Digital Multimeter HDM3055 Series Manual

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High-Voltage Self-Heating Errors

If you apply more than 300 Vrms, self–heating occurs in the multimeter's internal signal–conditioning

components. These errors are included in the multimeter's specifications. Temperature changes inside

the multimeter due to self–heating may cause additional error on other AC voltage ranges. The additional

error is less than 0.02% and dissipates in a few minutes.

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Measurements Below Full Scale

You can make the most accurate AC measurements when the multimeter is at or near the full scale

of the selected range. Autoranging occurs at 10% (down–range) and 120% (up–range) of full scale.

This enables you to measure some inputs at full scale on one range and 10% of full scale on the

next higher range. In general, the accuracy is better on the lower range; for the highest accuracy,

select the lowest manual range possible for the measurement.

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For low frequencies, the loading error is

At high frequencies, the additional loading error is

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Loading Errors (AC volts)

In the AC voltage function, the input of the multimeter appears as a 1 M? resistance in parallel with

100 pF of capacitance. The cabling that you use to connect signals to the multimeter also adds

capacitance and loading. The table below shows the multimeter's approximate input resistance at

various frequencies.

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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.

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

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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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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.