What is the Basic Accuracy of an Impedance Analyzer?

This blog explains the meaning of basic accuracy when referring to impedance analyzers.

The accuracy of an impedance analyzer is a critical parameter that allows the user to know how close the measured impedance will be to the true impedance of the device or sample under test. Since the accuracy of an impedance analyzer varies with both frequency and impedance, a parameter called basic accuracy is used to specify the best accuracy possible with the instrument and also to enable a comparison of various instruments.

Basic accuracy is expressed as a percentage and refers to the best possible accuracy of an impedance measurement when the user is free to select both frequency and impedance values. Furthermore, it refers to the accuracy of impedance measured at the front panel of the instrument, unless explicitly stated otherwise. Adding a fixture or cables between the front panel and the device under test would reduce the accuracy unless compensated for by an additional calibration procedure (such as a short-load compensation).

Basic accuracy varies with instrument, and manufacturers are free to quote a basic accuracy that presents their instrument in the best light. The stated basic accuracy can be at a single frequency/impedance combination, or it can cover a wide area of frequency/impedance. The latter allows for a wider range of components to be characterized as a function of frequency while maintaining the basic accuracy.

.

Figure 1: Reactance chart with the accuracy of the MFIA Impedance analyzer overlaid. Basic accuracy is 0.05% as highlighted by the orange lozenge. The white area where the basic accuracy is valid is also labeled.

The reactance chart becomes an accuracy chart

Reactance charts are the traditional way to nicely visualize how capacitance, inductance vary with frequency. The reactance chart is also the perfect way to display the accuracy of an impedance analyzer as it can be overlaid to show the various accuracies the user can expect for a given frequency and impedance (see figure 1; MFIA reactance chart). Once the accuracy has been overlaid, the reactance chart is then also known as an accuracy chart. The highest accuracy region (or basic accuracy region) can be found in the center of the chart (white region in figure 1). Outside of this basic accuracy region, the accuracy tails off and can be read from the accuracy chart for a given frequency and impedance combination.

 

Wider basic accuracy offers more flexibility

The reactance chart of the MFIA in figure 1 offers a clear overview of the accuracy which can be expected for a given frequency and impedance. The basic accuracy region of the MFIA is wide, allowing flexibility for characterization of components and materials with impedance up to 1 MOhm from 1 mHz to 500 kHz with a basic accuracy of 0.05%.