Digital Multimeter HDM3055 Series Manual

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In applications where sample–to–sample levels vary widely, but the DC offset level does not change,

the MEDIUM filter settles at 2 to 4 readings per second (depending upon the lowest frequency

component in the waveform) as shown in the following table:

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For AC voltage, additional settling time may be required when the DC level varies from sample to

sample. The default sample delays allow for a DC level change of 3% of range for all filters. If the

DC level change exceeds these levels, additional settling time is required. The multimeter's DC

blocking circuitry has a settling time constant of 0.2 seconds. This settling time only affects

measurement accuracy when DC offset levels vary from sample to sample. If maximum

measurement speed is desired, you may want to add an external DC blocking circuit for circuitry

with significant DC voltages present. This circuit can be as simple as a resistor and a capacitor.

For AC current, additional settling time is not required when the DC level varies sample to sample.

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Making High–Speed DC and Resistance Measurements

To make the fastest (but least accurate) DC or resistance measurements:

l Set the integration time (NPLC or aperture) to minimum

l Select a fixed range (autorange off)

l Disable autozero

l Disable offset compensation (resistance measurements)

Refer to the particular measurement type in Measurements for more information on the above

functions.

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Capacitance

The multimeter makes capacitance measurements by applying a known current to charge the

capacitance and then a resistance to discharge as shown below:

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An illustration of the response curve while charging is shown below:

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Capacitance is calculated by measuring the change in voltage (DV) that occurs over a “short aperture”

time, (Dt). This measurement is repeated at two different times during the exponential rise that occurs.

An algorithm takes the data from these four points, and by linearizing that exponential rise over these

“short apertures”, accurately calculates the capacitance value.

The measurement cycle consists of two parts: a charge phase (shown in the graph) and a discharge phase.

The time–constant during the discharge phase is longer, due to a 100 kΩ protective resistor in the measurement

path. This time–constant plays an important role in the resultant reading rate (measurement time). The

incremental times (or “sample times”) as well as the width of the “short apertures”, vary by range, in order

to minimize noise and increase reading accuracy.

For the best accuracy, take a zero null measurement with open probes, to null out the test lead capacitance,

before connecting the probes across the capacitor to be measured (see Capacitance Measurements for details).

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Capacitance Measurement Considerations

Capacitors that have a high dissipation factor or other non-ideal characteristics will affect capacitance

measurements. Capacitors with high dissipation factors may exhibit a variance between the measured

value using the multimeter versus the single frequency method of some other LCR meters. The single

frequency method will also see more variation at different frequencies. For example, some inexpensive

capacitance substitution boxes, when measured with the multimeter, are almost 5% different compared

to the same capacitance measured with the single frequency method of an LCR meter. The LCR meter

will also show different values at different frequencies.

Capacitors with long time constants (dielectric absorption) will result in slow measurement settling time,

and will take a number of seconds to stabilize. You may see this when first connecting a capacitor or

when the delay time to make a measurement is varied. A high quality film capacitor typically shows the

least of this and an electrolytic capacitor the most, with ceramic capacitors typically in between.

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Other Sources of Measurement Error

Settling Time Effects

The multimeter can insert automatic measurement settling delays. These delays are adequate for

resistance measurements with less than 200 pF of combined cable and device capacitance. This is

particularly important when measuring resistances above 100 k?. Settling due to RC time constant

effects can be quite long. Some precision resistors and multi–function calibrators use large parallel

capacitors (1000 pF to 0.1 ?F) with high resistor values to filter out noise currents injected by their

internal circuitry. Non–ideal capacitances in cables and other devices may have much longer settling

times than expected just by RC time constants due to dielectric absorption (soak) effects. Errors are

measured when settling after the initial connection and after a range change.