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For accurate high cap measurements, the recommended meters are the HP4284A, HP4278A, or equivalent.
The HP4284A with the high power option (option 001) increases the rms source current level from 10 mArms to 100 mArms. With a higher current range, the meter has more power to maintain the rms voltage level applied to the capacitor. The HP4284A also has Voltage and Current Level Monitors which show the actual voltage and current levels seen at the capacitor. One drawback from using the HP4284A is extended measurement time. The HP4284A uses a feedback configuration with the level monitor to maintain the full rms levels.
Measurement time for the HP4284A can be calculated from the following formula:
[(meas_timeshort) + (～ 115msec.)] x n (Formula 2)
where n = 2 to 6.
If measurement speed is a greater concern than accuracy, the HP4278A is recommended. This meter does not have the ALC function or the Voltage and Current Level Monitors but AC Voltage can still be verified by using a voltmeter. The HP4278A is an earlier version of the HP4284A and is sometimes referred to as a “production “ meter due to the faster measurement speed.
The important point is that the meter must be able to supply the correct voltage to the capacitor. This can be accomplished either by switching to a low output impedance range in order to compensate for the low impedance of high capacitance ceramic capacitors, or by having a feature which automatically maintains the test level at the device under test.
Further, the HP4284 has a feature known as Automatic Level Control (ALC). By activating this function, the set voltage level can be held constant at the DUT. When measuring high capacitance ceramic capacitors with this instrument, make sure that the ALC feature is “ON” (Fig.1-2). Failing to turn this function on can result in capacitance readings that are mistakenly read low.
Fig. 1: Setting ALC On.
Fig. 2: ALC on indicator.
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