1 5. sources of errors. 5.6. disturbances: interference noise 5.6. disturbances: interference noise...
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15. SOURCES OF ERRORS. 5.6. Disturbances: interference noise
5.6. Disturbances: interference noise
Measurement errors can occur due to the undesirable
interaction between the measurement system and:
E n v i r o n m e n t
Measurement System
Mat
chin
g
Mat
chin
g
Disturbance
yx
the environment,
the object under test,
observer.
y
25. SOURCES OF ERRORS. 5.6. Disturbances: interference noise
To quantify the effect of additive disturbances on the
measurement system, the disturbance sensitivity
(or sensitivity factor) is used:
d y
d dSd
x
Measurement System
Disturbance, d (VCC )
x 0 y
y
dx
.
additive disturbance,
multiplicative disturbance.
There are two types of disturbances (interference noise):
d y Sd d d
(SVCC y/V]supply voltage sensitivity)
35. SOURCES OF ERRORS. 5.6. Disturbances: interference noise
Additive disturbances can be written as the equivalent
disturbing input signal
Sd
Sx
xeq d,
where Sx is the sensitivity of the measurement system:
d y
d xSx .
Measurement System
x xeq y y d y Sx xeq
Disturbance, d
45. SOURCES OF ERRORS. 5.6. Disturbances: interference noise
Multiplicative disturbances affect the sensitivity Sx of the
measurement system.
xSx
Measurement system
To quantify the effect of multiplicative disturbances, the
disturbance coefficient is used:
d Sx / Sx
ddCd 106[ ppm /dd .]
y Cd d d ) · xy y
Disturbance, d (T )
Sx
dd
(CT ppm/]temperature coefficient)
55. SOURCES OF ERRORS. 5.6. Disturbances: interference noise
Example 1: Supply voltage sensitivity SVCC
VIN 0 Vout
VCC
DC-voltagenull detector
VIN Veq VoutDC-voltage
null detector
SVCC
SVINVeq V
Vout SVCC VCC
Vout SVIN Veq
6
G
d T
5. SOURCES OF ERRORS. 5.6. Disturbances: interference noise
Example 2: Temperature coefficient CT
VSVout 1
RG 1
Instrumentationamplifier, G
T1
VSVout 2
RG 2
Instrumentationamplifier, G
T2
106 ]ppm/º[ CT
Vout CT T ) ·VS
Vout 2 Vout 1
Vout 1
Vout
75. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.1. Reduction of the influence of disturbances
5.2.1. Reduction of the influence of disturbances
1. Isolate the measurement system. For example, use
electro-magnetic shielding, stabilize the ambient
temperature, etc.
2. Separate the effect of disturbances on the output of
measurement system to correct the measurements. For
example, suppress the input signal and measure the
output signal due to the additive disturbance only. Then
correct the measurements with the input signal applied.
3. Change the input signal in such away to avoid the
disturbance. For example, translate a dc signal into ac
one to avoid dc offset and drift and flicker noise.
4. Split the measurement system (or only its critical part)
into two parallel or series channels and use parallel,
series, or ratio compensation to compensate the
disturbance.
85. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.1. Reduction of the influence of disturbances
S1 S1
S2 S2
S1 S1
S2 S2
S1 S1 S2
S2
y
y
y
x
x
x
d
d
d d
d
d
ratio
series
parallel
S1 Cd 1 S2 Cd 2
Sd 1 Sd 2
Cd 1 Cd 2
Sd 1 S2 Sd 2
Cd 1 Cd 2
not effectiveAny ratio
measurement system
SensorSensorObjectObject
Example Compensation:
95. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.1. Reduction of the influence of disturbances
5. Use feedback against multiplicative disturbances.
SOL SOL
yx
T
SOL SOL yx
T
105. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.1. Reduction of the influence of disturbances
SOL
1 SOL 1. Sf
SOL /SOL
T2. CT OL
Sf /Sf
T3. CT f
1
1 SOL 4. dSfdSOL
SOL )1 SOL )2
1
)1 SOL ) 1
)1 SOL ) SOL
SOL
1
1 SOL 5. dSfSfdSOLSOL
1
1 SOL 6. CT fCT OL
115. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.1. Reduction of the influence of disturbances
Note that negative feedback reduces additive disturbances by
the same factor as it reduces the sensitivity of the system.
This means that the ratio of the measurement signal and the
disturbances (both referred to the output or the input) will not
change due to the application of feedback.
