Download A Typical DC Voltage Calibration Sequence

®
Understanding Basic
DMM Calibration
For Electrical
Calibration Sales
1
®
Chapter Objectives:
Type of DMMs
DMM Functional Block Diagram
Analog to Digital Converter Types
AC-DC Converters
Resistance Converters
Theory of DMM Calibration
Calibration Strategies
DMM Terminology
2
®
Is technical knowledge required?
What if the cal procedure tests things in the wrong order?
What if a parameter fails to meet the specification?
What if the instrument requires troubleshooting?
The technician who understands the technology is an asset to the lab!
3
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Types of DMMs
Laboratory DMMs
Handheld DMMs
4
Bench DMMs
®
Types of DMMs
Laboratory DMMs
6 1/2 to 8 1/2 digits resolution
Usually 5 functions
DC Volts
AC Volts
DC Current
AC Current
Resistance
May be configured for IEEE
remote operation
5
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Types of DMMs
Bench/System Meters
Less accurate than laboratory DMMs
Generally 4 1/2 to 6 1/2 digit resolution
Generally same 5 basic functions
May be configured for IEEE remote operation
Depending on model, may be software calibrated
6
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Types of DMMs
Handheld DMMs
Most versatile of the DMMs, but least accurate
Generally 3 1/2 or 4 1/2 digit resolution
Basic 5 functions, but may also include:
Frequency
Continuity
Diode Test
Peak hold
Temperature
Capacitance
7
Waveform
®
DMM Terminology
Function - Functional options of a DMM. e.g. DC Voltage,
AC Voltage, Resistance, Current, Frequency, etc.
Range - A DMM's measurement capability is split into
ranges such that the input signal can be scaled to a level
appropriate for its ADC or RMS converter.
Resolution & Scale Length -- A way of describing how
many figures a DMM can display. Resolution is the
number of digits e.g. 1.000 000 V is a 6½ digit display
where the ½ digit is the "1" and there are 6 places after
the decimal point.
Sensitivity is the smallest measurable amount (Lowest
Significant Digit) on any range.
8
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DMM Terminology
Linearity - Linearity is a description of how the gain of an instrument's
ADC might vary with the amplitude of the
measured signal.
Zero or Offset - All DC measurements are affected by residual Offsets
that may be in Volts, Amps or Ohms.
Frequency Response -- Frequency Response is interpreted as the -3dB
point i.e. the high frequency point at which the reading has reduced to
70.7% of its nominal low frequency value.
A more meaningful measure of response is Flatness.
Flatness is a measure of the deviation from an assumed flat reference
level.
9
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DMM Terminology
Input Resistance - Is important because it describes the potential loading
effect of the DMM on the test circuit.
For DC voltage up to 20 V, the input resistance may be >1012 W.
At higher voltages up to 1 kV, an attenuator is switched in to
divide the applied voltage down to 10 V or 1 V.
The attenuation resistor is typically 10 MW.
For AC voltage, the input resistance will usually be lower – typically 1 MW
with 150 pF in parallel.
10
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DMM Terminology
Input Bias Current – Amplifiers are usually designed for high gain, high
bandwidth, high input impedance, low output impedance and low input
voltage offsets.
Bi-polar semiconductor devices require an input current to "bias" them into a
desirable (usually linear) portion of their operating curve.
There will be some residual input current, which will flow in the external
measurement circuit.
A small current of 10 pA in 1 MW will develop 10 µV.
A modern sensitive DMM would be expected to have an input bias current of
<50 pA.
11
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DMM Terminology
Crest Factor - Is the peak-to rms ratio capability of a DMM's
AC converter.
A pure sine-wave has a crest factor of 1.414:1, a square wave
has a crest factor of 1:1.
12
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DMM Terminology
Common Mode Rejection - A measure of how the
DMM responds to the presence of a particular kind
of interference (unwanted) signal.
Common mode errors can be reduced by careful guarding
techniques to reduce the effects of unwanted leakages to ground.
Normal Mode Rejection - A measure of how the DMM
responds to an interference signal that is not common to
both inputs.
13
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DMM Terminology
Guard - A guard is a circuit that may be used to intercept and divert, or
control leakage currents.
It may be passive, where it is simply a conductive screen or case around
sensitive components and circuits, or active where amplifiers actively
sense and control the screen potential.
When line powered instruments are connected together, circulating
leakage currents may flow around the "loop" formed by their respective
power supplies and input lead connections.
14
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DMM Terminology
Guard - A guard is a circuit that may be used to
intercept and divert, or control leakage currents.
Such currents usually manifest themselves as noisy
or inflated readings in sensitive measurements and
may be eliminated by connecting the guard of the DMM
to the "earth" terminal (not directly to earth) of the
"source" instrument.
15
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DMM Terminology
4 Wire Resistance – A measurement technique
used to minimize the effects of the resistance of
the connecting leads.
In a 4-wire resistance measurement, separate
conductor pairs are used to connect the current and
voltage circuits.
True RMS - In high accuracy AC voltage measurement,
the most useful description of the amplitude is the
Root-Mean-Square value.
