Chopping: a technique for noise and offset reduction

Prev TOC Next

5.4. Noise modulation

From the previous paragraph we have seen the modulation effects of chopping on a input process x(t). It is important to notice the difference between a narrow-band process and a broadband process after chopper modulation. The input stationary random process x(t) can be white noise or 1/f noise applied to the amplifier A(f) which has a band limiting effect on the noise.

5.4.1. White noise modulation

For the beginning, assume that x(t) is a broadband random process white noise like with a power spectral density Swhite. In order to simplify the analysis, the gain of the amplifier is taken to be 0dB and the frequency transfer has a first order behavior:


The power spectral density of the noise at the output can be found from (5.11) with assumption that Sxx(f)=Swhite:


The series from (5.13) can be computed by using Poisson summation rule [9]. For large values of fLPT (fLPT>>1) we get:


In conclusion, the chopper modulator has a small influence on the white noise when the bandwidth of the amplifier is larger than the chopping frequency. This is not the case for sampling where undersampling phenomena actually increases the noise in the baseband [1]. The power spectral density of the white noise will be unchanged as long as the bandwidth of the amplifier is larger than the chopping frequency. Chopping at frequencies higher than fLP will reduce the power spectral density of the white noise as explained in reference [4]. Although white noise is a fundamental limitation it can be reduced by chopping. Oversampling in D/A and A/D converters has about the same effect on the baseband white noise.

5.4.2. 1/f noise modulation

When narrow band random processes are applied at the input of the chopper modulator the situation will change. The input power spectral density of 1/f noise is:


In the constant c we have included the 1/f noise constant k1/f process dependent and the geometry factor, dependent on the dimensions of the transistors. After chopping the PSD of the modulated 1/f noise becomes:


Fig.5.5 shows the normalized PSD of the flicker noise after chopping when fLPT>>1. The effect of chopper modulation on the flicker noise will be the reduction of the PSD of the output noise at low frequencies. At odd multiples of the chopper frequency the

Fig.5.5: PSD of 1/f noise after chopper modulation

PSD of the flicker noise increases. Thus, under the same condition fLPT>>1 the PSD of the white noise after chopping remains the same and the PSD of the flicker noise at low frequencies will be reduced.

IMTS 2018 Register Now>>

Featured Video
GIS Specialist for Metro Wastewater Reclamation District at Denver, Colorado
Upcoming Events
34th Annual Coordinate Metrology Society Conference 2018 at Grand Sierra Resort, 2500 East Second Street Meeting & Covention Center Reno NV - Jul 23 - 27, 2018
Design of Experiments (DOE) for Process Development and Validation (NTZ) 2018 at DoubleTree by Hilton San Diego Downtown 1646 Front St San Diego CA - Aug 2 - 3, 2018
FARO Measure And Inspect 2018 at Maple Hotel Bangkok Thailand - Aug 8, 2018
COMSOL Conference 2018 Bangalore at ITC Gardenia 1, Residency Road, Shanthala Nagar, Ashok Nagar, Bengaluru, Karnataka India - Aug 9 - 10, 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