Chopping: a technique for noise and offset reduction

Prev TOC Next

5.5. Chopped amplifiers and offset reduction

We have analyzed so far the effect of chopper modulation on white noise and 1/f noise. In order to be able to process signals without changing the baseband information, we have to modulate signals and noise differently. The principle of chopper amplifiers is illustrated in fig.5.6. For simplicity the 1/f noise has been represented on a logarithmic scale. The input signal is multiplied with a rectangular signal m(t) with unity amplitude and 50% duty-cycle. As a result, the signal is once modulated at odd harmonics of the chopper frequency. The signal will be amplified and/or filtered, modulated back, leaving spectral contributions at even harmonics of the chopper frequency.

The amplitude of the modulation signal decreases with 1/n where n is the harmonic number. Offset and 1/f noise are modulated at odd harmonics leaving the baseband free of 1/f noise. In the ideal chopping case, the bandwidth of the amplifier should be infinity. As long as this is true, multiplying the signal twice with m(t) will reconstruct the input signal ideally. If the bandwidth of the amplifier is limited, the result is a high frequency residue centered around the even harmonics and the signal in the baseband is attenuated.

To recover the signal, the output has to be low-pass filtered as shown in fig.5.7. Given the corner frequency of the 1/f noise fcorner and the cutoff frequency of the low-pass filter at the output, BWsignal the necessary condition to have complete reduction of the flicker noise in the baseband is found from:



Fig.5.6: The chopper technique


Fig.5.7: The baseband spectrum

To analyze the effect of chopping on the offset of the amplifier, the offset has been represented in fig.5.6 at the input of the amplifier A. As long as the frequency response of the amplifier is flat, the output voltage Vout(f) is found from the following convolution:


This sequence of Dirac pulses has no DC component and the offset at the output has a theoretical value of 0V. Obviously, any temperature drift of the offset voltage is also cancelled out after chopper modulation.



Featured Video
Mid-Level Mechanical Engineer for Kiewit at lenexa, Kansas
Mechanical Engineer for Allen & Shariff Corporation at Pittsburgh, Pennsylvania
System Designer/Engineer for Bluewater at Southfield, Michigan
Director of Process Engineering. for Tekni-Plex at Toledo, Ohio
GIS - Systems Analyst for City of Olathe at Olathe, Kansas
Senior Structural Engineer for Wiss,Janney, Estner Assoicates, Inc at houston, Texas
Upcoming Events
ACE 2018 Conference at The Westin Indianapolis 241 W Washington St Indianapolis IN - Mar 20 - 22, 2018
ESPRIT World 2018 at Indianapolis Marriott Downtown 350 West Maryland Street Indianapolis IN - Jun 11 - 15, 2018
HxGN LIVE 2018 at The Venetian Las Vegas NV - Jun 12 - 15, 2018
IMTS2018 International Manufacturing Tech Show at McCormick Place Chicago IL - Sep 10 - 15, 2018
Kenesto: 30 day trial

Internet Business Systems © 2018 Internet Business Systems, Inc.
25 North 14th Steet, Suite 710, San Jose, CA 95112
+1 (408) 882-6554 — Contact Us, or visit our other sites:
AECCafe - Architectural Design and Engineering EDACafe - Electronic Design Automation GISCafe - Geographical Information Services TechJobsCafe - Technical Jobs and Resumes ShareCG - Share Computer Graphic (CG) Animation, 3D Art and 3D Models
  Privacy PolicyAdvertise