LTC1068-50CG#TRPBF Linear Technology, LTC1068-50CG#TRPBF Datasheet - Page 11

• Manufacturer Part Number: LTC1068-50CG#TRPBF
• Description: IC FILTR BUILDNG BLK QUAD 28SSOP
• Manufacturer: Linear Technology

Specifications of LTC1068-50CG#TRPBF

• Filter Type: Universal Switched Capacitor
• Frequency - Cutoff Or Center: 50kHz
• Number Of Filters: 4
• Max-order: 8th
• Voltage - Supply: 3.14 V ~ 11 V, ±3.14 V ~ 5.5 V
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

W
Linear Technology’s universal switched-capacitor filters
are designed for a fixed internal, nominal f
f
200, 50 for the LTC1068-50 and 25 for the LTC1068-25.
Filter designs often require the f
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 arrange-
ments to realize different f
proper mode, the f
creased 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 lowpass
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
modes available. To make the design process simpler and
quicker, Linear Technology has developed the FilterCAD
for Widows design software. FilterCAD is an easy-to-use,
powerful and interactive filter design program. The de-
signer 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 (www.linear.com) or you can
order the FilterCAD CD-ROM by contacting Linear Tech-
nology Marketing.
MODES OF
CLK
/f
CLK
O
ratio is 100 for the LTC1068, 200 for the LTC1068-
/f
O
ratio. Listed here are four of the nearly twenty
CLK
OPERATION
/f
O
ratio can be increased or de-
CLK
/f
CLK
O
ratios. By choosing the
/f
O
ratio of each section
U
CLK
/f
O
ratio. The
Mode 1
In Mode 1, the ratio of the external clock frequency to
the center frequency of each 2nd order section is inter-
nally fixed at the part’s nominal ratio. Figure 4 illustrates
Mode 1 providing 2nd order notch, lowpass and band-
pass outputs. Mode 1 can be used to make high order
Butterworth 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
Mode 1b
Mode 1b is derived from Mode 1. In Mode 1b (Figure 5) 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 main-
tains the speed advantages of Mode 1 and should be
considered an optimum mode for high Q designs with f
to f
nal ratio.
The parallel combination of R5 and R6 should be kept
below 5k.
Please refer to the Operating Limits paragraph under Applica-
tions Information for a guide to the use of capacitor C
V
IN
CUTOFF
Figure 4. Mode 1, 2nd Order Filter Providing Notch,
Bandpass and Lowpass Outputs
R1
(or f
AGND
CENTER
–
+
R3
R2
f
Q =
H
O
OLP
=
) ratios greater than the part’s nomi-
R3
R2
RATIO
f
= H
CLK
; H
ON
ON
; f
n
= –
N
= f
+
R2
R1
LTC1068 Series
O
C
C
Σ
; H
–
OBP
S
= –
R3
R1
DEVICE
LTC1068
LTC1068-200
LTC1068-50
LTC1068-25
BP
11
RATIO
1068 F04
100
200
C
LP
50
25
.
1068fb
C
CLK
.