9
LTC1067/LTC1067-50
ODES OF OPERATIO
U
W
Mode 1
In Mode 1, the ratio of the external clock frequency to the
center frequency of each 2nd order section is internally
fixed at the part’s nominal ratio. Figure 3 illustrates Mode
1 providing 2nd order notch, lowpass and bandpass
outputs. Mode 1 can be used to make high order Butter-
worth lowpass filters; it can also be used to make low Q
notches and for cascading 2nd order bandpass functions
tuned at the same center frequency. Mode 1 is faster than
Mode 3.
Please refer to the Operating Limits paragraph under Appli-
cations Information for a guide to the use of capacitor C
C
.
Linear Technology’s universal switched-capacitor filters
are designed with a fixed internal, nominal f
CLK
/f
O
ratio.
The LTC1067 has a 100:1 f
CLK
/f
O
ratio and the
LTC1067-50 has a 50:1 f
CLK
/f
O
ratio. Filter designs often
require the f
CLK
/f
O
ratio of each section to be different from
the nominal ratio and in most cases different from each
other. Ratios other than the nominal value are possible
with external resistors. Operating modes use external
resistors, connected in different arrangements to realize
different f
CLK
/f
O
ratios. By choosing the proper mode, the
f
CLK
/f
O
ratio can be increased or decreased from the part’s
nominal ratio.
The choice of operating mode also effects the transfer
function at the HP/N pins. The LP and BP pins always give
the lowpass and bandpass transfer functions respectively,
regardless of the mode utilized. The HP/N pins have a
different transfer function depending on the mode used.
Mode 1 yields a notch transfer function. Mode 3 yields a
highpass transfer function. Mode 2 yields a highpass-
notch transfer function (i.e., a highpass with a stopband
notch). More complex transfer functions, such as low-
pass-notch, allpass or complex zeros, are achieved by
summing two or more of the LP, BP or HP/N outputs. This
is illustrated in sections Mode 2n and Mode 3a.
Choosing the proper mode(s) for a particular application
is not trivial and involves much more than just adjusting
the f
CLK
/f
O
ratio. Listed here are six of the nearly twenty
modes available. To make the design process simpler and
quicker, Linear Technology has developed the FilterCAD
TM
for Windows
®
design software. FilterCAD is an easy-to-
use, powerful and interactive filter design program. The
designer can enter a few filter specifications and the
program produces a full schematic. FilterCAD allows the
designer to concentrate on the filter’s transfer function
and not get bogged down in the details of the design.
Alternatively, those who have experience with the Linear
Technology family of parts can control all of the details
themselves. For a complete listing of all the operating
modes, consult the appendices of the FilterCAD manual or
the Help files in FilterCAD. FilterCAD can be obtained free
of charge on the Linear Technology web site (http://
www.linear-tech.com) or you can order the FilterCAD
CD-ROM by contacting Linear Technology’s marketing
department.
FilterCAD is a trademark of Linear Technology Corporation.
Windows is a registered trademark of Microsoft Corporation.
Mode 1b
Mode 1b is derived from Mode 1. In Mode 1b (Figure 4)
two additional resistors R5 and R6 are added to lower the
amount of voltage fed back from the lowpass output into
the input of the SA (or SB) switched-capacitor summer.
This allows the filter’s clock-to-center frequency ratio to
be adjusted beyond the part’s nominal ratio. Mode 1b
maintains the speed advantages of Mode 1 and should be
considered an optimum mode for high Q designs with f
CLK
to f
CUTOFF
(or f
CENTER
) ratios greater than the part’s
nominal ratio.
Figure 3. Mode 1, 2nd Order Filter Providing Notch,
Bandpass and Lowpass Outputs
–
+
Σ
∫ ∫
AGND
NOTE: RATIO = 100 FOR LTC1067
= 50 FOR LTC1067-50
R1
N
BP
LP
V
IN
1067 F03
+
–
S
R2
R3
C
C
f
O
= ; f
n
= f
O
Q = ; H
ON
= – ; H
OBP
= –
H
OLP
= H
ON
R2
R1
R3
R1
R3
R2
f
CLK
RATIO
