LTC1264CS Linear Technology, LTC1264CS Datasheet - Page 12

• Manufacturer Part Number: LTC1264CS
• Description: High Speed/ Quad Universal Filter Building Block
• Manufacturer: Linear Technology

IN
LTC1264
12
A
Figures 12 and 13 show the gain response graphs of the
40kHz Butterworth bandpass design described above. The
passband gain response graph (Figure 12) shows a 40kHz
gain of – 0.4dB and a tilted passband from 37kHz to 43kHz.
These errors are due to the 1% resistors used and the side-
to-side matching of the LTC1264 f
which typically is 0.4%. To adjust for 0dB gain at 40kHz,
reduce the value of R1 in the first stage by 5%. To adjust
for a flat passband, adjust by 1% the value of R6 in stages
3 and 4. Adjusting R6 compensates for the side-to-side
matching errors. Please refer to Figure 5 equations defin-
ing f
The sequence of 2nd order stages and the bandpass gain
H
filter’s intermediate outputs. A given internal output can
have several dB more gain than the final filter output. Gain
peaks occur around the corners of the passband. The gain
peaks can be reduced by increasing the R1 resistor of the
STAGE 1
STAGE 3
OBP
PPLICATI
R1
O
of each stage will determine the gain peaks at the
and Q as a function of R6.
R1
R3
R5
R6
R6
R5
R2
R3
R2
Figure 11. Mode 1b Bandpass Filter
O
U
INV B
HPB/NB
BPB
LPB
SB
AGND
V
SA
LPA
BPA
HPA/NA
INV A
+
S
LTC1264
I FOR ATIO
R1
R1
U
HPD/ND
HPC/NC
INV C
INV D
BPD
BPC
LPC
CLK
LPD
SD
SC
V
–
W
CLK
f
CLK
-to-f
R2
R3
R5
R6
R6
R5
R3
R2
CENTER
U
STAGE 4
STAGE 2
1264 F11
ratio
OUT
first stage and decreasing the R1 resistor of the last stage
by the same amount (multiplying the R1 resistor of the
first stage and dividing the R1 resistor of the last stage by
2 for narrowband filter, and by 5 for wideband filter is a
good rule of thumb). This adjustment may, however,
increase the filter’s passband noise.
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
–10
–20
–30
–40
–50
–60
–70
–80
–90
Figure 12. Passband Gain vs Frequency
40kHz Butterworth Bandpass
1.0
0.5
10
0
0
10
30
MODE 1b
V
f
f
Figure 13. Gain vs Frequency
40kHz Butterworth Bandpass
CLK
CLK
18
32
S
= ±7.5V
/f
= 1MHz
CENTER
26
34
34
36
FREQUENCY (kHz)
FREQUENCY (kHz)
= 25:1
42
38
50
40
MODE 1b
V
f
f
CLK
CLK
S
58
42
= ±7.5V
/f
= 1MHz
CENTER
66
44
74
46
= 25:1
82
48
1264 F13
1264 F12
90
50