The term True RMS was coined to distinguish between modern
RMS sensing instruments and the older or lower cost DMMs
that used mean or average sensing, but were adjusted to indicate
the RMS value.
16
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DMM Terminology
Calibration Uncertainty - The total uncertainty of the
calibration standard traceable to national and international
standards.
The uncertainty will include the contribution of the DMM
during the calibration process.
Relative Accuracy – The accuracy of the DMM relative
to but not including calibration standards.
Absolute Accuracy - The combination of Calibration Uncertainty
and Relative Accuracy.
The combination may be a simple arithmetic summation of the
two terms, or a more complex Root-Sum-of-Squares (RSS)
combination at a specified confidence level.
17
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DMM Block Diagram
18
A Simplified Block Diagram of a DMM
®
DMM Block Diagram
The ADC is the heart of the
DMM
19
A Simplified Block Diagram of a DMM
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DMM Block Diagram
Measurement
Section
Multifunction Analog
Input
Display
Control Section
Electrically Isolated
Communication and Control
Channels
Computer Interface
IEEE-488
A. TYPICAL BLOCK DIAGRAM
20
RS-232
Typical computer Interface
®
DMM Block Diagram
Measurement
Section
Multifunction Analog
Input
Display
Control Section
Electrically Isolated
Communication and Control
Channels
Computer Interface
IEEE-488
A. TYPICAL BLOCK DIAGRAM
21
No calibration
required
RS-232
Typical computer Interface
®
DMM Block Diagram
Measurement
Section
Display
Multifunction Analog
Input
Control Section
Electrically Isolated
Communication and Control
Channels
Computer Interface
IEEE-488
A. TYPICAL BLOCK DIAGRAM
22
RS-232
Checked during power up,
or with self Typical
test computer Interface
®
Multifunction Analog
Input
DMM Block Diagram
DCV
DC Volt Ranges &
Filtering
DCV
ALL OTHERS
A to D Converter
(ADC)
To Control
Section for
Display.
ACV
AC Range Amp
and AC-DC Converter
ALL
OTHERS
ACI
ACV & ACI
DC Reference
Current Shunts
DCI
DCI & ACI
Ohms
OHMS
B. TYPICAL MEASUREMENT
SECTION
23
OHMS
Ohms Reference
®
Multifunction Analog
Input
DMM Block Diagram
DCV
DC Volt Ranges &
Filtering
DCV
ALL OTHERS
A to D Converter
(ADC)
To Control
Section for
Display.
ACV
AC Range Amp
and AC-DC Converter
ALL
OTHERS
ACI
ACV & ACI
DC Reference
Current Shunts
DCI
DCI & ACI
Ohms
OHMS
B. TYPICAL MEASUREMENT
SECTION
24
OHMS
The ADC is the
heart of the DMM.
All measurement
functions go through the ADC
Ohms Reference
®
Multifunction Analog
Input
DMM Block Diagram
DCV
DC Volt Ranges &
Filtering
DCV
ALL OTHERS
A to D Converter
(ADC)
To Control
Section for
Display.
ACV
AC Range Amp
and AC-DC Converter
ALL
OTHERS
ACI
ACV & ACI
DC Reference
Current Shunts
DCI
DCI & ACI
Ohms Reference
Ohms
OHMS
B. TYPICAL MEASUREMENT
SECTION
25
OHMS
An ADC is usually a single range
circuit, and it can handle only a
narrow voltage range of zero to ±10
V
®
Multifunction Analog
Input
DMM Block Diagram
DCV
DC Volt Ranges &
Filtering
DCV
ALL OTHERS
A to D Converter
(ADC)
To Control
Section for
Display.
ACV
AC Range Amp
and AC-DC Converter
ALL
OTHERS
ACI
ACV & ACI
DC Reference
Current Shunts
DCI
DCI & ACI
Ohms Reference
Ohms
OHMS
B. TYPICAL MEASUREMENT
SECTION
26
OHMS
Circuits in the form of amplifiers and
attenuators scale the input voltage to
levels that can be measured by the
ADC
®
Multifunction Analog
Input
Contributions to the Spec
DCV
DC Volt Ranges &
Filtering
DCV
ALL OTHERS
A to D Converter
(ADC)
To Control
Section for
Display.
ACV
AC Range Amp
and AC-DC Converter
ALL
OTHERS
ACI
ACV & ACI
DC Reference
Current Shunts
DCI
DCI & ACI
Ohms
OHMS
B. TYPICAL MEASUREMENT
SECTION
27
OHMS
Ohms Reference
®
Multifunction Analog
Input
Contributions to the Spec
DCV
DC Volt Ranges &
Filtering
DCV
ALL OTHERS
A to D Converter
(ADC)
To Control
Section for
Display.
