When measuring current across the shunt, as can be seen on this slide it is necessary to break the circuit, where the X is below the drain. A very low resistance resistor, probably 1 Ω, is installed across the break. The voltage drop across the shunt is measured and then the current is calculated. This presents a low inductance which is good. It does have some disadvantages though; it requires differential measurement, using two probes and a differential amplifier for example. Also, as mentioned it is necessary to break the circuit, and that may not always be convenient. Shunt resistors can provide the method with the highest accuracy for measuring current. Being a resistor, the transfer equation for the shunt is Ohm’s law: Vout = Iunkown X RShunt. Coaxial shunts, which must be used in the grounded leg of the circuit, can provide calibration laboratory precision. In fact, they are used to calibrate current probes. Having very low reactance, coaxial shunts can measure current frequency components from DC to several tens or hundreds of MHz. A shunt resistor can accurately measure currents with large crest factors (ratio of peak to RMS). The shunt is the only current transducer which is electrically connected to the circuit. A differential amplifier is needed to convert the voltage signal to a ground referenced voltage which can be measured by the oscilloscope. Like many current transducers, shunts require that a conductor in the circuit be broken. In many cases, the biggest drawback to using a shunt to measure current is the amount of impedance it adds to the circuit. The user needs to trade off sensitivity for insertion impedance (and power rating) when selecting the shunt value. Occasionally an engineer will try to eliminate the problems resulting from insertion impedance by using an impedance already present in the circuit, such as a run on a circuit board or a bus bar. By driving a known current through the conductor, the voltage across two points can be measured and the resistance determined. This will work for DC and very low frequency current measurements. At higher frequencies, the inductance of the conductor adds a reactive element which increases the impedance, resulting in a measurement error. A shunt will have a maximum average (RMS) current limit which is set by its power dissipation capability. It may also have a peak current specification which is usually determined by the maximum voltage it can withstand.