# Low TC resistors for current measurement

I have a back-burner project to take an old bench power supply and retro-fit it with two 4 digit LED displays for current and voltage. The power supply is 3A/30V, so four digits means 3000 count and 12-bit conversions (4096 = 2^12).

I've been trying to design an analogue current measuring block that just has a pair of current shunt resistors with a relay and some range detection circuitry etc. But I've bumped into a weird question about which resistors to use.

Ignoring the range selection, let's assume that I choose resistor and current sense amplifier so that 3A => 3V. Then, in order to get 1mA accuracy, I have a maximum error of 333ppm. Round that down to 200ppm to give the later ADC some headroom. Now, it seems that a reasonable temperature range is maybe 10°C - 30°C, which basically corresponds to "rooms I can function in". The 15°C range from the midpoint then says that, assuming I null out errors correctly, I can have a maximum TC of 13.3ppm/°C.

Even ignoring any errors from the current sense amplifier, I don't think that one can buy such resistors! (Or not easily). Looking hard at Farnell/Digikey/Mouser, I can find a few 20ppm/°C resistors but that's it.

So the question: Have I messed up my calculation horribly? If not, how do people build multimeters...?

• Farnell sell 0.1% resistors that are 10ppm. I bought some yesterday. The common ones are 5ppm, 10ppm, 15ppm and 25ppm. Jul 12 '13 at 10:40
• Ah, looking again I see that they do but the SMD ones start at 10 ohm and the through hole ones at 5 ohm, which explains why I didn't spot them. Of couse a 5 ohm resistor is not really suitable for a 3A current shunt... Jul 12 '13 at 10:49
• Have you checked the total error of your ADC? I suspect it is much larger than the resistor error. Jul 12 '13 at 10:51
• Welcome to the wonderful world of high-precision measurement of anything. Anyways, if you really need that high precision, you should look into adding a temperature sensor, and having calibration tables across the temperature range over which you expect to use the device. Jul 12 '13 at 11:22
• Joe: No, the "cunning plan" was to build the analogue section first (so I could insert it and carry on using the bench supply!). Connor: Hmm, that seems likely. I think I'll probably add a temperature sensor to the board with analogue out and when I design the ADC/uC/display part I can decide what to do with the information! Jul 12 '13 at 12:08