Filtering analog

It is always advisible to filter out the noise, e.g., noise caused by rectification, by means of a 30 Hz fourth-order analog low-pass filter of the Butterworth type. This is also recommended if analog signals are to be AD converted. 

Still lower frequencies coming from other sources can be cut off by analog filters with cut-off frequencies between 10 and 0.5 Hz, but it must be carried out very carefully and stepwise to hold the change of the signal shape in tolerable limits. This is demonstrated in Table 4-3 and Fig. 4-12. 

In the example above, it can be seen that the signal height decreases significantly if T is higher than 22 ms and becomes distorted and shifted if t becomes higher than 235 ms (Fig. 4-12). 

R resistance C capacitance A peak height (without filtering, A is 100 mm) FWHM full width at half maximum amplitude A shift, (seen from left to right). 

Experimental analog filters tend to skew the input function more or less toward longer times (in the direction of scanning). This is the price paid for an improved signal-to-noise ratio (SNR). For optimization of smoothing, one should start with the lowest possible time constant of the device and observe both the SNR and the attenuation as well as the A shifting of the main maximum (AA). The latter is particularly important if the true position of the extrema is desired. 

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This basic sampling theorem has profound implications. It says that any high-frequency components in the signal (for example, 60-cycle-per-second electrical noise) can necessitate very fast sampling, even if the basic process is quite slow. It is, therefore, always recommended that signals be analog-filtered before they are sampled. This eliminates the unimportant high-frequency components. 

As discussed in Chapter 1, any linear analog filter network may be used. Its performance may always be described by convolution with a filter function, here called sE. Many modern digital filters may be thus described. The instrument employed in the present work used a simple single-stage RC filter of the type analyzed in Section II.D of Chapter 2. 

Van Valkenburg, M. E. 1982. Analog Filter Design. New York Harcort Brace Jovanovich College Publishers. 

The experimental set-up is shown in Fig. 7-1 an electrochemical interface with low level noise and a transfer function analyzer (TFA) were used for measurements of the EHD impedance. A matched two-channels 24 db/octave low pass filter (F) was used to remove HF noise and the ripple due to electric network supply, this analog filtering allows the TFA to operate with an increased sensitivity. These instruments were controlled by a computer, which recorded the data. 

Analog-filtering procedure was applied, based on the molecular physicochemical descriptors and optimum property ranges calculated for the set of known HIV PR inhibitors (Table 4.1), which permitted the selection of 100 most diverse cyclic urea analogs with suitable molecular properties. 

Analog filter — An electronic circuit that handles continuous signals and provides high impedance to a desired range of frequencies. The actual amount of attenuation for each frequency varies from filter to filter [i,ii]. Analog filters can be divided into two broad groups ... 

Recursive filter — A sort of - analog filter or - digital filter based on employing feedback to determine the current filtered output. In digital filters, the software technique is based on calculating the current filtered output (yn) from input values (x ,xn-, x -2,...) and previous filtered outputs (y -, y -2,...). Thus, previously calculated output values go back into the calculation of the... 

Signal filtering — A process used to eliminate unwanted frequencies, generated by electronic devices during the detection procedure, from a measured signal. Different analog filters and digital filters have been developed... 

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