ACV
AC Range Amp
and AC-DC Converter
ALL
OTHERS
ACI
ACV & ACI
DC Reference
Reference Stability (% of Reading)
Current Shunts
DCI
DCI & ACI
Ohms
OHMS
B. TYPICAL MEASUREMENT
SECTION
28
OHMS
Ohms Reference
®
Multifunction Analog
Input
Contributions to the Spec
DCV
DC Volt Ranges &
Filtering
DCV
ALL OTHERS
A to D Converter
(ADC)
To Control
Section for
Display.
ACV
AC Range Amp
and AC-DC Converter
ALL
OTHERS
ACI
ACV & ACI
DC Reference
ADC Linearity
(% of Scale)
Current Shunts
DCI
DCI & ACI
Ohms
OHMS
B. TYPICAL MEASUREMENT
SECTION
29
OHMS
Ohms Reference
®
Multifunction Analog
Input
Contributions to the Spec
DCV
DC Volt Ranges &
Filtering
DCV
ALL OTHERS
A to D Converter
(ADC)
To Control
Section for
Display.
ACV
AC Range Amp
and AC-DC Converter
ALL
OTHERS
ACI
ACV & ACI
DC Reference
Attenuator Stability (% of Reading)
Current Shunts
DCI
DCI & ACI
Ohms
OHMS
B. TYPICAL MEASUREMENT
SECTION
30
OHMS
Ohms Reference
®
Multifunction Analog
Input
Contributions to the Spec
DCV
DC Volt Ranges &
Filtering
DCV
ALL OTHERS
A to D Converter
(ADC)
To Control
Section for
Display.
ACV
AC Range Amp
and AC-DC Converter
ALL
OTHERS
ACI
ACV & ACI
DC Reference
Voltage Offsets
(Absolute)
Current Shunts
DCI
DCI & ACI
Ohms
OHMS
B. TYPICAL MEASUREMENT
SECTION
31
OHMS
Ohms Reference
®
Multifunction Analog
Input
Contributions to the Spec
DCV
DC Volt Ranges &
Filtering
DCV
ALL OTHERS
A to D Converter
(ADC)
To Control
Section for
Display.
ACV
AC Range Amp
and AC-DC Converter
ALL
OTHERS
ACI
ACV & ACI
DC Reference
Current Shunts
DCI
DCI & ACI
Input Bias Current (Absolute)
Ohms
OHMS
B. TYPICAL MEASUREMENT
SECTION
32
OHMS
Ohms Reference
®
Multifunction Analog
Input
Contributions to the Spec
DCV
DC Volt Ranges &
Filtering
DCV
ALL OTHERS
A to D Converter
(ADC)
To Control
Section for
Display.
ACV
AC Range Amp
and AC-DC Converter
ALL
OTHERS
ACI
ACV & ACI
DC Reference
Current Shunts
DCI
DCI & ACI
Noise
Ohms
OHMS
B. TYPICAL MEASUREMENT
SECTION
33
OHMS
(Absolute)
Ohms Reference
®
Multifunction Analog
Input
Contributions to the Spec
DCV
DC Volt Ranges &
Filtering
DCV
ALL OTHERS
A to D Converter
(ADC)
To Control
Section for
Display.
ACV
AC Range Amp
and AC-DC Converter
ALL
OTHERS
ACI
ACV & ACI
DC Reference
Current Shunts
DCI
DCI & ACI
Resolution (Absolute)
Ohms
OHMS
B. TYPICAL MEASUREMENT
SECTION
34
OHMS
Ohms Reference
®
Contributions to the Spec
35
±%R ±%FS ±µV
®
Contributions to the Spec
The basic DC linearity needs
only to be verified on the
basic (10V) range
36
±%R ±%FS ±µV
®
Contributions to the Spec
The 100 mV and 1 V ranges will have a
slightly worse performance.
Noise and voltage offsets will
be the dominant factor
37
±%R ±%FS ±µV
®
Contributions to the Spec
The 100 mV and 1 V ranges will have a
slightly worse performance.
InNoise
the higher
voltageoffsets
ranges,
and voltage
willthe
effects
power dissipation
be theofdominant
factor
The basic
DC
linearity
needs
in the attenuators will be the dominant
only to be verified onfactor.
the
basic (10V) range
38
±%R ±%FS ±µV
®
Contributions to the Spec
In the higher voltage ranges, the
effects of power dissipation
in the attenuators will be the dominant
factor.
