Digital to OI Analog

[Dave K.]

Originally Posted by Mike Copioli

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.

I agree that having to sort parts is not a cost effective way to go about manufacturing.

Having been at this a while, I've had designs that despite our best efforts to design to worst case tolerances, the manufacturer's let us down and provided parts well outside of their worst case specifications... and these are large, well respected companies that had the pipelines full of parts before we flagged the problem for them.

When it comes to semiconductors, design to worst case, and if possible, even a bit beyond that.


One of the reasons the AD5314 was a good fit was that its own power on reset set's the output to zero. I'm sure there are other, less costly, devices that would also work well and also provide a power on reset to zero... but this was the one that was available quickly.

Thanks,