Digital Multimeter HDM3055 Series Manual

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Estimating High–Frequency (Out–of–Band) Error

A common way to describe signal waveshapes is to refer to their "Crest Factor". Crest factor is the

ratio of the peak value to rms value of a waveform. For a pulse train, for example, the crest factor

is approximately equal to the square root of the inverse of the duty cycle.

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Notice that crest factor is a composite parameter, dependent upon the pulse width and repetition

frequency; crest factor alone is not enough to characterize the frequency content of a signal.

Traditionally, DMMs include a crest factor derating table that applies at all frequencies. The

measurement algorithm used in the DMMs is not inherently sensitive to crest factor, so no such

derating is necessary. With this multimeter, as discussed in the previous section, the focal issue

is high–frequency signal content which exceeds the multimeter’s bandwidth.

For periodic signals, the combination of crest factor and repetition rate can suggest the amount of

high– frequency content and associated measurement error. The first zero crossing of a simple

pulse occurs at f1 = 1/tp .

This gives an immediate impression of the high-frequency content by identifying where this crossing

occurs as a function of crest factor: f1=(CF2)(prf).

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The following table shows the typical error for various pulse waveforms as a function of input pulse

frequency.

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This table gives an additional error for each waveform, to be added to the value from the accuracy

table provided in the instrument's data sheet.

The specifications are valid for CF ≤ 10, provided there is insignificant signal energy above the 300 kHz

bandwidth for voltage, or the 10 kHz bandwidth for current. Multimeter performance is not specified for

CF > 10, or when significant out-of-band signal content is present.

Example

A pulse train with level 1 Vrms, is measured on the 1 V range. It has pulse heights of 3 V (that is, a Crest Factor of 3) and duration 111 ?s. The prf can be calculated to be 1000 Hz, as follows:

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Thus, from the table above, this AC waveform can be measured with 0.18 percent additional error.

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Other Primary Measurement Functions

Frequency and Period Measurement Errors

           

The multimeter uses a reciprocal counting technique to measure frequency and period. This method

generates constant measurement resolution for any input frequency. The multimeter's AC voltage

measurement section performs input signal conditioning. All frequency counters are susceptible to

errors when measuring low–voltage, low–frequency signals. The effects of both internal noise and

external noise pickup are critical when measuring "slow" signals. The error is inversely proportional

to frequency. Measurement errors also occur if you attempt to measure the frequency (or period)

of an input following a DC offset voltage change. You must allow the multimeter's input DC blocking

capacitor to fully settle before making frequency measurements.

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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.

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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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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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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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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).