 x 50k's in parallel = 25k source impedance
which (in the case of PIC16F88) more than the
specified 10k max source value.
So my advice would be to use mostly 10-20kB pots
with a 0.1 uf cap at the wiper pin for A/D readings.

Best,
J-Pierre





Le 2015-02-02 08:04, Jack Jackson a écrit :
> Hi all, 
> 
> Is there any difference in operation when using a 10Kb potentiometer
> into an ADC instead of a 100Kb?
> 
> Also any source out there for 9mm ALPs pots with a centre detent? also
> 9mm ALPS with 4 detents?
> 
> Best Regards,
> Jack 		 	   		
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