MF6 datasheet(14/20 Pages) NSC

14 / 20 page

1.0 MF6 Application Hints (Continued)

(where T equals one clock period) or

The equivalent input resistor (R

in) then can be defined as

The input capacitor is 2 pF for the MF6-50 and 1 pF for the

MF6-100, so for the MF6-100

and

for the MF6-50. As shown in the above equations for a given

cutoff frequency (f

c) the input impedance remains the same

for the MF6-50 and the MF6-100. The higher the clock to

center frequency ratio, the greater equivalent input resis-

tance for a given clock frequency. As the cutoff frequency in-

creases the equivalent input impedance decreases. This in-

put resistance will form a voltage divider with the source

impedance (R

source). Since Rin is inversely proportional to

the cutoff frequency, operation at higher cutoff frequencies

will be more likely to load the input signal which would ap-

pear as an overall decrease in gain to the output of the filter.

Since the filter’s ideal gain is unity its overall gain is given by:

If the MF6-50 or the MF6-100 were set up for a cutoff fre-

quency of 10 kHz the input impedance would be:

In this example with a source impedance of 10k the overall

gain, if the MF6 had an ideal gain of 1 or 0 dB, would be:

Since the maximum overall gain error for the MF6 is ±0.3 dB

with a R

s ≤ 2kΩ the actual gain error for this case would be

+0.21 dB to −0.39 dB.

1.5 CUTOFF FREQUENCY RANGE

The filter’s cutoff frequency (f

c) has a lower limit caused by

leakage currents through the internal switches discharging

the stored charge on the capacitors. At lower clock frequen-

cies these leakage currents can cause millivolts of error, for

example:

The propagation delay in the logic and the settling time re-

quired to acquire a new voltage level on the capacitors in-

creases as the MF6 power supply voltage decreases. This

causes a shift in the f

CLK/fc ratio which will become notice-

able when the clock frequency exceeds 250 kHz. The ampli-

tude characteristic will stay within tolerance until f

CLK ex-

ceeds 500 kHz and will peak at about 0.5 dB at the corner

frequency with a 1 MHz clock. The response of the MF6 is

still a reasonable approximation of the ideal Butterworth low-

pass characteristic as can be seen in

Figures 7, 8, 9, 10.

DS005065-20

FIGURE 7. MF6-100 ±5V Supplies

Amplitude Response

DS005065-21

FIGURE 8. MF6-50 ±5V Supplies

Amplitude Response

DS005065-22

FIGURE 9. MF6-100 ±2.5V Supplies

Amplitude Response

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