Quote:
Originally Posted by marcan
Thanks everyone for the comments. This certainly clarifies a lot of the details about the OI.
Right now, I'm considering two options: a 100K digipot in rheostat mode (the "hack") or the 10+1K divider. I'll probably end up trying both, and see how they behave.
On the subject of DACs and op-amps (or op-amps with the digipots), how would you deal with the range limitations of the op-amp? Even rail-to-rail op-amps aren't truly rail-to-rail (they just get close). Would you be able to use the full input range using an op-amp?
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Well one op-amp you could consider would be something like the Burr-Brown (now TI) OPA363 series. Worst case, with a 10k load, which is more than the OI effectively presents, it will get within 20mV (worst case) of V+ and typically within 10mV. That's one I spotted, and I'm sure there are others that would work OK.
As I think I mentioned earlier, you'd need something capable of handling the dynamic loading presented by the multiplexing that the OI uses to scan the channels. There are of course capacitive loading effects as the analog switch selects the channel, and then the load of the current sink.
In the case of the OPA363, it is specified driving a 10k load, which is an order of magnitude greater than the load presented by the OI. Other op-amps that come this close to the rails, are specified at even higher resistances, such as 100k, and may not handle the switched load without introducing some additional error.
As I wrote earlier, I chose not to go down this road for different reasons, and you've certainly highlighted one additional concern that further justifies not pursuing this approach.
Quote:
Originally Posted by marcan
As for the +5V Aux supply, I'm pretty sure it'll meet my needs, based on Dave's info and my own testing, as far as the power required goes. A more important aspect is how the voltage drop due to the current limitation affects the analog inputs when the current is higher and the voltage drops. I'll do some testing once the components arrive.
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If you can make good use of your microprocessor's sleep mode, and provide some local capacitance buffered by a series resistor, I think its probable that one could succeed in not loading the +5V aux supply enough for the OI to notice.
For my own interface, I'm faced with powering a 12-13mA device in addition to my interface electronics, so that will already pull the supply down by a few A/D counts. In a practical sense, it doesn't really matter if we loose some resolution in joystick position. Heck we already toss out more bits in the deadband region just to avoid robo-creep when the joystick is a couple knat hairs shy of being truely centered.