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Originally Posted by ConKbot of Doom
Ahhh sorry, I didn't make it clear, I meant to use that circuit so you can use joysticks with non-100k pots in them (there are issues with using pots that aren't 100k, correct?)
Though, now that you mention the architecture of the internal circuitry, you could just get away with an op-amp with enough output current so that the pull-down resistor doesn't affect the voltage to the ADC.
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Actually, the OI is looking for a resistance, rather than a voltage, at its inputs. The conversion is made using a simple RC circuit. Capacitance being a constant inside the OI, the time (and this is what's actually read by the OI) it will take for this circuit to reach a certain threshold level (charge/discharge) will be proportional to the resistance that can be changed with joystick movement.
That's why the 100k value is so important, to achieve full scale (see Jack Jones' above post for an example).
That's the concept used, but I'm not sure how it really is implemented inside the OI. Someone willing to void their warranty and snap some pictures?
For reference, the PC game port implementation can be found
here and an implementation using an Atmel microprocessor (which I assume is similar to what is done inside the OI) can be found
here , courtesy of Larry Barello, mentor of FRC team 492.