39
±%R ±%FS ±µV
®
Analog-to-Digital Converter (ADC)
S3
S2
VRef
•
S1
Vmeas
OP Amp
Integrator
De-integration Process
– S2 closed, S1 and S3 Open
– Vref applied to Cint in opposite
polarity via Rin
– Cint discharges to 0V
– Time for discharge monitored by
counter
•
Zero Process
– S3 closed, S1 and S2 Open
40
Cint
Integration process
– S1 closed, S2 and S3 Open
– Cint charges to conditioned
voltage level “Vmeas” via Rin
•
Rin
Used in low
resolution
DMMs
Larger Vmeas
Vref discharges
Cint at same rate
Smaller Vmeas
Variable
Charge
Slope
Constant
Discharge
Slope
Constant
Time - T1
Variable
Time - T2
Basic Dual-Slope Converter
Zero Volts
Comparator
®
DC Voltage Converter
Amplifiers Or Attenuators
Scale the input to the proper level for the ADC
Amplifies the low input ranges
Attenuates the high input ranges
May include filtering circuits
41
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Alternating Voltage (AC to DC) Converters
Range amplifier attenuates or amplifies input to proper range for
conversion
Applied to AC to DC Converter
DC output applied to ADC for digitizing
ACVmeas
AC to DC Converter
Range Amp
42
ADC
®
Main Types of Alternating Voltage Converters
Average Responding Converter
True RMS Voltage Converters
Thermal Sensing Technique
Log/Antilog Analog Computation
Digital Sampling
ACVmeas
AC to DC Converter
Range Amp
43
ADC
®
Average Responding Converter
Operational Rectifier
ADC and
DMM
Display
ACVmeas
Range Amp
Capacitance Voltage is
Proportional to
Average Current from
Range Amp.
DMM
Common
44
®
Average Responding Converter
The Average Responding Converter
must have a SINE wave input.
Operational Rectifier
ADC and
DMM
Display
ACVmeas
Range Amp
Capacitance Voltage is
Proportional to
Average Current from
Range Amp.
DMM
Common
45
®
Average Responding Converter
Operational Rectifier
ADC and
DMM
Display
ACVmeas
Range Amp
Capacitance Voltage is
Proportional to
Average Current from
Range Amp.
DMM
Common
46
The Operational Rectifier
determines the average after
rectification
®
Average Responding Converter
Operational Rectifier
ADC and
DMM
Display
ACVmeas
Range Amp
Capacitance Voltage is
Proportional to
Average Current from
Range Amp.
DMM
Common
47
The Average (0.637 of peak) is
scaled up by 1.11 to RMS
equivalent
®
Average responding AC-DC Converters
Converts the AC input signal to an equivalent DC voltage
Usually the average voltage is converted to the corresponding RMS
value through multiplication by a constant
The conversion is made with the assumption that the applied signal is
pure sine-wave
Otherwise the conversion factor is not correct and the meter indicates
an incorrect value
For a pure sine wave the conversion is as follows:
Vrms 
48
2  π  Vavg
 1.11  Vavg
4
®
True RMS Converters
The function of a True-RMS converter is to convert the AC input signals to a
DC voltage value proportional to the RMS value of the input signal
True-RMS values are obtained mathematically by averaging the squared
value of the input, then finding the square root of the average square
49
®
True RMS Voltage Converters
Three general types of “True RMS” Converters
Thermal Sensing
Log/Anti-Log Analog Computation
Digital Sampling
50
®
Thermal Sensing Technique
RMS Sensor
DMM
Display
ADC
ACVmeas
Range Amp
Capacitance Voltage is
Proportional to
Average Current from
Range Amp.
DMM
Common
51
®
Thermal Sensing Technique
The Range Amp scales
the basic operating range
up or down
RMS Sensor
DMM
Display
ADC
ACVmeas
Range Amp
Capacitance Voltage is
Proportional to
Average Current from
Range Amp.
DMM
Common
52
®
Thermal Sensing Technique
RMS Sensor
DMM
Display
ADC
ACVmeas
Range Amp
Capacitance Voltage is
Proportional to
Average Current from
Range Amp.
DMM
Common
The resistor heats the
transistor, increasing
conduction
53
®
Thermal Sensing Technique
RMS Sensor
DMM
Display
ADC
ACVmeas
Range Amp
Capacitance Voltage is
Proportional to
Average Current from
Range Amp.
DMM
Common
54
Collector current increase
causes voltage to decrease,
unbalancing differential
amplifier
®
Thermal Sensing Technique
RMS Sensor
DMM
Display
ADC
ACVmeas
Range Amp
Capacitance Voltage is
Proportional to
Average Current from
Range Amp.
DMM
Common
55
The differential amplifier
changes current through
resistor, heats transistor until
balance is achieved
®
Thermal Sensing Technique
The differential amplifier output
potential level is sensed to ADC
RMS Sensor
DMM
Display
ADC
ACVmeas
Range Amp
Capacitance Voltage is
Proportional to
Average Current from
Range Amp.
DMM
Common
56
®
Log/Antilog Analog Computation
57
®
Log/Antilog Analog Computation
A True RMS converter consists of a precision rectifier, Logarithmic and
Exponential amplifiers.
58
®
Log/Antilog Analog Computation
A True RMS converter consists of a precision rectifier, Logarithmic and
Exponential amplifiers.
The logarithmic and Anti-log amplifiers
effectively square and square-root the
input signal
59
®
Log/Antilog Analog Computation
A True RMS converter consists of a precision rectifier, Logarithmic and
Exponential amplifiers.
The output of the multiplier Vx has a DC
ripple content that is averaged (to obtain
a mean value) by applying it to an active
low-pass filter
60
®
Log/Antilog Analog Computation
A True RMS converter consists of a precision rectifier, Logarithmic and
Exponential amplifiers.
