Switched capacitor filters (by Gerard Larooy)

Component: MF4

Category: Special function IC's

These chips were supposed to be the bee's knees when National Semiconductors announced them in an age where fast opamps were expensive and difficult to obtain. They allowed high order filter to be configured where the cut frequency could be readily changed, unlike existing schemes using passive components. They also allowed low cut frequencies without inconveniently large component values.

However the clock noise problems were terrible. At a first glance one would think that it would be easy to filter with simple components because the clock frequency was 50 or 100 times the cut frequency. The noise on these filters increases markedly at the cut frequency of a low pass where there is usually only 0.3dB attenuation and when a band pass configuration is employed the noise peak moves to the centre frequency. The effective dynamic range of these filters is a small fraction of an opamp circuit running the same supply voltage. This precludes these filters for any serious communications use.

When a high Q bandpass filter is wanted, the chip centre frequency moves significantly from the stated value and resistor values become critical to avoid self oscillation. In other words, the component tolerance problem that the switched capacitor filter was supposed to circumvent is back with a vengeance.

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