Digital Multimeter HDM3055 Series Manual

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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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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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With a few precautions, you can perform AC measurements at speeds up to 500 readings

per second.

Use manual ranging to eliminate auto-ranging delays. By setting the trigger delay to 0, the

FAST, MEDIUM, and SLOW filters allow up to 500, 150, and 50 readings per second, but with

reduced accuracy because the filter may not fully settle. If the sample–to–sample levels are

similar, little settling time is required for each new reading. Under this specialized condition,

the MEDIUM filter provides reduced accuracy results at 20 readings per second, and the FAST

filter provides reduced accuracy results at 200 readings per second.

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Making High–Speed AC Measurements

The multimeter's AC voltage and AC current functions implement three low–frequency filters.

These filters allow you to trade–off minimum measured frequency for faster reading speed.

The FAST filter settles in 0.025 seconds, and is useful for frequencies above 200 Hz. The

MEDIUM filter settles in 0.625 seconds for voltage and 0.25 seconds for current, and is useful

for measurements above 20 Hz. The SLOW filter settles in 2.5 seconds for voltage and 1.66

seconds for current, and is useful for frequencies above 3 Hz.

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NULL Reading

The DMM allows a separate null setting to be saved for the temperature function. When making

null measurements, each reading is the difference between a stored null value and the input signal.

One application of NULL is to increase accuracy of two-wire resistance measurements by first nulling

the closed–circuit test lead resistance.

Autozero On/Off

Enabling the autozero feature (ON) provides greater accuracy; however, the additional measurement

(of zero) reduces the reading speed.

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Important: Use only a 5 k? 44007-type thermistor. This type thermistor has constants of A = 1.285e-3,

B = 2.362e-4, C = 9.285e-8. Using an incorrect type of thermistor can result in errors greater than 20 °C

for a temperature being measured of 100 °C.

2-Wire vs. 4-Wire Measurements

As with resistance measurements, 4-wire temperature measurements are more accurate, because errors

due to lead wire resistance are completely eliminated. Alternatively, you can use the multimeter’s Null function

to remove the test lead resistance from the measurement (see NULL Reading below).

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Thermistor Requirements

The DMM converts the measured thermistor resistance to temperature using the Steinhart-Hart

thermistor equation:

1?T = A + B (Ln(R)) + C (Ln(R))3

Where:

A, B, and C are constants provided by the thermistor manufacturer and derived from three

temperature test points.

R = Thermistor resistance in Ω.

T = Temperature in degrees K.

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Temperature Measurements

Temperature measurements require a temperature transducer probe. The supported probes are

2-wire and 4-wire RTDs, 2-wire and 4-wire thermistors.

Probe Type Choice

RTD's provide very accurate, highly linear relationships between resistance and temperature,

over a range of roughly –200 to 500 °C. There is very little conversion complexity for an RTD

because it is so intrinsically linear. The multimeter provides measurement for the IEC751 standard

RTD, which has a sensitivity of 0.385%/°C.

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Avoid applying signals to unused current input terminals

If signal inputs are applied to terminals not needed for the current measurement, measurement

errors may occur. The unused terminals are still protected but the un-needed signals may interfere

with current measurement. For example, applying inputs to the 3A terminals while making

measurements on the 10A terminals will typically cause errors.

The Hi and Lo sense terminals are not used for many measurements. Applying signals here when

not needed can also cause errors. AC or DC voltages above 15 volts peak on the un-needed sense

terminals are likely to cause measurement errors. If unexpected errors are occurring, signals on the

un-needed terminals is an area to check.

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DC Current

When you connect the multimeter in series with a test circuit to measure current, a measurement

error is introduced. The error is caused by the multimeter's series burden voltage. A voltage is

developed across the wiring resistance and current shunt resistance of the multimeter, as shown

below.