Vout is also used to provide the feedback signal Vf,
which provides the square-root element of the
computation and controls the gain of the circuit to
61
give a wide dynamic range
®
Log/Antilog Analog Computation
A True RMS converter consists of a precision rectifier, Logarithmic and
Exponential amplifiers.
62
The low-pass filter determines the low
frequency response of the instrument.
Some log-feedback DMM's are capable
of operating down to 0.01 Hz.
®
Digital Sampling
In this technique, portions of the input waveform are sampled during short
intervals of time relative to the period of the waveform.
Each sample is quickly digitized by a fast ADC.
Its value is squared by the DMM’s microprocessor and the result is stored in
its memory.
When a sufficient number of samples have been taken, the CPU takes the
square root of the average value of the squares.
Waveform Sampling
ACVmeas
Sampling
Gate
Range Amp
63
Microprocessor
Fast
ADC
ADC and DMM
Display
DMM COMMON
®
Digital Sampling
Mathematically the rms value of samples in the sampling technique is
expressed as:
N
Vrms 
2
V
i
i 1
N
Where:
64
N =
Total number of samples
Vi =
i th voltage sample
i
sample number such that 1 i  N
=
®
Resistance Converters
65
®
Resistance Converters
Two different resistance conversion schemes are commonly employed in
today’s DMMs
The constant current method
Ratio method
66
®
Constant current method of Resistance
Conversion
67
®
Constant current method of Resistance
Conversion
In the constant current method, constant current sources, are built into the
DMM.
DMM
19.9992
Range Amp
ADC
Precision
Constant
Current
Source in
DMM
Rmeas
DMM COMMON
68
The DMM measures the voltage
across this resistor.
This constant current source is
applied through Rmeas, the unknown
resistor.
®
Ratio Resistance Measurement
69
®
Ratio Resistance Measurement
Vref from Rstd
S2
S3
Rstd
Floating
Current
Source in
DMM
Rin
Buffer
Amp
Cint
Rmeas
Internally, an approximately
known current is applied through
Rmeas, the unknown resistor.
S1
Vmeas from Rmeas
It is connected in series to a
known reference resistor Rstd.
A simple
voltage
70 divider is
formed.
0V
Vmeas from Rmeas
Operation
Integrator
Vref from Rstd
Zero Volts
Comparator
®
Ratio Method of Resistance Conversion
The unknown resistance value can be determined by solving:
Rmeas
71
 Vmeas 

 Rstd  
 Vstd 
®
Current Converters
72
®
DMM Current Converters
+
Iin
R1
RShunt
R2
Shunt Current Converter
E out 
Either the dc or ac voltage section
senses the voltage across the
shunt
Iin  R shunt  (R1  R 2 )
R2
Two methods are commonly used for DMM Current Converters
73
®
DMM Current Converters
+
Ifb
Iin
R1
-
E out
-
Iin
R1
+
RShunt
R2
Feedback Current Converter
An unknown current flows to an
operational amplifier circuit which
generates a proportional output
voltage
Shunt Current Converter
E out 
Iin  R shunt  (R1  R 2 )
R2
E out  Iin  R f
Two methods are commonly used for DMM Current Converters
74
®
DMM Current Converters
+
Ifb
Iin
R1
-
E out
-
Iin
R1
+
RShunt
R2
Feedback Current Converter
Shunt Current Converter
E out 
Iin  R shunt  (R1  R 2 )
R2
E out  Iin  R f
Two methods are commonly used for DMM Current Converters
75
®
General Calibration Requirements
Regardless of the type of the DMM, calibration adjustments are performed
to reduce instability in the offset, gain and linearity of the transfer functions
of the signal processing units
Each of the functional blocks in a DMM’s measurement section is subject to
these sources of variation in performance
76
®
Theory of Calibration Adjustment
Assume that all of the DMM’s signal processing blocks have a transfer function
of:
y = mx + b
Where:
y = the output
m = the gain
x = the input
b = the offset
77
®
Theory of Calibration Adjustment
y = mx + b
The DMM is designed in such
way that ‘m’ has an exact nominal
value, such as 10, and ‘b’ should
be zero (i.e.; no zero offset).
When ‘m’ is not equal to its
nominal value, all values of ‘y’
will deviate from nominal by
the same percentage.
78
®
Theory of Calibration Adjustment
y = mx + b
When ‘b’ is not equal to zero, all
values of ‘y’ will be offset from
their correct value by a constant
amount.
79
®
Contributions to the Spec
Display
+ Full Reading
0
- Full Reading
0
Input
80
±%R ±%FS ±µV
®
Contributions to the Spec
Display
+ Full Reading
0
- Full Reading
0
Input
81
Even though ‘m’ and ‘b’
deviate from their nominal
values, the slope of ‘y’, as a
function of ‘x’, should be a
straight line.
