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• Internal noise:
– Johnson noise (thermal)
[volts2/Hz] 4
– Shot noise (quantization)
2 [volts2/Hz] – Flicker noise (low frequency)
[volts2/Hz] • Noise through a system
2
ME365 Noise and Noise Rejection 27 Sources of Noise
External
• AC Interference
60/120/180/240 Hz sinusoidal interference due to power lines,
fluorescent lights, fans, etc. • Communication Interference
Radio/TV (1100 MHz), cordless phone, cellular communication
interference. • Switching Interference
Switching interference due to switching power supplies, relays,
lightning, arc welders, auto ignition systems, etc. • Mechanical/Structural Vibration
Normally under 20 Hz. ME365 Noise and Noise Rejection 28 Noise Spectrum ME365 Noise and Noise Rejection 29 Modes of Interference
• Series Mode Interference
Noise or interference voltage VSM “enters” the measurement system in
series with the measurement signal voltage ETH .
ZOUT VSM RC / 2
ZL ETH VL RC / 2
Source Load ⇒ SNR
ME365 Noise and Noise Rejection 20 log
30 Modes of Interference
• Common Mode Interference
The potentials on both sides of the signal circuit are raised by a
common voltage VCM relative to a common earth ground .
ZOUT RC / 2 ZL ETH
RC / 2
VCM VA Source VL = VA − VB VB
Load Differential measurement
⇒ ME365 Noise and Noise Rejection 31 Noise Coupling
• Shielding & Grounding
– Inductive Coupling
AC Power line in close proximity to
one of the measurement leads will
generate a series mode interference: where i is the power line current
M is the mutual inductance.
Solution:
(1) Physical separation ∝ 1 (2) Shorter cable length
(3) Twisted pair ME365 Noise and Noise Rejection 32 Noise Coupling
• Shielding & Grounding
– Capacitive Coupling
Induce both common mode and
series mode noise. VAC Solution:
(1) Physical separation
(2) Shielding Measurement Circuit Screen
(Shield)
GND ME365 Noise and Noise Rejection 33 Noise Coupling
• Shielding & Grounding
– Ground Loop ⇒ Forms loop TUPS 0 2 Solution:
Single Ground for all devices!!
using lowimpedance grounding 2
ME365 Noise and Noise Rejection ⇒ 2
34 Noise Rejection
• Filtering ME365 Noise and Noise Rejection 35 Noise Rejection
• Filtering 60 60
60
ME365 Noise and Noise Rejection 36 Noise Rejection
• Modulation
60 60 ME365 Noise and Noise Rejection 37 Noise Rejection
• Averaging
The S/N ratio of a periodic signal can be increased by taking
repeated measurements of the signal. 1,2,3, … Periodic signal 1st Volt Sample: 2nd Sample:
3rd Sample:
Nth Sample: 2
1
T Sum:
Average:
ME365 Noise and Noise Rejection Time 1
38 Noise Rejection
• Averaging
Average after N repeated measurements: 1
If nk(t) is zero mean, is zero mean
⇒ SNR [SNR]
SNR ⋅ N ⋅ [SNR] ME365 Noise and Noise Rejection 39 Noise Rejection  Common Mode Noise
• Differential Amplifier (Instrumentation Amplifier)
ZOUT Ri VA  ETH
Ri
VB
VCM RF + VOUT
RF Source Differential measurement
⇒ ME365 Noise and Noise Rejection 40 Noise Rejection  Common Mode Noise
More realistic Op Amp model:
If a common mode voltage VCM exists V  V+ ⋅ VO +
AOL Common Mode Rejection Ratio (CMRR)
CMRR Differential Gain
Common Mode Gain CMRR ⇓
V ⋅  VO + V+ VCM
CMRR AOL CMRR
ME365 Noise and Noise Rejection 41 Noise Rejection  Common Mode Noise
• Differential Amplifier considering Common Mode Rejection
VA Ri RF Ri ZOUT +  ETH VB
VCM VOUT RF Source 1 CMRR Ex: Let VA − VB = 1 mV, RF /Ri = 1000, VCM = 1V and CMRR = 10 4 , find VOUT . ME365 Noise and Noise Rejection 42 ...
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This note was uploaded on 12/26/2011 for the course ME 365 taught by Professor Merkle during the Fall '07 term at Purdue University.
 Fall '07
 MERKLE

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