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Using current shunt monitors

Posted: 17 Apr 2009     Print Version  Bookmark and Share

Keywords:current measurement  loop regulation  shunt resistor 

Many of today's applications need current measurement for power estimation, regulation purposes, or for diagnostics. This article focuses on circuits with analogue 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 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 per cent and the TCR (thermal coefficient of resistane) in parts per million (ppm). 20ppm/°C can be regarded as a good shunt, while 500ppm/°C would be deemed average.

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