Many of today's applications require current measurement for power estimation, regulation purposes, or for diagnostics. This article focuses on circuits with analog outputs that are needed for loop regulation or for signal conditioning for ADCs.
In general there are two ways to measure currents - high side or low side, as shown in Figures 1 and 2. Low side current measurement seems to be the easier choice since the signal chain, in this case, is ground related. In practice, however, it might be impractical if the application circuitry cannot be raised from ground because ground integrity has to be obtained.
Both methods have one thing in common - a voltage difference, which is measured across a precision resistor or, a shunt resistor. Typically, these resistors have very low resistance (in the lower mOhms range), high accuracy and low thermal drift. It is important to know these figures in order to make an error budget calculation. A shunt is specified by its tolerance, for example, +/-1%, and the TCR (thermal coefficient of resistane) in ppm (parts per million). 20ppm/°C can be regarded as a very good shunt, while 500ppm/°C would be deemed average.
The following calculations give the TCR errors:
T = (85°C - 25°C) = 60°C
TCR Error (20ppm shunt) = 60°C * 20ppm/°C * (10exp-4)% = 0.12%
TCR Error (500ppm shunt) = 60°C * 500ppm/°C * (10exp-4)% = 3%
Because the shunt itself is expensive, the more accuracy and low drift that is required, so a compromise between the tolerance and TCR has to be found. In addition, errors in other parts of the signal conditioning circuitry need to be as low as possible. Both parameters form part of the error budget along with the offset and offset drift of the active components used and the errors caused by the resistors, which are often overlooked parts in the chain.
Error = Shunt + TCR + Offset OP + Drift OP + Gain Error R + CM Error R
