ECEN474: (Analog) VLSI Circuit Design Fall 2011

Size: px

Start display at page:

Download "ECEN474: (Analog) VLSI Circuit Design Fall 2011"

Ruby Bond
5 years ago
Views:

1 ECEN474: (Analog) LSI Circuit Design Fall 011 Lecture 1: Noise Sebastian Hoyos Analog & Mixed-Signal Center Texas A&M Uniersity

2 Announcements Reading Razais CMOS Book Chapter 7

3 Agenda Noise Types Noise Properties Resistor Noise Model Diode Noise Model 3

4 Noise Significance Why is noise important? Sets minimum m signal leel el for a gien performance e parameter Directly trades with power dissipation and bandwidth d Reduced supply oltages in modern technologies degrades noise performance Signal Power dd SNR P sig / P Noise is often proportional to kt/c noise dd noise Increasing capacitance to improe noise performance has a cost in increase power consumption for a gien bandwidth 4

5 Interference Noise Interference Man-Made Noise Deterministic signal, i.e. not truly random Could potentially be modeled and predicted, but practically this may be hard to do Examples Power supply noise Electromagnetic interference (EMI) Substrate coupling Solutions Fully differential circuits Layout techniques Not the focus of this lecture Unless the deterministic noise is approximated as a random process 5

6 Inherent Noise Electronic or Deice Noise Random signal Fundamental property of the circuits Examples Thermal noise caused by thermally-excited random motion of carriers Flicker (1/f) noise caused by material defects Shot noise caused by pulses of current from indiidual carriers in semiconductor junctions Solutions Proper circuit topology More power!!! Is the focus of this lecture 6

7 Noise Properties Noise is random [Johns] Instantaneous t noise alue is unpredictable and the noise must be treated statistically Can only predict the aerage noise power Model with a Gaussian amplitude distribution Important properties: mean (aerage), ariance, power spectral density (noise frequency spectrum) 7

8 RMS alue If we assume that the noise has zero mean (generally alid) RMS or sigma alue is the square-root of the noise ariance oer a suitable aeraging time interal, T T 1 1 nrms n t dt T 0 Indicates the normalized noise power, i.e. if n (t) is applied to a 1 resistor the aerage power would be nrms P n n 1 rms 8

9 Signal-to-Noise Ratio (SNR) SNR signal power 10log noise power For a signal with normalized power of x rms SNR 10log 0log n rms nrms x rms x rms Quantified in units of db 9

10 Thermal Noise Spectrum The power spectral density (PSD) quantifies how much power a signal carries at a gien frequency Thermal noise has a uniform or white PSD The total aerage noise power P n in a particular frequency band is found by integrating ti the PSD P n f f 1 PSD f df For white noise spectrum : P n f f n f n

11 Thermal Noise of a Resistor The noise PSD of a resistor is f PSD n 4kT where k is the Boltzmann constant and T is the absolute 0 temperature (K) The total aerage power of a resistor in a gien frequency band is P n f f f kt f 4 ktdf 4 kt 4 1 f 1 Example: f=1hz P =4x10-1 n W=-174dBm 11

12 Resistor Noise Model An equialent oltage or current generator can model the resistor thermal noise 4kT R n Rn Pn R 4kTRf f Rn I P R [Johns] h Recall the PSD is white (uniform w/ frequency) 1

13 Noise Summation no t t t no n 1 n 1 T T rms n1 t n t T no T 0 0 dt rms n 1rmsnrms n 1 t nt Same procedure applies to noise current summing at a node dt 13

14 Correlation Last term describes the correlation between the two signals, defined d by the correlation coefficient, C C T 1 T 0 n1 n1 t n t rms nrms no C rms n1rms nrms n1rms n rms Correlation always satisfies -1 C 1 C=+1, fully-correlated in-phase (0) C=-1, fully-correlated out-of-phase o (180) C=0, uncorrelated (90) dt 14

15 Uncorrelated Signals For two uncorrelated signals, the mean- squared sum is gien by no Add as though they were rms n1rms nrms ectors at right angles For two fully correlated signals, the meansquared sum is gien by no rms n1rms nrms Sign is determined by phase relationship RMS alues add linearly (aligned ectors) 15

16 Noise Example #1: Two Series Resistors n C rms n1rms nrms n1rms n rms The noise of the two resistors is uncorrelated or statistically independent, so C=0 rms n 1 rms n rms 4kT R 1 R f n Always add independent noise sources using mean squared alues Neer add RMS alues of independent sources 16

17 TAMU-ELEN Noise Example #: oltage Diider id Jose Sila-Martinez Lets compute the output oltage: Apply superposition (noise sources are small signals, you can use small signal models)! n1 R n in R - R R1 R R R1 R R1 R1 R 0 in n1 n Aboe is what you do for deterministic signals, but we cannot do this for the resistor noise But noise is a random ariable, power noise density has to be used rather than oltage; then the output referred noise density (noise in a bandwidth of 1 Hz) becomes 0n R R1 R n1 R1 R1 R R R1 0 4kTR1 R1 R R1 R n n -17-4kTR f H s f on, T x General Case : x s jf x

18 Diode Noise Model Shot noise in diodes is caused by pulses of current from indiidual carriers in semiconductor junctions White spectral density [Johns] Where q=1.6x10-19 C and I D is the diode DC current 18

19 Next Time Noise in MOSFETs Noise Analysis 19

ECEN474/704: (Analog) VLSI Circuit Design Spring 2016

ECEN474/704: (Analog) VLSI Circuit Design Spring 2016 ECEN474/704: (Analo) LSI Circuit Desin Sprin 016 Lecture 11: Noise Sam Palermo Analo & Mixed-Sinal Center Texas A&M Uniersity Announcements HW3 is due today HW4 is due Mar 30 Exam is on Apr 4 9:10-10:35PM