[Mike Copioli]

QUOTE=Dave K.;567119]A more traditional R-2R, multiplying DAC coupled with an external op-amp and offset voltage would be another way to accomplish the same thing at about the same cost and component count.

In the end, I chose to use a 10bit, voltage output, quad DAC. In particular, an Analog Devices AD5314. I chose this part more for size and immediate availability, than for reasons of cost. The higher resolution allows for the 500mV offset to be removed in software without loosing the ability to hit each of the 8 bit A/D converter 'bins' as seen by the OI.[/quote]


Dave, using an ADC is the best way to interface to the O/I. The Rev2 Chicklet will use an ADC. We realized the short commings of using the digpot when we got our first batch of pots in the lower end of the spectrum. Although we anticipated it, we thought the tolerances were on the conservative side. We developed a way to test the pots prior to assembly. We sorted them out into seperate groups based on there A to B resistance. This was tedious and not very cost effective due to the fact that we needed to purchase about 33% more pots than we actually needed in order to get enough that were within our spec. As far as elaborating on the voltage divider. It is very simple to overcome the tolerance issues. The 10k pots are actually a little better than the 100k. They vary from 8k to 12k. The object is to keep the offset resistor value within 10% of the pots AB resistance. One way to do this is to use another 10k pot as the offset resistor. That way you can adjust the offset value in software. The offset resistor is not just for allowing full range, actually it's primary purpose is to prevent the input of the O/I from going to ground. If this condition is allowed the O/I will read 127. Another way to calibrate with software is to use a second digpot to adjust the input voltage to the voltage divider pot. The idea is to make it so the center is 127. That is more important than getting full range. Most digpots have a hardware/software shutdown mode. This puts the wiper to center. This is very usefull. But only if the center is 127 on the O/I. If for some reason your device is not connecting to your controller you need a safe state for the interface to go to. This is an effective way to interface to the O/I. But I would still suggest the 10 bit ADC.