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Publication Date: 2017-Jan-27
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Current probes are widely used to measure electrical currents propagating through various parts of an electrical circuit.  The type of current probe used to carry out a current measurement can be generally based on the nature of the current to be measured. When the current is primarily an AC current, an AC current probe having an inductive coupling element (such as a coil of wire) can be brought close to an electrical conductor through which the AC current is flowing and a tiny portion of the current that is picked up by the inductive coupling element is converted into a voltage in the AC current probe.  The voltage can be amplified in an amplifier inside the AC current probe before being coupled (via a connector of the AC current probe) into a measurement instrument such as a voltmeter or an oscilloscope.  On the other hand, when the current to be measured is either a purely DC current or is an AC current having a DC component, a current probe having a Hall effect sensor can be used.  The Hall effect sensor allows measurement of both DC as well as AC currents.

Unfortunately, many conventional current probes such as the ones referred to above, can suffer from certain limitations as a result of various factors such as non-ideal response characteristics of current sensing elements (inductive coupling element and Hall effect sensor, for example) and non-ideal behavior of electronic circuitry incorporated into the current probe.  These limitations, which can have an adverse impact upon performance parameters such as measurement sensitivity, measurement resolution, signal distortion, and component non-linearity, can sometimes prevent these types of current probes from being used to measure very small currents and/or can introduce measurement errors when measuring certain types of currents. 

Consequently, a different type of current probe can be used when a higher level of measurement accuracy is desired and at least some of the limitations associated with current probes incorporating an inductive coupling element or a Hall effect sensor, can be addressed.  Figure 1 below shows a current measuring set-up in which such a current probe is used to measure both AC currents as well as DC currents that can propagate through an element in a device-under-test (DUT). 

In the example shown in Figure 1, the element is a resistor that is connected to a high-voltage DC supply or an AC signal having a very large amplitude, but in other cases, the element can be any type of impedance coupled to a voltage source.  The resistor is labeled as a “DUT element” in Figure 1 for the sake of convenience.  The current propagating through the DUT element is arranged to also propagate through a “sense” resistor (RSENSE) that is connected in series with the DUT element.  The sense resistor is selected to have a relatively low resistance value in comparison to the DUT element, such as 1 mΩ for exam...