lmf380 National Semiconductor Corporation, lmf380 Datasheet - Page 5

• Manufacturer Part Number: lmf380
• Description: Triple One-third Octave Switched-capacitor Active Filter
• Manufacturer: National Semiconductor Corporation

Connection Diagrams
Pin Description
GND
N C
OUT1 OUT2
OUT3
XTAL1
XTAL2
V
Order Number LMF380CIJ LMF380CMJ or LMF380CIN
b
See NS Package Number J16A or N16E
See NS Package Number V20A
Plastic Chip Carrier Package
Order Number LMF380CIV
This is the analog ground reference for the
LMF380 In split supply applications GND
should be connected to the system ground
When operating the LMF380 from a single
positive power supply voltage pin 1 should
be connected to a ‘‘clean’’ reference volt-
age midway between V
These pins are not connected to the inter-
nal circuitry
These are the outputs of the filters
This is the crystal oscillator input pin When
using the internal oscillator the crystal
should be tied between XTAL1 and XTAL2
XTAL1 also serves as the input for an exter-
nal CMOS-level clock
This is the output of the internal crystal
oscillator When using the internal oscilla-
tor the crystal should be tied between
XTAL1 and XTAL2
This is the negative power supply pin It
should be bypassed with at least a 0 1 mF
ceramic capacitor
Dual-In-Line Package
Top View
Top View
For best results
a
and V
TL H 11123 – 2
TL H 11123 – 3
b
a
5
Figure 1 shows the magnitude versus frequency curves for
CLOCK OUT
INPUT1
INPUT2
INPUT3
V
Functional Description
The LMF380 contains three fourth-order Chebyshev band-
pass filters whose center frequencies are spaced one-third
of an octave apart making it ideal for use in ‘‘real time’’
audio spectrum analysis applications
switched-capacitor filters the center frequencies are pro-
portional to the clock frequency applied to the IC the center
frequencies of the LMF380’s three filters are located at
f
The three filters in an LMF380 cover a full octave in fre-
quency so that by using several LMF380s with clock fre-
quencies separated by a factor of 2n a complex audio pro-
gram can be analyzed for frequency content over a range of
several octaves To facilitate this the CLK OUT pin of the
LMF380 supplies an output clock signal whose frequency is
one-half that of the incoming clock frequency Therefore a
single clock source can provide the clock reference for all of
the 30 filters (10LMF380s) in a real time analyzer that cov-
ers the entire 10-octave audio frequency range The
LMF380 contains an internal clock oscillator that requires
an external crystal and two capacitors to operate Since the
clock divider is on-board only a single crystal is needed for
the top-octave filter chip the remaining devices can derive
their clock signals from the master If desired an external
oscillator can be used instead
the three filters in the LMF380 Separate input and output
pins are provided for the three internal filters The input pins
will normally be connected to a common signal source but
can also be connected to separate input signals when nec-
essary
CLK
FIGURE 1 Response curves for the three filters in the
a
50 f
LMF380 The clock frequency is 250 kHz
CLK
62 5 and f
1 0 mF to 10 0 mF tantalum capacitor
should also be used For single-supply op-
eration connect this pin to system ground
This is the clock output pin It can drive the
clock inputs (XTAL1) of additional LMF380s
or other components The clock output fre-
quency is one-half the clock frequency at
XTAL1
These are the signal inputs to the filters
This is the positive power supply pin It
should be bypassed with at least a 0 1 mF
ceramic capacitor For best results a 1 0
mF to 10 0 mF tantalum capacitor should
also be used
CLK
80
As with other
TL H 11123 – 6