Digital Multimeter Measurement Techniques and Definitions

Publish Date: May 30, 2010 | 49 Ratings | 2.82 out of 5 |  PDF

Overview

This tutorial recommends tips and techniques for using a National Instruments digital multimeter (DMM) to build the most accurate measurement system possible. Specifically, this tutorial focuses on advanced considerations for precision measurement applications.

For more information return to the Complete Digital Multimeter Measurement Tutorial.

Table of Contents

  1. Normal and Common-Mode Signals
  2. Normal-Mode Rejection Ratio (NMRR)
  3. Common-Mode Rejection Ratio (CMRR)
  4. Effective Common-Mode Rejection Ratio (ECMRR)
  5. Input Resistance
  6. Burden Voltage
  7. Thermal Voltages
  8. Settling Time
  9. Resistor Self Heating
  10. Dielectric Absorption

1. Normal and Common-Mode Signals

A normal-mode signal is a signal applied differentially to the inputs, for the purpose of measuring that signal, as shown in the following figure:

A common-mode signal is the component of an input, "common" to the differential input as shown in the following figure:


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2. Normal-Mode Rejection Ratio (NMRR)


Normal-mode rejection ratio (NMRR) describes the ability of the digital multimeter to reject an AC normal-mode signal usually at line frequencies. NMRR is given by the following formula:

NMRR = 20*log(Vin/Verror)

where Verror is the value returned by the digital multimeter for an applied AC normal-mode voltage Vin.

NMRR is useful for measurement systems that can eliminate signals at a given frequency or over a range of frequencies. NMRR, which is often used to indicate the capability of the instrument to reject a powerline noise of 50 or 60 Hz, is valid only at the specified frequency and is useful when making DC measurements.

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3. Common-Mode Rejection Ratio (CMRR)


Common-mode rejection ratio (CMRR) is a measure of the capability of the digital multimeter to reject a common-mode signal and is often specified with a 1 kΩ resistance (ohms) in the LO input lead, as shown in the following figure:

CMRR is important because it indicates how much of the common-mode signal affects your measurement. The CMRR is defined by the following equation:

CMRR = 20*log10(Differential Gain/Common-Mode Gain)

For example, if you are measuring a thermocouple in a noisy environment, the noise from the environment appears on both input leads. Therefore, this noise is a common-mode signal that is rejected by the CMRR of the instrument.

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4. Effective Common-Mode Rejection Ratio (ECMRR)


Effective common-mode rejection ratio (ECMRR) is the sum of CMRR and NMRR at a given frequency and is only valid for DC measurements. It is the effective rejection on a given noise signal that is applied to both input leads because it is rejected first by the CMRR capability of the instrument and then again by its NMRR capability. This specification is useful at powerline frequencies, particularly for laboratory and manufacturing floor environments. An equivalent equation to represent ECMRR is as follows:

ECMRR = 20*log10(VCM/Verror)

where Verror is the value returned by the digital multimeter in response to an applied common mode voltage VCM.

For example, if you are measuring 1 mVDC with a digital multimeter that specifies an ECMRR of 130 dB at 60 Hz, and you have a common-mode interference (noise) of 100 mVrms, then the resulting measurement error is:

Measurement Error = 10(-130/20) X 100 mV = 316 nV

which is 0.03% of your measured signal instead of the 10,000% error that the 100 mV interference would otherwise imply.

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5. Input Resistance


Resistance (ohms) is a measure of the opposition to the flow of current (amps). The input resistance (ohms) of the digital multimeter can affect the accuracy of voltage measurements particularly when the resistance of the source voltage is comparable to the input resistance of the digital multimeter.

For example, assume a digital multimeter with a 10 MΩ input resistance (ohms) measures a 9 V source voltage with a series resistance of 50 Ω while in a 10 V DC range, as represented in the following figure:

Using the following formula, you can determine the approximate value returned by the digital multimeter:

VM = VS *[(Rin)/(RS + Rin)]
where VM is the voltage measured by the digital multimeter, VS is source voltage, Rin is the input resistance (ohms) of the digital multimeter, and RS is external source resistance (ohms).

VM = (9 V * 10 M Ω)/(50 Ω + 10 M Ω)

VM = 8.99996 V

Another example is to assume a digital multimeter with a 10 MΩ input resistance (ohms) measures a 9 V source voltage with a series resistance of 1 M Ω while in a 10 V DC range as represented in the following figure:

Using the same formula, you can determine the approximate value returned by the digital multimeter.

VM = (9 V * 10 M Ω)/(1 M Ω + 10 M Ω)
VM = 8.18182 V

When measuring the source voltage with a low source resistance (ohms), the digital multimeter returned a value that was accurate to within ±0.0005%, or 5 ppm, and the same source voltage with a high resistance returned a value that was accurate to within ±10%, or 100,000 ppm.

