Specifications

5-6

5.1.3 Causes of negative D problem

When measuring the dissipation factor (D) of a low loss capacitor, the impedance measuring instru-

ment may sometimes display a negative D value despite the fact that the real dissipation factor must

be a positive value. A negative D measurement value arises from a measurement error for a small

resistance component of the measured impedance. In this section, we discuss the causes of negative

D and the methods for minimizing the measurement errors that lead to the negative D problem. Five

typical causes of negative D problem are:

• Instrument inaccuracy

• Contact resistance in the 4TP or 5T configuration

• Improper short compensation

• Improper cable length correction

• Complicated residuals

Note: The following discussion also applies to a negative Q problem because the Q factor is the

reciprocal of D.

D measurement error due to instrument inaccuracy

If a DUT has a low D value compared with the D measurement accuracy (allowable D measurement

error) of the instrument, a measured dissipation factor may become a negative value. Figure 5-8

shows how the D measurement accuracy of instrument impacts a negative D value. For example,

when D measurement accuracy (of instrument A) is ±0.001 for a low-loss capacitor that has a dissi-

pation factor of 0.0008, the impedance measurement error is represented by a dotted circle on the

vector plane as shown in Figure 5-8. The shaded area of the dotted circle exists on the left side of

reactance axis (X axis.) This shaded area represents the negative D area in which the resistance

component of the measured impedance is a negative value. The allowable D value range is from

–0.0002 to 0.0018. In this case, there is possibility that a negative D value is displayed. If the D mea-

surement accuracy (of instrument B) is ±0.0005, the measured impedance vector is within the solid

circle as shown in Figure 5-8. The negative D value is not displayed because the allowable D value

range is from 0.0003 to 0.0013. Accordingly, an impedance measuring instrument with the best pos-

sible accuracy is required for avoiding negative D display in low dissipation factor measurements.

Figure 5-8. Negative D measurement value due to measurement inaccuracy

Example:

D = 0.0008

(at specific measurement conditions)

Instrument D accuracy Possible readout

0.001 –0.0002 to 0.0018

0.0005 0.0003 to 0.0013

Negative D

0.001 accuracy

0.0005 accuracy

Impedance

vector