1. 5.6. Disturbances: interference noise
5. SOURCES OF ERRORS Disturbances: interference noise
5.6. Disturbances: interference noise
Measurement errors can occur due to the undesirable interaction between the measurement system and:
the environment.
E n v i r o n m e n t
Disturbance
+Dy
+D x x y 1
Measurement
Object
Matching
Measurement
System
Matching
Observer
Influence
Influence

2. Sd = ___  ___ . x=0 x=0 d y = Sd d d
5. SOURCES OF ERRORS Disturbances: interference noise
There are two types of disturbances:
additive disturbance,
multiplicative disturbance.
To quantify the effect of additive disturbances on the measurement system, the disturbance sensitivity (or sensitivity factor) is used:
d y
d d Dy Dd
Sd = ___
 ___ .
x=0
x=0
Disturbance, d
x=0
Measurement
System Dy
d y = Sd d d

3. Sd Sx xeq = ___ Sx = ___ . d y = Sx xeq
5. SOURCES OF ERRORS Disturbances: interference noise
Additive disturbances can be written as the equivalent disturbing input signal Sd Sx
xeq = ___
Dd ,
where Sx is the sensitivity of the measurement system:
d y
d x
Sx = ___ .
Disturbance, d
x + xeq
Measurement
System Dy
d y = Sx xeq

4. Sx dy = (Cd d d )·x Cd = ______ 106 [ppm/ d ].
5. SOURCES OF ERRORS Disturbances: interference noise
Multiplicative disturbances affect the sensitivity Sx of the measurement system.
Disturbance, d Dy x Sx
dy = (Cd d d )·x
Measurement system
To quantify the effect of multiplicative disturbances, the disturbance coefficient (or influence function) is used:
 Sx / Sx
 d
Cd = ______
106 [ppm/ d ].

5. the supply voltage sensitivity SV [Dy/V],
5. SOURCES OF ERRORS Disturbances: interference noise
Nomenclature: When dealing with a specific disturbances, their names are used in the just described characteristics of measurement systems.
For example, we speak of
the supply voltage sensitivity SV [Dy/V],
and the temperature coefficient CT [ppm/º],
of a measurement system.

6. Example 1: Supply voltage sensitivity SV
5. SOURCES OF ERRORS Disturbances: interference noise
Example 1: Supply voltage sensitivity SV
DVCC
VS =0
DC-voltage
null detector
DVout
DVout = Sd DV Sd Sx
VS +Veq
DC-voltage
null detector
DVout
Veq = ___ DV
DVout = Sx Veq

7. ______ 106 [ppm/º] __________ Example 2: Temperature coefficient CT
5. SOURCES OF ERRORS Disturbances: interference noise
Example 2: Temperature coefficient CT T1
RG 1 VS
Instrumentation
amplifier
Vout 1
 G/G
 T
CT =
______ 106 [ppm/º] T2
RG 2
Vout 2 - Vout 1
Vout 1
__________
dVout = VS
Instrumentation
amplifier
Vout 2
dVout = (CT DT )·VS

8. 5.2.1. Reduction of the influence of disturbances
5. SOURCES OF ERRORS Disturbances: interference noise Reduction of the influence of disturbances
Reduction of the influence of disturbances
1. Isolate the measurement system. For example, use electro-magnetic shielding, stabilize the ambient temperature, etc.
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.

9. d Sd 1 = - Sd 2 x y d S1 Cd 1 = - S2 Cd 2 d d Sd 1 S2 = - Sd 2 x y
5. SOURCES OF ERRORS Disturbances: interference noise Reduction of the influence of disturbances
Example
Compensation d
parallel S1
Sd 1 = - Sd 2 x y d
S1 Cd 1 = - S2 Cd 2 S2
series d d
Sd 1 S2 = - Sd 2 x y
Object
Sensor S1 S2
Cd 1 = - Cd 2 d
ratio S1
Any ratio measurement
system
not effective x y d
Cd 1 = Cd 2 S2

10. DT x y DT x y 5. Use feedback against multiplicative disturbances. SOL
5. SOURCES OF ERRORS Disturbances: interference noise Reduction of the influence of disturbances
5. Use feedback against multiplicative disturbances. DT x y
SOL DT x y
SOL b

11. _______ ________ ______ _______ _________ _________ _________ ____ SOL
5. SOURCES OF ERRORS Disturbances: interference noise Reduction of the influence of disturbances
SOL
1 + SOL b
_______
1. Sf =
DSOL /SOL DT
________
2. CT OL =
DSf /Sf DT
______
3. CT f = 1
1 + SOL b
SOL b
(1 + SOL b )2 1
(1 + SOL b ) 1
(1 + SOL b )
SOL
_______
_________
_________
_________
____
4. d Sf /d SOL = = 1
1 + SOL b
5. d Sf /Sf = _______ d SOL /SOL 1
1 + SOL b
6. CT f = _______ CT OL

12. 5. SOURCES OF ERRORS. 5. 6. Disturbances: interference noise. 5. 6. 1
5. SOURCES OF ERRORS Disturbances: interference noise 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.
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.)
Reference: [1]

13. 5.2.2. Sources of disturbances
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
Sources of disturbances
A. Thermoelectricity
Metal A
Junction at T1
V = ST (T1- T2 )
Metal B
Metal A
Junction at T2
Reference: [1]