±%R ±%FS ±µV
®
Theory of Calibration Adjustment
A DMM operates under this
y = (mx + b)
relationship
82
®
Theory of Calibration Adjustment
It takes the unknown input x
(whether in voltage, current or resistance)
and converts it into y
(in Volts, Amperes or Ohms)
83
®
Theory of Calibration Adjustment
Voltmeter specifications are based primarily on offset, gain and linearity
The % of reading specification is the gain uncertainty
“Zero” uncertainty is due to voltage offsets in the ADC amplifiers and
comparators, specified in the specifications as the range error, or the floor
Linearity uncertainty is due to the secondary error sources, such as
mismatches in resistive ladders of the ADC, or dielectric absorption errors
by storage capacitors
84
®
Theory of Calibration Adjustment
In addition to these sources of uncertainty, some DMMs have spurious
voltage spikes, DC bias currents, and pump-out currents present at their
input terminals
These can introduce errors in the measurements
85
®
Internal References
Every DMM has a voltage reference
The DC voltage is used as the reference for the ADC and is the limiting
factor for the DMM’s best accuracy for all the voltage and current
measurements, and therefore also the resistance measurements
There are also one or more resistance references that determine the
accuracy of the resistance converter
86
®
Direct Voltage Reference
+ 15 V Power
+7 V Main
Reference
- 1 Gain
0 Volts
REF
AMP
-7 V Main
Reference
- 15 V Power
87
®
Resistance Reference
Reference
Current
+
-
54.6k
- 10V
From Precision
Voltage Reference
88
Precision resistor along with precision dc
voltage gives a known resistance current.
®
DC Voltage Calibration Strategy
Functional Tests
Prior to starting the calibration process:
Run Self – Test if available
Check each function and range using a copper short or known
voltage or resistance or current
89
®
DC Voltage Calibration Strategy
A Typical DC Voltage Calibration Sequence
90
10 V Zero,
+10 V Gain, -10 V Gain
10 V Linearity:
±1 V, ±5 V, ±15 V, ±19.9 V
1 V Zero,
+1 V Gain, -1 V Gain*
100 mV Zero,
+100 mV Gain, -100 mV Gain*
100 V Zero,
+100 V Gain, -100 V Gain*
1 kV Zero,
+1 kV Gain, -1 kV Gain*
®
DC Voltage Calibration Strategy
A Typical DC Voltage Calibration Sequence
The 10V range is the first one
measured.
91
10 V Zero,
+10 V Gain, -10 V Gain
10 V Linearity:
±1 V, ±5 V, ±15 V, ±19.9 V
1 V Zero,
+1 V Gain, -1 V Gain*
100 mV Zero,
+100 mV Gain, -100 mV Gain*
100 V Zero,
+100 V Gain, -100 V Gain*
1 kV Zero,
+1 kV Gain, -1 kV Gain*
®
DC Voltage Calibration Strategy
A Typical DC Voltage Calibration Sequence
92
10 V Zero,
A short is used to establish
the zero
+10 V Gain, -10 V Gain
10 V Linearity:
±1 V, ±5 V, ±15 V, ±19.9 V
1 V Zero,
+1 V Gain, -1 V Gain*
100 mV Zero,
+100 mV Gain, -100 mV Gain*
100 V Zero,
+100 V Gain, -100 V Gain*
1 kV Zero,
+1 kV Gain, -1 kV Gain*
®
DC Voltage Calibration Strategy
A Typical DC Voltage Calibration Sequence
93
10 V Zero,
+10 V Gain, -10 V Gain
10 V Linearity:
1 V Zero,
The
of 1 V and
±1 V, ±5 V, ±15
V,lower
±19.9ranges
V
100 mV would be measured
next
+1 V Gain, -1 V Gain*
100 mV Zero,
+100 mV Gain, -100 mV Gain*
100 V Zero,
+100 V Gain, -100 V Gain*
1 kV Zero,
+1 kV Gain, -1 kV Gain*
®
DC Voltage Calibration Strategy
A Typical DC Voltage Calibration Sequence
94
10 V Zero,
+10 V Gain, -10 V Gain
10 V Linearity:
±1 V, ±5 V, ±15 V, ±19.9 V
1 V Zero,
+1 V Gain, -1 V Gain*
100 mV Zero,
Then the ranges of 100 V and
+100 mV Gain, -100
mV
Gain*be measured
1 kV
would
100 V Zero,
+100 V Gain, -100 V Gain*
1 kV Zero,
+1 kV Gain, -1 kV Gain*
®
DC Voltage Calibration Strategy
A Typical DC Voltage Calibration Sequence
10 V Zero,
+10 V Gain, -10 V Gain
10 V Linearity:
±1 V, ±5 V, ±15 V, ±19.9 V
1 V Zero,
+1 V Gain, -1 V Gain*
100
mV
+100 mV Gain, -100 mV Gain*
To
test
forZero,
linearity errors in the
range amplifier, the voltage is
applied
at both + and – range
100 V Zero,
+100 V Gain, -100 V Gain*
limits
95
1 kV Zero,
+1 kV Gain, -1 kV Gain*
®
AC Voltage Calibration Strategy
The AC functions have the added dimension of frequency.
This complicates calibration by introducing additional test points for each
amplitude range.
