Electronic Systems by Zappa Franco

Author:Zappa, Franco
Language: eng
Format: epub
Publisher: Società Editrice Esculapio
Published: 2014-11-10T16:00:00+00:00

Fig. 6.74: Frontispiece of a data-sheet of the double time-continuous UAF MAX270 (Maxim).

Fig. 6.75: Usage example for a universal filter to create a sinusoid starting from sinusoidal steps.

Fig. 6.76: Sinusoidal steps spectrum (on the left) of Fig. 6.75 and the sinusoid sine (on the right) obtained after the UAF.

Fig. 6.77: 20kHz signal recovery with noise, using an IC with 4 second order-continuous-time filters. The circuit realizes an eight-order band-pass filter.

Fig. 6.78: Example of an IC containing a couple of switching capacitor filters.

A simple example of the use of these ICs is reported in the scheme in Fig. 6.75: a digital counter (74HC163) drives an eight-channel analog multiplier (DG508A) to obtain calibrated steps with a sinusoidal trend (see the steps shown in the upper trace of the oscilloscope shown in Fig. 6.75).

To obtain the continuous sine (and without distortion), the steps go towards the universal filter MAX270, connecting in cascade two second-order internal filters, obtaining a low-pass fourth-order Sallen-Key filtering. Spectral purity is more satisfactory (observe the lower trace in Fig. 6.75 and the output spectrum in Fig. 6.76). The THD is better than –80dB. The frequency is adjustable between 1kHz and 25kHz, simply modifying the counter command frequency (equal to eight times that required at the output).

The circuit in Fig. 6.77 shows another example of a circuit that employs, this time, a quadruple UAF (Maxim MAX274) to realize an eight-order (!) band-pass centered around 20kHz, simply with an IC and some external resistors. In this way, it is possible to recover the useful input signal at 20kHz, which is completely corrupted by white noise.

To obtain more selective filters, i.e. with a higher order, it should be possible to cascade many second order circuits, as done in the preceding examples, with a consequent higher dimension of the overall circuit. Producers offer ICs with a new type of high order filters that are not based on OpAmps, time-continuous capacitance values, and resistance values, but on switched-capacitors. In these circuits, resistance values are realized with capacitance which is switched at high frequency (higher than the input signal bandwidth).

An example of an IC with a couple of switched capacitor filters is shown in Fig. 6.78; note that it is possible to arbitrarily configure the filters by means of appropriate digital programming lines.

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