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As capacitance and frequency increase, the impedance of the capacitor decreases. To further illustrate this point, the following example mathematically shows how the impedance of the capacitor affects the actual voltage seen across the capacitor.
The capacitor (C3216Y5V1A106Z) is tested using both the HP 4263B cap meter and the HP4278A cap meter. Substituting a frequency of 1kHz and capacitance of 10 µF into Formula 1 yields a capacitor impedance of approximately 16 Ω.
When 1.0 Vrms is applied from the test equipment to the capacitor, the voltage is divided between the meter impedance and the capacitor impedance. The impedance of the HP4263B remains at 100 Ω(Fig.1) but the impedance of the HP4278A changes to 1.5 Ω for the calculated capacitor impedance (Fig.2). The result is that for the HP4263B the majority of the applied voltage is dropped across the cap meter impedance, while the HP4278A enables the capacitor to receive most of the voltage. The outcome is that the HP4263B will show an indicated value that is lower than the true value
Setting the OSC voltage of a test meter to 1.0Vrms does not guarantee that the full-applied voltage is delivered across the capacitor. It is not surprising to find that the voltage across the capacitor is around 10% of the set value. The following graph shows how a lower VAC effects the measured capacitance (Fig.3).
Fig. 5: HP4263B Impedance.
Fig. 6: HP4278A Impedance.
Fig. 7:Capacitance vs. AC Voltage
The following pictures illustrate the difference between ALC on and off. With the ALC function off, the actual voltage across the capacitor is approximately 10% of the 1.0Vrms set voltage. With the ALC on, the voltage across the capacitor is almost 100% of the set voltage. The HP4284A has voltage and current level monitors, but the actual voltage also can be verified using a voltmeter.
Automatic level COntrol (ALC) Function of HP4284A Capacitance Meter