Digital Multimeter HDM3055 Series Manual

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Loading Errors (DC Voltage)

Measurement loading errors occur when the resistance of the DUT is an appreciable percentage of

the multimeter's input resistance, as shown below.

To reduce the effects of loading errors and to minimize noise pickup, set the multimeter's input

resistance to >10 G? (high-impedance) for the 100 mVDC, 1 VDC, and 10 VDC ranges. The

input resistance is maintained at 10 M? for the 100 VDC and 1000 VDC ranges.

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Noise Rejection

Rejecting Power–Line Noise Voltages

A desirable characteristic of integrating analog–to–digital (A/D) converters is their ability to reject

power– line related noise present with DC input signals. This is called normal mode noise rejection,

or NMR. The multimeter achieves NMR by measuring the average DC input by "integrating" it over a

fixed period. If you set the integration time to a whole number of power line cycles (PLCs), these

errors (and their harmonics) will average out to approximately zero.

The multimeter provides three integration selections (1, 10, and 100 PLCs) that achieve NMR. The

multimeter measures the power–line frequency (50 Hz or 60 Hz), and then determines the corresponding

integration time. For a complete listing of NMR, approximate added rms noise, reading rate, and resolution

for each integration setting, see the Performance vs. Integration Time table.

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Common Mode Rejection (CMR)

Ideally, a multimeter is completely isolated from earth–referenced circuits. However, there is finite

resistance between the multimeter's input LO terminal and earth ground, as shown below. This can

cause errors when measuring low voltages which are floating relative to earth ground.

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Noise Caused by Magnetic Loops

When making measurements near magnetic fields, avoid inducing voltages in the measurement

connections. You should be especially careful when working near conductors carrying large currents.

Use twisted–pair connections to the multimeter to reduce the noise pickup loop area, or dress the

test leads as close together as possible. Loose or vibrating test leads will also induce error voltages.

Tie down test leads

securely when operating near magnetic fields. Whenever possible, use magnetic shielding materials

or increased distance from magnetic sources.

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Noise Caused by Ground Loops

When measuring voltages in circuits where the multimeter and the DUT are referenced to a common

earth ground, a "ground loop" is formed. As shown below, any voltage difference between the two

ground reference points (Vground) causes current to flow through the measurement leads. This

causes noise and offset voltage (usually power–line related), which are added to the measured voltage.

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The best way to eliminate ground loops is to isolate the multimeter from earth by not grounding

the input terminals. If the multimeter must be earth–referenced, connect it and the DUT to one

common ground point. Also connect the multimeter and DUT to the same electrical outlet

whenever possible.

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Resistance Measurement Considerations

The multimeter offers two resistance measurements: 2-wire and 4-wire ohms. For both methods,

the test current flows from the input HI terminal, through the resistor being measured. For 2-wire

ohms, the voltage drop across the resistor being measured is sensed internal to the multimeter.

Therefore, test lead resistance is also measured. For 4-wire ohms, separate "sense" connections

are required. Because no current flows in the sense leads, the resistance in these leads does not

give a measurement error.

The errors mentioned earlier in this chapter for DC voltage measurements also apply to resistance

measurements. Additional error sources unique to resistance measurements are discussed below.

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The 4-wire ohms method provides the most accurate way to measure small resistances because

it reduces test lead and contact resistances. This is often used in automated test applications

where resistive and/or long cable, numerous connections, or switches exist between the multimeter

and the DUT. The recommended connections for 4-wire ohms measurements are shown below.

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Removing Test Lead Resistance Errors

To eliminate offset errors associated with test lead resistance in 2-wire ohms measurements,

follow these steps:

Short the test lead ends together and read the displayed test lead resistance.

Press Null. The multimeter will store the test lead resistance as the 2-wire ohms null value,

and subtract that value from subsequent measurements.

See also "Null Measurements."

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Minimizing Power Dissipation Effects

When measuring resistors designed for temperature measurements (or other resistive devices

with large temperature coefficients), be aware that the multimeter will dissipate some power in

the DUT. The following table shows several examples.

If power dissipation is a problem, you should select a higher fixed range (all multimeter models).