lmf60 National Semiconductor Corporation, lmf60 Datasheet - Page 11

• Manufacturer Part Number: lmf60
• Description: High Performance 6th-order Switched Capacitor Butterworth Lowpass Filter
• Manufacturer: National Semiconductor Corporation

1 0 LMF60 Application Hints
If the LMF60-50 or the LMF60-100 were set up for a cutoff
frequency of 10 kHz the input impedance would be
In this example with a source impedance of 10k the overall
gain if the LMF60 had an ideal gain of 1 (0 dB) would be
Since the maximum overall gain error for the LMF60 is
dB
case would be
1 5 CUTOFF FREQUENCY RANGE
The filter’s cutoff frequency (f
leakage currents through the internal switches discharging
the stored charge on the capacitors At lower clock frequen-
b
0 3 dB with a R
A
V
FIGURE 7c LMF60-100
e
FIGURE 7a LMF60-100
10 kX
a
R
1 MX
0 21 dB to
Amplitude Response
Amplitude Response
IN
a
e
S s
1 MX
1
10 kHz
c
2 kX the actual gain error for this
e
10
b
C
) has a lower limit caused by
10
0 99009 (
0 39 dB
g
e
g
2 5V Supplies
1 MX
5V Supplies
b
86 4 mdB)
(Continued)
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a
0 1
11
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 LMF60 power supply voltage decreases
This causes a shift in the f
noticeable when the clock frequency exceeds 500 kHz The
amplitude characteristic will stay within tolerance until f
exceeds 750 kHz and will peak at about 0 4 dB at the cutoff
frequency with a 2 MHz clock The response of the LMF60
is still a reasonable approximation of the ideal Butterworth
lowpass characteristic as can be seen in Figure 7
f
CLK
FIGURE 7d LMF60-50
FIGURE 7b LMF60-50
e
100 Hz I
V
e
Amplitude Response
Amplitude Response
1 pF (100 Hz)
LEAKAGE
1 pA
CLK
f
C
g
e
g
e
ratio which will become
2 5V Supplies
5V Supplies
1 pA C
10 mV
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e
1 pF
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TL H 9294 – 20
CLK