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参数资料
| 型号： | MF6 |
| 厂商： | National Semiconductor Corporation |
| 英文描述： | 6th Order Switched Capacitor Butterworth Lowpass |
| 中文描述： | 6阶巴特沃斯低通开关电容 |
| 文件页数： | 14/20页 |
| 文件大小： | 878K |
| 代理商： | MF6 |
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
) 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
). Since R
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
≤
2 k
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
) 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
/f
ratio which will become notice-
able when the clock frequency exceeds 250 kHz. The ampli-
tude characteristic will stay within tolerance until f
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
www.national.com
14
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