Datasheet

2.0 Modes of Operation (Continued)
TABLE 1. Summary of Modes. Realizable filter types (e.g. low-pass) denoted by asterisks.
Unless otherwise noted, gains of various filter outputs are inverting and adjustable by resistor ratios.
Mode BP LP HP N AP Number of Adjustable Notes
Resistors f
CLK
/f
0
1
** *
3No
(2) May need input buffer.
1a H
OBP1
=−Q H
OLP
= + 1 2 No Poor dynamics
H
OBP2
=+1 for high Q.
1b
** *
3 No Useful for high
frequency applications.
Yes (above
2
** *
3f
CLK
/50 or
f
CLK
/100)
Universal State-
3
***
4 Yes Variable Filter. Best
general-purpose mode.
As above, but also
3a
****
7 Yes includes resistor-
tuneable notch.
Gives Allpass res-
4
** *
3 No ponse with H
OAP
=−1
and H
OLP
= −2.
Gives flatter allpass
5
** *
4 Yes response than above
if R
1
=R
2
= 0.02R
4
.
6a
**
3 Yes Single pole.
(2)
6b H
OLP1
= + 1 2 Yes Single pole.
6c
**
3 No Single pole.
7 2 Yes Summing integrator with
adjustable time constant.
3.0 Applications Information
The LMF100 is a general purpose dual second-order state
variable filter whose center frequency is proportional to the
frequency of the square wave applied to the clock input
(f
CLK
). The various clocking options are summarized in the
following table.
Clocking Options
Power Supply Clock Levels LSh V
D
+
−5V and +5V TTL (0V to +5V) 0V +5V
−5V and +5V CMOS (−5V to +5V) 0V +5V
0V and 10V TTL (0V to 5V) 0V +10V
0V and 10V CMOS (0V to +10V) +5V +10V
−2.5V and
+2.5V
CMOS
0V +2.5V
(−2.5V to +2.5V)
0V and 5V TTL (0V to +5V) 0V 0V
Power Supply Clock Levels LSh V
D
+
0V and 5V CMOS (0V to +5V) +2.5V +5V
By connecting pin 12 to the appropriate dc voltage, the filter
center frequency, f
0
, can be made equal to either f
CLK
/100 or
f
CLK
/50. f
0
can be very accurately set (within
±
0.6%) by us-
ing a crystal clock oscillator, or can be easily varied over a
wide frequency range by adjusting the clock frequency. If de-
sired, the f
CLK
/f
0
ratio can be altered by external resistors as
in
Figures 10, 11, 12, 13, 14, 15 and Figure 16 . This is useful
when high-order filters (greater than two) are to be realized
by cascading the second-order sections. This allows each
stage to be stagger tuned while using only one clock. The fil-
ter Q and gain are set by external resistor ratios.
All of the five second-order filter types can be built using ei-
ther section of the LMF100. These are illustrated in
Figures
1, 2, 3, 4
and Figure 5 along with their transfer functions and
some related equations.
Figure 6 shows the effect of Q on
the shapes of these curves.
LMF100
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