>a) Average Voltage to Power circuit. 6V-24V >b) Average Current in Power circuit. Average current during the last 100mS. >c) Voltage of Control circuit. Control is a 3-6V source >d) Voltage of Monitor circuit. Monitor is a 4.5-5.5V source >All data points will be sampled at 10-30 Hz (selected by customer at >runtime, with no hardware changes permitted). 8-bit data is sufficient at >this stage, but they may want 10-12 bit in the future. >The Monitor circuit is supposed to be small (as small as possible) and >needs to run at least 1 hour on batteries (NiCd rechargeable or Li coin >cells). Of course it's mobile, and will use an RS-232 interface to >transfer data to a laptop, either during operation or at the end of a >testing period. >How do I hook these separate analog signals up without sharing the ground >with the Power circuit? Depends on what your design constraints are, obviously - how much additional cost is acceptable, etc... Do you intend to use the internal PIC A/D converters or an external part? If you are using an external A/D, is that set in stone or flexible? This post intregued me because I've spent the last four years working on EV telemetry and power systems, and we often face similar problems in terms of noise, coupling, floating voltages, and you name it... Personally, I'd recommend you use true differential delta-sigma A/D converters - and no, before everyone jumps on me, these aren't necessarily out of the price range on any reasonable project - Analog's AD7705 is a relatively inexpensive (~$4 in small quantity, I think) dual differential 16 bit D-S A/D that has an SPI interface and works very nicely under extremely noisy conditions. Because of the integration-deintegration performed in the D-S process, noise and random jitter in the data is reduced significantly - to the point that one of these was used on the output of a large (500W) switching power supply to measure current through a shunt- and had consistent and accurate average currents out to 10 to 12 bits without a problem. And with a main ground bus that can have ~2-3 volts of ripple on it due to currents and inductive kickback, this is no small feat! Power consumption on these is very reasonable for battery-based applications - I believe they can be standby'd to 10uA or so... If that isn't an option or you would prefer to use some other A/D converter, just put an instrumentation amp on the front end - these usually have two drawbacks: they're expensive if you go for really high-quality versions, and otherwise their common mode rejection isn't the greatest at higher frequencies. Since you're only looking for averages, though, you could R-C integrate off all the high frequencies and leave the in-amp to only convert the differential input signal you're trying to monitor down to a "ground" referenced voltage for your circuit to read. I'm personally a fan of Burr Brown INA2128s, but these are costly if you actually have to use them in a production circuit. Analog also makes some very nice ones at lower cost, I believe. For your application, they also have one other drawback I just thought of - they're usually current hogs. On the order of ~20mA. Just my two bits for the day... ND Holmes -------------------------------------------------------------------------- Nathan D. Holmes maverick@drgw.net, ndholmes@iastate.edu 122 Shepard #3 Box 328 Gilbert, IA 50105 Iowa State University - EE http://www.drgw.net/~maverick PH: 515-663-9368 -------------------------------------------------------------------------