Digital Multimeter HDM3055 Series Manual

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Introduction to the Instrument

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When you change any of these settings, the sample interval (sample timer in continuous mode) is

increased to be greater than the calculated measurement time. In the continuous mode, attempting

to reduce the sample interval below the calculated value results in an error message. You must then

choose among the various ways you can achieve a smaller measurement time to achieve the smaller

sample interval, such as decreasing the NPLC setting.

Another way to put it is that for continuous mode, sample timer/interval is always controlled by the other

measurement settings.

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How Sample Rate/Interval is Determined

The DMM always attempts to make the sample interval greater than the expected time required

to take the measurements. A number of settings go into the calculation of the minimum allowable

sample interval. These settings include the measurement function, NPLC, aperture, autorange,

autozero, offset compensation, AC filter, TC open check and gate time.

For example, when autorange is on, the assumption is that no more than one range change will

be required. If more than one level of change occurs, the measurement may be delayed and an

error will be issued.

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Unnecessary Signal Errors

Both 3A and 10A terminals are available for AC and DC current measurements. If signals are applied

to terminals not being used for the current measurement, measurement errors may occur. The

unused terminals are protected but the additional signals may interfere with current measurements.

For example, applying inputs to the 3A terminals while making measurements on the 10A terminals

will typically cause errors.

Unnecessary signals applied to the Hi and Lo Sense terminals can also cause errors. AC or DC

voltages above 15 volts peak on the sense terminals are likely to cause measurement errors.

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Leakage Current Errors

The DMM's input capacitance will "charge up" due to input bias currents when the terminals are

open–circuited (if the input resistance is >10 G?). The DMM's measuring circuitry exhibits

approximately 30pA of input bias current for ambient temperatures from 0 to 30 °C. Bias

current doubles for every 8 °C change in ambient temperature above 30 °C. This current

generates small voltage offsets dependent upon the source resistance of the DUT. This effect

becomes evident for a source resistance of greater than 100 k?, or

when the DMM's operating temperature is significantly greater than 30 °C.

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Common Mode Errors

Errors are generated when the multimeter's input LO terminal is driven with an AC voltage relative

to earth. The most common situation where unnecessary common mode voltages are created is

when the output of an AC calibrator is connected to the multimeter "backwards." Ideally, a multimeter

reads the same regardless of how the source is connected. Both source and multimeter effects can

degrade this ideal situation. Because of the capacitance between the input LO terminal and earth

(approximately 200 pF), the source will experience different loading depending on how the input is

applied. The magnitude of the error is dependent upon the source's response to this loading.

The DMM's measurement circuitry, while extensively shielded, responds differently in the backward

input case due to slight differences in stray capacitance to earth. The DMM's errors are greatest for

high– voltage, high–frequency inputs. Typically, the DMM exhibits about 0.06% additional error for a

100 V, 100 kHz reverse input. You can use the grounding techniques described for DC common mode

problems to minimize AC common mode voltages.

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Most extraneous noise is not correlated with the input signal. You can determine the error as shown

below.

Correlated noise, while rare, is especially detrimental because it always adds directly to the input signal.

Measuring a low–level signal with the same frequency as the local power line is a common situation that

is prone to this error.

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Connect the DMM and the AC source to the same electrical outlet whenever possible.

You should also minimize the area of any ground loops that cannot be avoided. A

high–impedance source is more susceptible to noise pickup than a low–impedance

source. You can reduce the high–frequency impedance of a source by placing a

capacitor in parallel with the DMM's input terminals. You may have to experiment

to determine the correct capacitor for your application.

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Low–Level Measurement Errors

AC voltage measurements less than 100 mV are especially susceptible to errors introduced by

extraneous noise sources. An exposed test lead acts as an antenna and a properly functioning

DMM will measure the signals received. The entire measurement path, including the power line,

act as a loop antenna. Circulating currents in the loop create error voltages across any

impedances in series with the DMM's input. Therefore, you should apply low–level AC

voltages to the DMM through shielded cables, with the shield connected to the input LO terminal.

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AC Current Measurement Errors (Burden Voltage)

Burden voltage errors, which apply to DC current, also apply to AC current measurements. However,

the burden voltage for AC current is larger due to the multimeter's series inductance and your

measurement connections. The burden voltage increases as the input frequency increases.

Some circuits may oscillate when performing current measurements due to the multimeter's

series inductance and your measurement connections.