PSN-L Email List Message

Subject: Flicker noise
From: Tangazazen@.......
Date: Fri, 13 Aug 2010 08:46:29 EDT


Hi All,
 
 
This note concerns the flicker noise in pre-amplifiers which may be of  
interest to those who have a coil resistance that is greater than 5 K ohms.  
Chopper amplifiers such as the LMV2011 etc.are said to be free of flicker  
noise but the shot noise voltage is large at 35nv / root Hz. For a 5k ohm 
source  resistance the noise figure (N.F.) is about 12 db and for a band width of 
0.05  to 5 Hz gives a total noise of about 100 nv. 
The LT1007 op amp has a flicker noise voltage level of 10-17 volts squared  
/ f and a flicker noise current of 2*10-23 amps squared / f , this gives an 
 optimum source resistance of about 700 ohms and a N.F. of 15.5 db at 0.05 
Hz,  though at 5 K ohms this rises to 21 db. This particular IC has two 
pairs of  transistor and 6 noise current sources which give it the high current 
noise  value and are unchangeable.
The Hitachi transistor 2SC2545 noise performance  is very low indeed with 
regard to flicker noise and the data is given as N.F.  contour on a log of 
collector current versus log of source impedance graph at  1Kc/s and 10 c/s.  
It has been possible to extract the principle flicker  coefficients from 
this for a simple model. The flicker noise voltage remains  constant but the 
flicker noise current depends on the collector current squared  and at a 
collector current of 0.055 ma, gives an optimum source  impedance equal to 5K 
ohms. A pair of these transistors  would give a  spot noise at 0.05 Hz equal to 
the chopper amplifier but the total noise  for a 0.05 to 5 c/s bandwidth of 
about 1/6.  This bandwidth is calculated  for a simple RC high and low pass 
filter.
It may appear a small improvement  but every bit may help. 


Martin





Hi All,
 
 
This note concerns the flicker noise in pre-amplifiers which may be= of=20 interest to those who have a coil resistance that is greater than 5 K ohms= ..=20 Chopper amplifiers such as the LMV2011 etc.are said to be free of fli= cker=20 noise but the shot noise voltage is large at 35nv / root Hz. For a 5k ohm= source=20 resistance the noise figure (N.F.) is about 12 db and for a band width of= 0.05=20 to 5 Hz gives a total noise of about 100 nv.
The LT1007 op amp has a flicker noise voltage level of 10-17 volts sq= uared=20 / f and a flicker noise current of 2*10-23 amps squared / f , this gives= an=20 optimum source resistance of about 700 ohms and a N.F. of 15.5 db at 0.05= Hz,=20 though at 5 K ohms this rises to 21 db. This particular IC has two pairs= of=20 transistor and 6 noise current sources which give it the high current nois= e=20 value and are unchangeable.
The Hitachi transistor 2SC2545 noise perfor= mance=20 is very low indeed with regard to flicker noise and the data is given as= N.F.=20 contour on a log of collector current versus log of source impedance graph= at=20 1Kc/s and 10 c/s.  It has been possible to extract the principle flic= ker=20 coefficients from this for a simple model. The flicker noise voltage remai= ns=20 constant but the flicker noise current depends on the collector current sq= uared=20 and at a collector current of 0.055 ma, gives an optimum source=20 impedance equal to 5K ohms. A pair of these transistors  would= give a=20 spot noise at 0.05 Hz equal to the chopper amplifier but the total no= ise=20 for a 0.05 to 5 c/s bandwidth of about 1/6.  This bandwidth is calcul= ated=20 for a simple RC high and low pass filter.
It may appear a small improve= ment=20 but every bit may help.
 
Martin

[ Top ] [ Back ] [ Home Page ]