lec29_filled - From last time… • Internal noise: –...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: From last time… • 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 (1-100 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 low-impedance 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 ...
View Full Document

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.

Ask a homework question - tutors are online