> From: Jim Ruxton > > Hi, > I'm trying to read current by using a low value shunt resistor. The > resistor value is .01 ohms. I want to be able to read .05 amp steps > from 0 to 10 Amps using a PIC 16c73 with a VREF of 2.5 volts. I'm > planning to use an OP AMP with a gain of 20 so that 1 Amp will yield .2 > volts into the PIC. Can anyone suggest a good single supply OP AMP that > I can run off of 5 volts that would have a fairly low input offset? Do I > need an instrumentation OP AMP? Are these available in a single package > or do I have to make them out of 3 OP AMPS? Any suggestions would be > greatly appreciated. While I'm asking for info. does anyone know where I > can buy .01 ohm 1 watt resistors. Thanks! > Jim > One of the CMOS type opamps would be the go, since the output swings rail-to-rail, and handles ground referenced inputs. I haven't kept up with the latest instrumentation amps, but I don't think one is justified. If needed it would be better to construct your own using a quad opamp pack, since I think IOA's are fairly pricey for a one-off. Regarding resistors: at this low level it is probably better to make your own using constantan wire. This is typically about 6 ohms per metre for 28SWG, and is solderable. Use a number of strands in parallel. Look at Analog Devices, Linear Technology, Burr Brown, Nat Semi etc. for info on op amps. Farnell and RS Components are UK-based companies which probably have a presence in Canada. You can get small qty of an extensive range of components from them (Farnell probably better range of opamps) for a premium price, however this is better than shopping around unless you get lucky. They also supply pretty cool current shunts if you don't want to make your own. Having said this, it may be better for you to get a proper linear hall effect current sensor. These are only $20 or so, and give the advantage of negligible voltage drop in the measured circuit, 150KHz bandwidth, and total isolation of the electronics. They take a +- 15V supply. You pass the conductor whose current is to be measured through the hole in the middle of the sensor. You can wrap several turns through it to increase sensitivity (and reduce range). Isn't it annoying when you ask a simple question then some smart-arse tells you that you're doing things the hard way in the first place? Regards, SJH Canberra, Australia