MF6 datasheet(10/20 Pages) NSC

10 / 20 page

1.0 MF6 Application Hints

The MF6 is comprised of a non-inverting unity gain lowpass

sixth order Butterworth switched capacitor filter section and

two undedicated CMOS Op-Amps. The switched capacitor

topology makes the cutoff frequency (where the gain drops

3.01 dB below the DC gain) a direct ratio (100:1 or 50:1) of

the clock frequency supplied to the lowpass filter. Internal in-

tegrator time constants set the filter’s cutoff frequency. The

resistive element of these integrators is actually a capacitor

which is “switched” at the clock frequency (for a detailed dis-

cussion see Input Impedance Section). Varying the clock fre-

quency changes the value of this resistive element and thus

the time constant of the integrators. The clock to cutoff fre-

quency ratio (f

CLK/fc) is set by the ratio of the input and feed-

back capacitors in the integrators. The higher the clock to

cutoff frequency ratio (or the sampling rate) the closer this

approximation is to the theoretical Butterworth response.

The MF6 is available in f

CLK/fc ratios of 50:1 (MF6-50) or

100:1 (MF6-100).

1.1 CLOCK INPUTS

The MF6 has a Schmitt-trigger inverting buffer which can be

used to construct a simple R/C oscillator. The oscillator’s fre-

quency is dependent on the buffer’s threshold levels as well

as on the resistor/capacitor tolerance (see

Figure 1).

DS005065-12

FIGURE 1. Schmitt Trigger R/C Oscillator

MF6 Datasheet(PDF) 10 Page - National Semiconductor (TI)