14. Cu-Au 0.3 mV/º Cu-Kovar 500 mV/º Cu-Cd/Sn Cu-CuO 1000 mV/º
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances T1 T2 Cu
Pb/Sn
Kovar
Cu-Ag Cu-Pb/Sn 3 mV/º
Cu-Au 0.3 mV/º Cu-Kovar mV/º
Cu-Cd/Sn Cu-CuO mV/º
Thermoelectricity is an additive disturbance.
Reference: [1]

15. B. Leakage currents V2 - V1 RL IL = ________ V2 V1 1 cm (100 MW)
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
B. Leakage currents
1 cm
(100 MW) V2 V1
Leakage current, IL
V2 - V1 RL
IL = ________
Reference: [1]

16. IL = ________ = __________________ = ______ ____
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
Active guarding
Vout
AOL V1
Leakage current, IL
V1 - Vout
R’L
V1 - V1 AOL /(1+AOL)
R’L 1
1+AOL V1
R’L
IL = ________ = __________________ = ______ ____
“Leakage currents cause multiplicative errors as they give rise to an extra equivalent loading impedance.” (?)
Reference: [1]

17. C. Capacitive injection of interference
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
C. Capacitive injection of interference Cp ZS vS
220 V
50 Hz
Vin
Zin Vd
Cable
Measurement
system
Vin = vd jw Cp(ZSIIZin)
1/jw Cp >> ZSIIZin
(ZSIIZin)  Vin
Inductive injection of interference is an additive disturbance.
Reference: [1]

18. Vd Electrical shielding Cp ZS vS Zin ZS < Rin Shielded cable
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
Electrical shielding Cp ZS vS
220 V
50 Hz
Zin
Shielded cable Vd
Measurement
system
ZS < Rin
Reference: [1]

19. Vd Electrical shielding Cp ZS iS Zin Zin < ZS Shielded cable
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
Electrical shielding Cp ZS iS
220 V
50 Hz
Zin
Shielded cable Vd
Measurement
system
Zin < ZS
Reference: [1]

20. D. Inductive injection of interference
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
D. Inductive injection of interference ZS
i(t) VS Vd
Area, A
H(t)
Zin
Wire loop
Measurement
system VS Vd
Vd  A, d i/d t;
___  f(ZS ,Zin)
Inductive injection of interference is an additive disturbance.
Reference: [1]

21. Reduction of the wire loop area
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
Reduction of the wire loop area ZS
i(t) VS
H(t) Vd
Zin
Wire loop
Measurement
system
A  Vd 
Reference: [1]

22. Employment of twisted pair
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
Employment of twisted pair ZS
i(t) VS
H(t) Vd
Zin
Twisted pair
Measurement
system
Aeq  Vd 
Reference: [1]

23. Magnetic shielding i(t) ZS VS Zin
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
Magnetic shielding ZS
i(t) VS
Zin
Single-shell or multi-shell magnetic shield
Reference: [1]

24. E. Injection of interference by imperfect grounding
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
E. Injection of interference by imperfect grounding
Grounding the measurement object and the measurement system at different points on a ground rail causes additive voltage disturbances due to stray ground currents. ~ N ZS vS
Measurement
system vd
Istray1
Istray2 Rg
Istray
Reference: [1]

25. Single-point grounding helps to reduce the disturbances.
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
Single-point grounding helps to reduce the disturbances. ~ N ZS vS
Measurement
system vd
Istray1
Istray2 A B
Istray
Reference: [1]

26. 5. SOURCES OF ERRORS. 5. 6. Disturbances: interference noise. 5. 6. 2
5. SOURCES OF ERRORS Disturbances: interference noise Sources of disturbances
Differential input and shielded twisted pair further reduce the disturbances. ~ N ZS vS
Measurement
system
(CMRR) vd
Istray1
Istray2 Rg
Istray
Reference: [1]

27. 6. MEASUREMENT SYSTEM CHARACTERISTICS
6. MEASUREMENT SYSTEM CHARACTERISTICS General structure of a measurement system
6. MEASUREMENT SYSTEM CHARACTERISTICS
General structure of a measurement system -.

28. 6.2. Measurement system characteristics
6. MEASUREMENT SYSTEM CHARACTERISTICS Measurement system characteristics Sensitivity threshold
Measurement system characteristics
Sensitivity threshold -.

29. 6.2.2. Signal shape sensitivity
6. MEASUREMENT SYSTEM CHARACTERISTICS Measurement system characteristics Signal shape sensitivity
Signal shape sensitivity -.

30. 6. MEASUREMENT SYSTEM CHARACTERISTICS. 6. 2
6. MEASUREMENT SYSTEM CHARACTERISTICS Measurement system characteristics Resolution
Resolution -.

31. 6. MEASUREMENT SYSTEM CHARACTERISTICS. 6. 2
6. MEASUREMENT SYSTEM CHARACTERISTICS Measurement system characteristics Non-linearity
Non-linearity -.

32. 6. MEASUREMENT SYSTEM CHARACTERISTICS. 6. 2
6. MEASUREMENT SYSTEM CHARACTERISTICS Measurement system characteristics System response
System response -.

33. Thank you and good luck in the final exam!