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 yd d
Sd x
Measurement System
Disturbance, d (VCC )
x 0 y
yd
x
.
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 yd x
Sx .
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-voltagenull detector
SVCC
SVINVeq V
Vout SVCC VCC
Vout SVIN Veq
6
Gd 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
S2
S1
S2
S1 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
SensorObject
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
yx
T
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
11 SOL
4. dSfdSOLSOL
)1 SOL )2
1)1 SOL )
1)1 SOL )
SOL
SOL
11 SOL 5. dSfSfdSOLSOL
11 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
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/ºCuAu 0.3 V/º CuKovar 500 V/ºCuCd /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
11+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
vS220 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
vS Measurement 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
vS Measurement 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
vS Measurement system
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