LTC1064 Linear Technology, LTC1064 Datasheet - Page 8

• Manufacturer Part Number: LTC1064
• Description: Low Noise/ Fast/ Quad Universal Filter Building Block
• Manufacturer: Linear Technology

APPLICATIONS
W
LTC1064
Offset Nulling
Lowpass filters may have too much DC offset for some
users. A servo circuit may be used to actively null the
offsets of the LTC1064 or any LTC switched-capacitor
filter. The circuit shown in Figure 4 will null offsets to better
than 300 V. This circuit takes seconds to settle because of
the integrator pole frequency.
PRIMARY MODES
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 50:1 or 100:1. Figure 5 illustrates Mode 1 provid-
ing 2nd order notch, lowpass and bandpass 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 with unity gain. Mode 1 is faster
than Mode 3. Note that Mode 1 can only be implemented
with three of the four LTC1064 sections because Section
D has no externally available summing node. Section D,
however, can be internally connected in Mode 1 upon
special request.
8
V
POSITIVE
V
SUPPLY
+
IN
TRACE FOR FILTER
ODES OF OPERATIO
Figure 3. Buffering the Output of a 4th Order Bandpass Realization
R11
SEPARATE V
0.1 F
R21
R31
U
7
+
POWER SUPPLY TRACE FOR BUFFER
INFORMATION
U
LTC1064
W
19
U
0.1 F
R22
R32
R12
10k
NEGATIVE
SUPPLY
U
0.1 F
–
+
LT
LT1007
LT1056
®
318
4
7
0.1 F
V
Noise
All the noise performance mentioned excludes the clock
feedthrough. Noise measurements will degrade if the
already described grounding bypassing and buffering
techniques are not practiced. The graph Wideband Noise
vs Q in the Typical Performance Characteristics section is
a very good representation of the noise performance of
this device.
10k
IN
+
1 F
R1
Figure 5. Mode 1: 2nd Order Filter Providing Notch,
Bandpass and Lowpass
1064 F03
f
+
O
1 F
=
100(50)
AGND
f
CLK
C1 = C2 = LOW LEAKAGE FILM
R1 = R2 = METAL FILM 1%
FIRST SUMMING
–
+
; f
TO FILTER
n
R3
R2
= f
(I.E. POLYPROPYLENE)
O
NODE
; H
OLP
Figure 4. Servo Amplifier
= –
100k
R3
R2
R1
N
1/4 LTC1064
+
; H
OBP
–
= –
S
0.1 F
R3
R1
LT1012
C2
; H
ON1
+
–
= –
FILTER OUTPUT
BP
R2
R1
R2
1M
FROM
; Q =
R1
1M
1064 F05
C1
0.1 F
R3
R2
1064 F04
LP