NI 4070 Digital Multimeter users can select the input resistance   for DC voltage measurements.

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6. Burden Voltage


Burden voltage is the voltage drop caused by current (amps) flowing through a current measuring device. A large burden voltage can affect the circuit being measured, corrupting the measurement. For this reason, it is desirable for burden voltage to be kept as low as possible. The following figure shows a current (amps) measuring device with a 0.5 V burden voltage in series with a 5 Ω resistor and a 1.5 V source:



With burden voltage, the current (amps) in this circuit equals:

Imeasured = (1.5 V - 0.5 V)/(5 Ω)

Imeasured = 0.2 A

Without burden voltage, the calculated current (amps) is:

Iactual = 1.5 V/5 Ω

Iactual = 0.3 A

In the figure above, burden voltage of the current (amps) measuring device subtracts from the voltage across the 5 Ω resistor. The result is a significant error in the measurement. In this case, the error caused by the burden voltage is 33%.

Techniques to reduce error caused by ammeter burden voltage:

  • When using the current function within the digital multimeter, keep leads to the ammeter short, and use appropriate gauge of interconnect to minimize voltage drops from leads.
  • Sense remotely with external shunts to eliminate the need for long current-carrying interconnects to the digital multimeter. Use the lowest value of shunt resistance (ohms) that the measurement allows.
  • Use wires in the circuit under test as resistance (ohms) shunts. Measure the voltage drop across the wire. Next, use the Offset Compensated Ohms function to measure the resistance (ohms) of the wire, then calculate I = Vwire/Roco.

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7. Thermal Voltages


Thermal voltages, also known as thermal EMFs, are voltages created by the junction of dissimilar metals when a temperature difference exists between these junctions. The generated voltage increases with temperature. The specific metal-to-metal junctions result in specific temperature coefficients (V/ºC) called Seebeck coefficients shown in the following table:

Junction
Seebeck Coefficient
(µV/ºC)
Copper–Copper <0.3
Copper–Gold 0.5
Copper–Silver 0.5
Copper–Brass 3
Copper–Nickel 10
Copper–Lead-Tin Solder 1–3
Copper–Aluminum 5
Copper–Kovar 40
Copper–Copper Oxide >500


You can determine thermal voltages using the following formula:

VT = K(T2 - T1)

where

T1 and T2 are temperatures measured at the junctions of dissimilar metals
K is the Seebeck coefficient of Copper—N (from table above), and N is a dissimilar metal


To reduce the effects of thermal voltages, use copper-to-copper connectors with gold-plating.

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8. Settling Time


Settling time is the time required for a measurement system to stabilize to a specified accuracy limit. The digital multimeter settle time is dictated by the measurement range, cable properties, source impedance, and change in input level. Use short cables with low dielectric absorption and minimal capacitance—NI recommends Teflon cable. Ideally, your source should have a low output impedance. Settling time becomes especially important in scanning systems . The scanner or multiplexer requires an additional settle time before the measurement can be taken. NI-DMM allows for a programmable delay between channels so that both the digital multimeter and the multiplexer can settle. Settling times are dependent on the input signal and your selected resolution.

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9. Resistor Self Heating


Resistor self-heating can occur when measuring large electrical currents (amps). Large current (amps) measurements can heat the current shunt resistor, which changes the resistance (ohms) value and causes the measurement accuracy to drift. Usually resistor self-heating does not present a problem because the maximum current (amps) range is relatively small compared with the shunt power rating and because the NI CSM-200mA and NI CSM-10A current (amps) shunts have excellent temperature coefficients of resistance (ohms). When using third-party current (amps) shunts in a system, select a resistor with a temperature coefficient of no more than 10 ppm/ºC and a power rating of at least twice the power you expect in the measurement. Refer to ni.com/catalog for more information on the NI CSM-200mA and NI CSM-10A current (amps) shunts.

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10. Dielectric Absorption


Dielectric absorption caused by cable materials can increase settle time. The figure below illustrates dielectric absorption due to cable resistance (ohms) and dielectric polarization capacitance where:

RDA = Insulation resistance of cabling (10 GΩ - 1014Ω)
CDA = Dielectric polarization capacitance (0.1 - 1 pF/ft)
CC = Cable capacitance (10 - 40 pF/ft)


The RC formed by RDA and CDA results in slow settle tails that significantly lengthen settle time.

The best way to avoid dielectric absorption effects is to use a high quality cable, such as Belden 83317 available at beldon.com. NI recommends cables with Teflon, polypropylene, or polyethylene insulation. For more information about cabling requirements, refer to Interconnects and Cables.

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