LTC1061 Просмотр технического описания (PDF) - Linear Technology
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LTC1061 Datasheet PDF : 16 Pages
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W
U
ODES OF OPERATIO
Another example of Mode 1b is illustrated on the front
page of the data sheet. The cascading sequence of this 6th
order bandpass filter is shown in block diagram form,
Figure 10a. the filter is geometrically centered around the
side B of the LTC1061 connected in Mode 1. This dictates
a clock-to-center frequency ratio of 50:1 or 100:1. The side
A of the IC operates in Mode 1b to provide the lower center
frequency of 0.95 and still share the same clock with the
rest of the filter. With this approach the bandpass filter can
SIDE A
VIN
MODE 1b
fO1 = 0.95
Q1 = 31.9
SIDE B
MODE 1
fO2 = 1
Q2 = 15.9
SIDE C
MODE 3
fO3 = 1.05
Q3 = 31.9
VOUT
1061 F10a
Figure 10a. Cascading Sequence of the Bandpass Filter Shown
on the Front Page, with (fCLK/fO) = 50:1 or 100:1
SIDE A
SIDE B
SIDE C
VIN
MODE 1b
fO1 = 0.95
Q1 = 31.9
MODE 1
fO2 = 1.05
Q2 = 31.9
MODE 3
fO3 = 1
Q3 = 15.9
VOUT
1061 F10b
Figure 10b. Cascading Sequence of the Same Filter for Speed
Optimization, and with (fCLK/fO) = 52.6:1
LTC1061
operate with center frequencies up to 24kHz. The speed of
the filter could be further improved by using Mode 1 to lock
the higher resonant frequency of 1.05 and higher Q or 31.9
to the clock, Figure 10b, thus changing the clock to center
frequency ratio to 52.6:1.
Mode 3a – This is an extension of Mode 3 where the
highpass and lowpass outputs are summed through two
external resistors Rh and Rl to create a notch, Figure 11.
Mode 3a is very versatile because the notch frequency can
be higher or lower than the center frequency of the 2nd
order section. The external op amp of Figure 11 is not
always required. When cascading the sections of the
LTC1061, the highpass and lowpass outputs can be
summed directly into the inverting input of the next
section. Figure 12 shows an LTC1061 providing a 6th
order elliptic bandpass or notch response. Sides C and B
are connected in Mode 3a while side A is connected in
Mode 1 and uses only two resistors. The resulting filter
response is then geometrically symmetrical around either
the center frequency of side A (for bandpass responses) or
the notch frequency of side A (for notch responses).
R1
VIN
R4
R3
R2
–
+
AGND
CC
HP
S
BP
LP
+
–
Σ
∫
∫
Rl
Rh
√ √ fO
= fCLK
100(50)
R2
R4
;
fn
= 10f0C(L5K0)
Rh
Rl
;
HOHP
=
–
R2 ;
R1
HOBP
=
–
RR31 ;
HOLP
=
–
R4
R1
HON1 (f → 0) = RR×gl ; HRRO41N2
( ) f → fC=2LK ×RRgh; HORRN21(f = fO) = Q
(HOLRRPgl–
HOHRRPgh
√ Q = R3 R2
R2 R4
NOTE:
FOR
Q
>
5
AND
fCLK
>
1MHz,
ADD
CC
SUCH
AS
CC
≅
0.16
R4 × 1.2MHz
Rg
–
NOTCH
+
EXTERNAL OP AMP OR
INPUT OP AMP OF THE
LTC1061, SIDES A, B, C
)
1061 F11
Figure 11. Mode 3a: 2nd Order Filter Providing Highpass, Bandpass, Lowpass, Notch
1061fe
9
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