John said:
Lets start at the beginning.
What is the voltage range of your A/D?
I can set it to whatever is best. It's a 10b ADC and gets maximum
accuracy with a 5V reference voltage, but I could also set it to be
lower.
What voltages are available?
5V. Though I can add other voltages if necessary - the size/cost of
this board is not an issue.
How often do you need to measure the resistance?
I'd like to get a full reading (which means reading in from 4 different
pots) every tenth of a second or so. Every hundredth would be nice, but
not necessary. Certainly nothing faster than that though.
Do you need maximum resolution across the resistance range, or could
you use a nonlinear method and linearize in software (with some loss
in resolution)?
I'm really hoping to avoid a lookup table or some other software
linearization. I like everything to be done in hardware, not software.
But worst case scenario that is possible.
Keep in mind that most joystick resistance measurements are made with
oscillators that charge a capacitor to some voltage with current
through the resistance connected to a fixed voltage, and then dump the
charge with a switch, and measure the period of the oscillation this
process produces as the resistance changes. A 555 timer can be
configured to perform the oscillation function and connect to a
digital input that includes an interrupt. The result is nonlinear,
but a lookup table (often updated with a calibration test) converts
the period to a fairly linear result.
I was reading up on that method of reading earlier today. For me
though, reading in the signal from the 555 would be a pain though, so
I'd rather use the ADC.
One thing I was just thinking about - I don't have to use the full
range of the ADC. I mean I don't need all 10b of accuracy. So if the
current source provided 100ua it would only need to operate properly
with a voltage drop between 2.5 and 5V across it, which I think is much
easier. With a 5V reference voltage to my ADC I'd have 9b of accuracy,
which is more than enough. 7-8b is really all I want.
Thanks,
-Mike