Within the document “PSM17xx + IAI Operating Guide”, the connection and basic operation of PSM1700 or PSM1735 instruments when used in conjunction with the Impedance Analysis Interface is described.
Here, we discuss in more detail the subject of compensation and the importance of this process whenmaking precision impedance analysis.
Why is compensation required?
While it is natural for engineers to recognise the importance of making a good electrical connection when measuring the impedance of a component, it is quite common to underestimate the impact that
impedance of the connection system itself may have on the total measurement.
Many high performance impedance measurement instruments (including the PSM + IAI) reduce the effect of cable impedance by using 4 wire Kelvin connections. Here, two sets of leads are used to connect to the component under test and this enables measurement of the voltage drop across thecomponent under test to be made directly, without including the additional voltage associated with current flowing through impedance of the power leads. However, it must be remembered that while this technique offers an effective solution to DC measurement errors, any phase shift associated with the reactive impedance of the connection leads is not accounted for using this technique.
While the value of parasitic impedance associated with connection leads may often be small relative to a component being tested, connection leads that may be thought to have negligible impact on
measurements (for example coaxial Kelvin leads) can in easily introduce significant errors. This effect is particularly evident when measurements are made over a wide frequency range because it is much more likely that tests will be made at a frequency where the parasitic impedance of connection cables ,becomes a large proportion or sometimes even a greater absolute value than the impedance being measured.
What is the principal of compensation?
Compensation is a process of measuring and recording the impedance of connection cables when no test component is in place so that this value of ‘connection cable impedance error’ can be removed or‘offset’ from the total measured value when a component is being measured with the same cables.
The maximum errors associated with the impedance of connection cables are usually associated with particularly high or low impedance measurements. For example, inductance measurements at low frequency where the sample impedance will be low relative to the cables or capacitor measurements at low frequency where the high sample impedance will be influenced by cable capacitance.