Reference: [1]
In the same way, the signal-to-noise ratio of the measurement
system will also not be improved by using negative feedback.
(It will be decreased due to the additional noise contribution by
the feedback network.)
SOL SOL
y+yx+xeq
125. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
5.2.2. Sources of disturbances
A. Thermoelectricity
Reference: [1]
Metal A
Metal A
Metal B
Junction at T1
Junction at T2
V ST )T1T2)
Thermoelectricity is an additive disturbance.
135. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
CuAg CuPb/Sn 3 V/º
CuAu 0.3 V/º CuKovar 500
V/º
CuCd /Sn CuCuO 1000
V/º
Reference: [1]
Cu Pb/Sn Kovar
T2T1
145. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
B. Leakage currents
Reference: [1]
1 cm )100 M)
Leakage current, IL
IL V2 V1
RL
V2 V1
15
V1
0.5RL
5. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
Active guarding
Leakage current, IL
AOL
IL
V1
Vout
V1 Vout
0.5RL
V1 V1 AOL /)1+AOL)
0.5RL
1
1+AOL
16
Measurement system
Zin
5. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
C. Capacitive injection of interference
Reference: [1]
ZS
vS
220 V 50 Hz
Cable
Cp
Vin
Vin Vd jCp)ZSIIZin)1/jCp >> ZSIIZin
Vd
)ZSIIZin)Vin
Inductive injection of interference is an additive disturbance.
175. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
Reference: [1]
ZS
vS
Measurement system
220 V 50 Hz
Shielded cable
Electrical shielding: grounding at the source
Cp
Zin
ZS < Zin
Vd
Prove that the grounding of the shield at the end of the cable
that is attached to the circuit with the lowest impedance keeps
as small as possible the interference voltage between the
shield and the signal conductors.
Home exercise:
185. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
Reference: [1]
ZS
iS
Measurement system
220 V 50 Hz
Shielded cable
Cp
Zin
Vd
Electrical shielding: grounding at the measurement sistem input
ZS > Zin
195. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
D. Inductive injection of interference
Reference: [1]
ZS
VS
i(t)
H(t)
Area, A
Zin
Measurement system
Wire loop
Inductive injection of interference is an additive disturbance.
Vd
f)ZS ,Zin)VS
Vd
VdA, d i/d t ;
205. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
Reference: [1]
ZS
VS
i(t)
Zin
Measurement system
Wire loop
Vd
AVd
H(t)
Reduction of the wire loop area
215. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
Reference: [1]
ZS
VS
i(t)
H(t)
Zin
Measurement system
Twisted pair
Vd
AeqVd
Employment of twisted pair
22
VS
5. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
Reference: [1]
ZS i(t)
Zin
Magnetic shielding
Single-shell or multi-shell magnetic shield
235. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
E. Injection of interference by imperfect grounding
Reference: [1]
1) Stray currents. Grounding the measurement object and the
measurement system at different points on a ground rail causes
additive voltage disturbances due to stray ground currents.
ZS
vS
Rg
Measurement system
~N
Istray
Istray1 Istray2 vd
245. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
Reference: [1]
Single-point grounding helps to reduce the disturbances.
ZS
vSMeasurement
system
~N
Istray
Istray1 Istray2 vd
Rg
255. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
Reference: [1]
Differential input and shielded twisted pair further reduce the
disturbances.
ZS
vS
Rg
Measurement system
)CMRR(
~N
Istray
Istray1 Istray2 vd
265. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
Reference: [2]
ZS
vS
2) Ground loops. If single-point grounding is impossible, ground
lops can be a significant source of interference noise:
The effect of multiple-point grounding can be minimized by
isolating the two circuits by: (1) transformers,(2) common-mode
chokes, (3) optical couplers, or (4) frequency-selective
grounding (hybrid grounds).
Ground loop)inductive injection of interference)
Measurement system
275. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
Reference: [2]
Isolation with:
ZS
vSMeasurement
system
Isolating device
(1) transformers (2) common-mode chokes (3) optical couplers
Common-mode current
Signal current
Balun )balanced, unbalanced signals)
285. SOURCES OF ERRORS. 5.6. Disturbances: interference noise. 5.6.2. Sources of disturbances
Reference: [2]
Isolation with: (4) frequency-selective grounding (hybrid
grounds) is used when the common-noise voltages are at very
different frequencies from the desired signal:
ZS
vSMeasurement
system
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