The AC converter module must provide gain and attenuation for a wide
range of signals from typically a few millivolts to 1 kV
The gain of a DMM's AC function will vary with frequency.
This is known as its Frequency Response and requires that
measurements are made at key points throughout each amplitude
range.
96
®
AC Voltage Calibration Strategy
Frequency Response
97
®
AC Voltage Calibration Strategy
Frequency Response
From 40 Hz to 5 kHz, the DMM's gain is
determined by resistor networks, its AC
to DC converter and DC reference
98
®
AC Voltage Calibration Strategy
Frequency Response
At higher frequencies, reactive effects,
primarily capacitive, will determine the
flatness of the DMM's frequency
response
99
®
AC Voltage Calibration Strategy
A Typical AC Voltage Calibration Sequence
100
1 V AC Zero,
±1 V DC Turnover*
Crest Factor:
(at 3:1 or 5:1 as required)*
1 V LF Gain,
1 V HF Gain, Check Frequency Response*
1 V LF Linearity,
1 V HF Linearity*
100 mV Zero,
response*
100 mV LF, 100 mV HF, Check Frequency
10 V Zero,
10 V LF, 10 V HF, Check Frequency Response*
100 V Zero,
Response*
100 V LF, 100 V HF, Check Frequency
1 kV Zero,
1 kV LF, 1 kV HF, Check Frequency Response*
*These adjustments will usually be iterative. Always follow the manufacturer's
recommendations for the adjustment sequence.
®
AC Voltage Calibration Strategy
A Typical AC Voltage Calibration Sequence
1 V AC Zero,
±1 V DC Turnover*
Crest Factor:
(at 3:1 or 5:1 as required)*
1 V LF Gain,
1 V HF Gain, Check Frequency Response*
1 V LF Linearity,
1 V HF Linearity*
100 mV Zero,
response*
10 V Zero,
100 V Zero,
Response*
1 kV Zero,
101
The first adjustment is DC
Turnover, where the gain of the
100 mV LF, 100 mV HF,
Check
Frequency
precision
rectifier
is adjusted to
be identical for both positive and
negative
excursions
of the input
10 V LF, 10 V HF, Check
Frequency
Response*
signal
100 V LF, 100 V HF, Check Frequency
1 kV LF, 1 kV HF, Check Frequency Response*
*These adjustments will usually be iterative. Always follow the manufacturer's
recommendations for the adjustment sequence.
®
AC Voltage Calibration Strategy
A Typical AC Voltage Calibration Sequence
1 V AC Zero,
±1 V DC Turnover*
Crest Factor:
(at 3:1 or 5:1 as required)*
1 V LF Gain,
1 V HF Gain, Check Frequency Response*
1 V LF Linearity,
1 V HF Linearity*
100 mV Zero,
response*
10 V Zero,
102
The second is Crest Factor,
which
canFrequency
be affected by non100 mV LF, 100 mV HF,
Check
symmetry of the rectifier and also
by the bias arrangements of the
10 V LF, 10 V HF, Check Frequency Response*
analog multiplier
100 V Zero,
Response*
100 V LF, 100 V HF, Check Frequency
1 kV Zero,
1 kV LF, 1 kV HF, Check Frequency Response*
*These adjustments will usually be iterative. Always follow the manufacturer's
recommendations for the adjustment sequence.
®
AC Voltage Calibration Strategy
A Typical AC Voltage Calibration Sequence
1 V AC Zero,
±1 V DC Turnover*
Crest Factor:
(at 3:1 or 5:1 as required)*
1 V LF Gain,
1 V HF Gain, Check Frequency Response*
1 V LF Linearity,
1 V HF Linearity*
100 mV Zero,
response*
AC zeroFrequency
measurements are
100 mV LF, 100 mV HF, Check
10 V Zero,
100 V Zero,
Response*
1 kV Zero,
103
often made with a short-circuit
applied to the DMM's input, but
10 V LF, 10 V HF, Check
Frequency
Response*
some
instruments
will require a
smallFrequency
AC bias voltage because
100 V LF, 100 V HF, Check
the RMS converter cannot
operate with a zero input
1 kV LF, 1 kV HF, Check Frequency Response*
*These adjustments will usually be iterative. Always follow the manufacturer's
recommendations for the adjustment sequence.
®
Resistance Calibration Strategy
The accuracy of the resistance function is dependent upon both the
resistance current source and the DC voltage ranges of the DMM
The resistance function is usually measured after the DC voltage function
Changes in the performance of the either the current defining resistors or
voltage gain-defining resistors will affect the resistance calibration
For sensitive long-scale DMMs, it is usually better to start with the high
resistance ranges and then work down. This is will allow a longer thermal
stabilization time for the connecting leads and the DMM's internal circuits
104
®
Resistance Calibration Strategy
105
®
Resistance Calibration Strategy
The resistance function of a DMM consists
primarily of a constant current source providing
a range of currents typically from 10 nA to 10
mA.
106
®
Resistance Calibration Strategy
Selecting a resistance range selects an
appropriate constant current to pass through the
unknown or standard resistance.
