Accurately measuring the inductance with a conventional LCR meter with no DC bias is
straightforward but becomes much more difficult if attempting to do this using an external
power supply to generate the DC Bias current. The difficulty here is stopping any AC current
flowing from the LCR through the DC current source.
There are four main factors causing an AC current leak through the DC Bias supply
1. Output capacitance of the source
A conventional bench PSU usually contains an output capacitor to smooth its supply.
This would sink the AC current from the LCR supply and result in a measurement error. See
2. Capacitance between the DC source and ground.
See (2) right
3. Nonlinear response of the transistors used in constant current PSUs
See diagram right, showing collector current to voltage.
In an ideal transistor the curves are flat, but in reality, there is a characteristic slope.
As the LCR AC signal is added to the DC voltage drop across the UUT, the collector current
varies slightly, causing error.
4. Change of transistor response over DC current.
As well as being nonlinear, this error current also changes over varying DC current output.
See diagram (Right)
The response curve at 1 A is different to 20 A, and so the current leak
is larger than
This in turn means that a conventional LCR compensation performed at 0 DC Amps will not be
valid at 5, 10, 20... DC Amps.