Agilent Technologies 4294A Specifications download pdf (Page 77)

When we measure capacitors, we have to consider these parasitics. Impedance measurement

instruments measure capacitance in either the series mode (Cs-D, Cs-Rs) or in the parallel mode

(Cp-D, Cp-Rp). The displayed capacitance value, Cs or Cp, is not always equal to the real capaci-

tance value C due to the presence of parasitic components. For example, when the capacitor circuit

shown in Figure 5-2 is measured using the Cs-Rs mode, the displayed capacitance value Cs is

expressed using the complicated equation shown in Figure 5-3. Cs is equal to C only when the value

of Rp is sufficiently high (1/Rp<<1) and the reactance of L is negligible (ωL<<1/ωC). Generally, the

effects of L are seen in the higher frequency region where L is not negligible. However, Rp can be

disregarded in many cases. For high-value capacitors, the reactance of the paralleled C value is

much lower than Rp. For low-value capacitors, the value of Rp itself is very high. Therefore, most

capacitors can be represented as shown in Figure 5-4. Figure 5-5 (a) and (b) shows the typical

impedance (|Z|∠θ) characteristics and Cs-D characteristics for ceramic capacitors. You can recog-

nize the existence of L from the resonance point seen in the higher frequency region.

Figure 5-3. Effects of parasitics in actual capacitance measurement

Figure 5-4. Practical capacitor equivalent circuit

Figure 5-5. Typical capacitor frequency response

5-3

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Agilent Technologies 4294A Specifications Page 77