107
®
Resistance Calibration Strategy
The voltage developed across the resistance is
then measured by the DMM's DC voltage
function
108
®
Resistance Calibration Strategy
Resistance Linearity Tests
For DMMs with a resistance function accuracy of not better than 100ppm
(4½-5½ digit resolution), a decade resistor box could be used
To measure linearity on the 10 kW range of a 7½ digit DMM with a
maximum indication of 19.000 000 kW would require several different
resistance standards
109
®
Resistance Calibration Strategy
If the DC Voltage range is linear, then the
resistance ranges are also linear
The
resistance
linearity can
be checked
by inserting a
resistance in
series with the
I+ lead and
verify the
current source
can deliver
the same
current
through a
range of
resistance
110
values
.
®
Current Calibration Strategy
Therefore
changes
in the DC
or AC 100
mV range
performa
nce will
also
affect the
current
ranges
From Fig 10-14
112
• The current ranges are linked together.
• The shunts are connected in series with suitable
tapping points for the voltage sensing
• To measure 0.1 mA, the current is passed through
all of the shunts in series and their values will be
chosen such that the sum of their resistive values
for any particular current range will develop 100
mV for the specified current
The voltage developed across the shunts will
be passed to the DC or AC 100 mV ranges
depending upon whether the DCI or ACI
function has been selected.
All of the shunts included will have an
affect on that range's performance
®
Current Calibration Strategy
A Typical Current Calibration Sequence
DC Current Calibration Sequence
100µA Zero,
+100µA,
-100µA
1mA Zero,
+1mA,
-1mA
10mA Zero,
+10mA,
-10mA
100mA Zero,
+100mA*,
-100mA*
1A Zero*,
+1A*,
-1A*
Minimize any heating
effects by measuring the
lowest current ranges first
*These measurements may require a longer settling time
due to self-heating and thermo-electric effects.
113
®
Current Calibration Strategy
A Typical Current Calibration Sequence
AC Current Calibration Sequence
100µA LF Gain, 100µA HF Gain**
1mA LF Gain, 1mA HF Gain**
Minimize any heating effects
by measuring the lowest
current ranges first
10mA LF Gain, 1mA HF Gain**
100mA LF Gain, 1mA HF Gain**
1A LF Gain, 1A LF Gain**
** Some DMMs do not have an HF adjustment
114
®
Zero Considerations
• All DC functions will need their zero offsets evaluated and
compensated before measurements of the gain values are
made
• Zero is only a baseline for the measurements and it is the gain
values that transfer traceability
• It is important that all DC measurements are referred to a
known offset state, but that state does not have to be exactly
zero
• An offset will affect all readings by a fixed amount
115
®
Zero Considerations
• It is very important that a "system" zero is performed when a DMM is being
calibrated.
-
A system zero means zeroing the DMM to the zero offset of the
calibration standard.
• For voltage measurements, the DMM should be zeroed to the source
"zero" output.
• For resistance measurements, the DMM should be zeroed to the resistor
such that the effects of Voltage offsets in the Hi and Lo circuit are removed.
• Current measurements require the DMM to be zeroed in the open-circuit
state, although some calibration sources have a "zero" current output to
which the DMM should be zeroed.
116
®
DMM Measurement Uncertainty
Measurement Uncertainty Contributions
• Uncertainty of the Standard
• Resolution of the DMM
• Short-Term Stability of the DMM with Time and Temperature
• Combined Noise of Standard and DMM
• The Calibration Procedure
117
®
DMM Uncertainty
Uncertainty of the Standard
• It is imperative that the calibration standard is of
sufficient accuracy to be able to calibrate the DMM
with confidence
• Sometimes Test Accuracy Ratios (TAR) are quoted as a
requirement
i.e. a TAR of 4:1 requires the accuracy of the
standard to be 4 times better than the specification of the DMM
118
®
DMM Uncertainty
The Resolution or Scale
• The resolution of the DMM could become a limiting factor in the
measurement regardless of how accurate the calibration standard might
be
Short-Term Stability
• The short-term stability of the DMM (and the calibration standard) with
time and temperature will also affect the uncertainty of measurement
• Usually the dominant factor is the temperature coefficient of the DMM and
stability of the calibration environment
• Secular drift is not usually significant unless the calibration takes several
hours or the instrument is defective.
119
®
DMM Uncertainty
The Combined Noise of the Standard and DMM
• The combined effect of the noise of the standard and the DMM can
be an important factor
• If individual readings can be easily observed, and if the noise is
predominantly random, the sample standard deviation of the
readings can be calculated and used as the uncertainty contribution
for combined noise.
120
®
DMM Uncertainty
The Calibration Procedure
• A poorly chosen test sequence, insufficient settling
time or poor interconnection techniques will all introduce
additional errors that may pass unnoticed by the operator
• The manufacturer's recommended procedure should be used
as the basis for DMM calibration. Note that this also applies
to the use of the calibration standards.
121
®
Understanding Basic DMM Calibration
For Electrical
Calibration Sales
Congratulations – you have completed this section of ECAL
Competency Training
122