I truly appreciate all the feedback and suggestions. What I've done is had my partner send a reply to the customer stating=20 that we can readily design something that has 0.1% accuracy over the=20 range of 100uA to 1A and anywhere from 0.25% to 0.1% accuracy over=20 the range of 10uA to 100uA (won't know until we try it). If the=20 customer can accept the reduction in accuracy, we might consider doing the = job. Russell - would you mind asking Ken if he is interested in the=20 project? If he is, we would just pass his contact info on to the=20 customer and let them deal directly with each other. I ask because its obvious that Ken has already done much of the=20 initial interfacing and making the leap from Voltage source to=20 Current source is not massively onerous, especially if the customer=20 is willing to relax their requirements down to the 10-bit range. Thanks again! dwayne At 08:50 PM 5/1/2013, RussellMc wrote: > > But: if I do have to design this beast, Does anyone have any favorite > > A/D and D/A convertors that are 14 bit accurate or better? I don't > > normally use parts that accurate and would prefer to have people whom > > I trust make suggestions rather than just starting to pour over > > data-books and find out that the parts that at first seem to make > > sense are now unobtainium. SPI preferred, I think, but willing to > > look at anything. > >Here is a very lightly edited comment from my friend Ken. >(I took out his description of a product of his). > >Note his very last paragraph re required DAC resolution. > >Russell: Need for a super DAC may be able to be relaxed if you have a >lower resolution DAC and can "nudge it up or down a bit" independently >and use the ADC to control what is happening. > > > > Russell > >____________ > >Linear Tech have pretty much all the IC's Dwayne should need - and=20 >a good SPICE >simulator so he can be pretty sure his design will work (and meet spec) be= fore >he buys or builds anything. > >We've just done a very high precision programmable voltage source with >programmable output resistance for an ATE system and pretty much=20 >used LT devices >exclusively - and the SPICE simulations we did were right on the money. > >We opted to use I2C-bus devices throughout for the programmable aspects. = That >meant the unit needed no uC and no firmware - just a USB-I2C bridge (easi= ly >done using FTDI's devices) with just a Windows DLL and app to do all the >control. That saved a lot of time, and also neatly solved the=20 >problem that the >unit (in fact each of the five sub-units it comprises) could be readily >galvanically isolated from ground and from each other (using I2C-bus isola= tors >from Analog Devices) - an essential requirement for the application. > >Achieving an accuracy of 0.01% would be a challenge, and would definitely >require that Dwayne know what he is doing - especially when it comes to P= CB >layout (1A flowing in the wrong place in a PCB can cause havoc for mV-leve= l >control and feedback signals). How much of a challenge depends on=20 >the external >environment (temperature range especially) as much as anything else,=20 >but proper >analog design techniques (to take account of opamp input bias currents etc= ..), >and proper component selection (with adequately low temperature coefficien= ts) >will also play an important part. The required closed-loop=20 >bandwidth will also >dictate the types of active components he will need to use (and the detail= ed >circuit topology), and he will have to watch out for nasty analog=20 >"gotchas" that >simulators don't ever pick up - things like thermal tails. > >Trim-on-test will also be essential if 0.01% precision is to be=20 >achieved - just >picking a 1-lsb accurate 14-bit DAC will definitely not get you there for = the >overall system (or anywhere near it). > >We have done analog designs to higher levels of precision than that - but= you >have to get creative - and you definitely have to know what you are doing= - >and have the time to re-spin prototype PCB's as it's very unlikely you'll = get >the layout exactly right the 1st time (it took three spins for our >..... thing .... - and that is working to an accuracy of 0.025%). > >I can't really comment too much on the circuit topology without=20 >knowing the full >spec but if he really needs bipolar compliance he should probably be=20 >looking at >a buffered Howland topology or derivations thereof. > >Note also that 10uA to 1A range mentioned implies a 10uA programming=20 >resolution >over 5 decades (a resolution of 1 part in 100k) - which would=20 >require a 17-bit >resolution DAC. That would significantly constrain the choice of DAC devi= ce. > > > Ken >-- >http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >View/change your membership options at >http://mailman.mit.edu/mailman/listinfo/piclist --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .