LTC1064 Datasheet, PDF (8/16 Pages) Linear Technology – Low Noise, Fast, Quad Universal Filter Building Block

LTC1064
APPLICATIONS INFORMATION
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 300µV. This circuit takes seconds to settle because of
the integrator pole frequency.
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.
SEPARATE V⁺ POWER SUPPLY TRACE FOR BUFFER
R12
R11
V_IN
R21
R22
R31
R32
10k
V⁺ TRACE FOR FILTER
POSITIVE
SUPPLY
7
0.1µF
LTC1064
19
0.1µF
NEGATIVE
SUPPLY
0.1µF
10k
–⁴
LT^®318
LT1007
LT1056
+₇
+
1µF
R3
TO FILTER 100k
FIRST SUMMING
NODE
0.1µF
1µF
+
C1 = C2 = LOW LEAKAGE FILM
(I.E. POLYPROPYLENE)
R1 = R2 = METAL FILM 1%
1064 F03
FROM
FILTER OUTPUT
LT1012
R1
1M
C1
0.1µF
C2
0.1µF
R2
1M
1064 F04
Figure 3. Buffering the Output of a 4th Order Bandpass Realization
Figure 4. Servo Amplifier
W
U
ODES OF OPERATIO
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.
R1
V_IN
R3
R2
–
+
AGND
N
S
+–
Σ∫
1/4 LTC1064
BP
LP
∫
f_O
=
f_{CLK ;}
100(50)
f
n
=
f_O;
H_OLP
=
–
R2
R1
;
H_OBP
=
–
R3
R1
;
H_ON1
=
–
R2
R1
;
Q
=
R3
R2
1064 F05
Figure 5. Mode 1: 2nd Order Filter Providing Notch,
Bandpass and Lowpass
8