On Fri, Apr 15, 2011 at 6:55 PM, Bob Blick wrote: > That's a simplified diagram, not a real schematic. > > Your high side amp could easily have one percent base inaccuracy, plus > temperature and common mode error. > > Typically one would not do high side sensing. If I did, it would be > because I did the math on it and found it to be worth it. It usually > isn't. A good monolithic diff amp is not cheap and making one that is > good still takes a decent opamp and some very good resistors. > > In your application I'd sense on the low side. And since you don't need > low saturation voltage, you could even triple-darlington for your active > load. Or use a FET. > Alright, I'll do a Darlington pair then, or even triple. I'll simulate it with LTSpice first. But I don't understand why the high side sensing is difficult. Isn't it the same idea? Just measure the voltage drop across the resistor? I'll just sample some nice diff amp from LT or something. With the low side sensing, I can just use a good opamp? --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .