Electronic noise sources

Fig. 6.1.13 Electronic noise sources in capacitive readout circuit...

For a state of art photodiode array UV absorbance detector based on a deuterium lamp source and 5 nm spectral bandwidth, I Is of the order of <3 nanoamps. For T =0.5 seconds. Equations 2, 6, 8, 9 evaluate to an rms shot noise current ng of 1.8xl0 amps, an rms noise fraction F of 5.8x10 and a peak-to-peak absorbance noise contribution Ng of 2.2xl0 au. This is approximately 2-8 fold less than the noise of current array detectors, indicating the presence of a major non-optlcal noise source or sources. This is an electronic noise source, called array readout noise ( ), which will be described In a later section. 

Practical problems and challenges associated with EEG signal recordings arise from physiological, environmental, and electronic noise sources. Physiological sources of interference are motion artifact, muscle noise, eye motion or blink artifact, and sometimes even heartbeat signals. Electrical interference arises from the usual sources 60 Hz power lines, RFs, and electrically or magnetically induced interference. Moreover, the electronic... 

Higher-quality (and, of course, more expensive) sound cards targeted at musicians and recording engineers are much more electronically quiet than the more widely available general-purpose cards. In addition to being internally quieter, these cards are frequently better shielded from external electronic noise sources, including other components within the PC. 

Every electronic system has some level of electromagnetic radiation associated with it. If this level is strong enough to cause other equipment to malfunction, the radiating device will be considered a noise source and usually be subjected to shielding regula-... 

This noise source is associated with the discrete nature of the electric current. When a certain current i is induced or generated in the photocathode, there is some uncertainty in the current, which arises from the quantum properties of electrons. It has previously been demonstrated that the fluctuations in any electrical current with a frequency between / and / + Af are given by... 

Sensitivity performance has improved greatly in the last two decades. The benchmark noise level of 1x10 AU/cm, thought at one time to be the physical limit of UVWis detection imposed by short-term source fluctuations, thermal flow noise and electronic noise, is now surpassed by... 

Noise in the UV-Vis measurement originates primarily from the light source and electronic components. Noise in the measurement affects the accuracy at both ends of the absorbance scale. Photon noise from the light source affects the accuracy of the measurements at low absorbance. Electronic noise from the electronic components affects the accuracy of the measurements at high absorbance [8]. A high noise level affects the precision of the measurements and reduces the limit of detection, thereby rendering the instrument less sensitive. 

In the experiment, the transmission intensities for the excited and the dark sample are determined by the number of x-ray photons (/t) recorded on the detector behind the sample, and we typically accumulate for several pump-probe shots. In the absence of external noise sources the accuracy of such a measurement is governed by the shot noise distribution, which is given by Poisson statistics of the transmitted pulse intensity. Indeed, we have demonstrated that we can suppress the majority of electronic noise in experiment, which validates this rather idealistic treatment [13,14]. Applying the error propagation formula to eq. (1) then delivers the experimental noise of the measurement, and we can thus calculate the signal-to-noise ratio S/N as a function of the input parameters. Most important is hereby the sample concentration nsam at the chosen sample thickness d. Via the occasionally very different absorption cross sections in the optical (pump) and the x-ray (probe) domains it will determine the fraction of excited state species as a function of laser fluence. 

This source works extremely well in combination with a FT spectrometer the continuous, high-density production of rotationally cold radicals is matched with the high resolution and sensitivity of the spectrometer. This setup avoids the problems with timing and electronic noise that often occur in pulsed radical experiments. The entire spectrum is measured at one time, eliminating the need for precise control over radical production conditions such as flow rates. 

Short-term noise consists of baseline perturbations that have a frequency that is significantly higher than that of the eluted peak. Short-term detector noise is usually not a serious problem in practice, as it can be easily removed by appropriate electronic noise filters that do not significantly affect the profiles of the peaks. The source of this noise is usually electronic, originating from either the detector sensor system or the amplifier electronics. 

Electronic noise from fluorescent hghts and other common sources is often called 60-cycle noise because it... 

Spikes in chromatograms can come from many sources, such as aging detector lamps or bubbles in the flow cell both can be easily corrected. External electrical noise sources, such as ovens, refrigerators, cellular telephones, and fluorescent lights, and other possible noise sources such as system electronics or from external electronic sources, and laboratory power feed may be beyond a chromatographer s control. 

We defined S as the signal, in electrons, resulting from the Raman scattering of the analyte of interest, as described in Chapter 3. In the absence of other noise sources, the standard deviation of S is determined by the shot noise limit ... 

This expression Impllclty assumes the same noise sources, mixing efficiencies within the detectors, and comparable electronic time constants. This expression can be rewritten as ... 

Instrumental errors can arise from several sources. Electronic noise in the detector, referred to as Johnson or shot noise, is a primary source of error. A less important source of error is flicker in the light source. 

Also shown explicitly in Fig. 6.1.6 is the interconnection resistance Rpm of the movable structure. In sensors with long and skinny mechanical suspensions it is often large and can contribute significant electronic noise. Highly doped substrates and connecting all suspensions electrically in parallel reduce this source of error. Other interconnection resistances are often negligible and omitted from the circuit diagram. Silicon resistivity should be kept sufficiently low also to prevent depletion of the capacitor electrodes. 

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