LTC1264-7
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Aliasing
Aliasing is an inherent phenomenon of sampled data
systems and it occurs when input frequencies close to the
sampling frequency are applied. For the LTC1264-7 case
at 50:1, an input signal whose frequency is in the range of
f
CLK
±10%, will be aliased back into the filter’s passband.
If, for instance, an LTC1264-7 operating with a 100kHz
clock and 2kHz cutoff frequency receives a 95kHz 10mV
input signal, a 5kHz 56µV
RMS
alias signal will appear at its
output. When the LTC1264-7 operates with a clock-to-
cutoff frequency of 25:1, aliasing occurs at twice the clock
frequency. Table 10 shows details.
Table 10. Aliasing (f
CLK
= 100kHz)
INPUT FREQUENCY OUTPUT LEVEL OUTPUT FREQUENCY
(V
IN
= 1V
RMS
, (Relative to Input, (Aliased Frequency
f
IN
= f
CLK
± f
OUT
) 0dB = 1V
RMS
)f
OUT
= ABS [f
CLK
± f
IN
])
(kHz) (dB) (kHz)
25:1, f
CUTOFF
= 4kHz
175 (or 225) –76 25
180 (or 220) – 69 20
185 (or 215) – 62 15
190 (or 210) – 43 10
195 (or 205) – 7 5
50:1, f
CUTOFF
= 2kHz
75 (or 125) –96 25
80 (or 120) – 90 20
85 (or 115) – 82 15
90 (or 110) – 72 10
95 (or 105) – 45 5
99 (or 101) 0 1
Table 9. Maximum V
IN
vs V
S
and Clock
POWER SUPPLY MAXIMUM f
CLK
MAXIMUM V
IN
±7.5V 5.0MHz 1.6V
RMS
(f
IN
≥ 160kHz)
4.5MHz 2.0V
RMS
(f
IN
≥ 160kHz)
4.0MHz 2.5V
RMS
(f
IN
≥ 160kHz)
≥ 3.5MHz 1.6V
RMS
(f
IN
≥ 500kHz)
±5V 3.0MHz 1.6V
RMS
(f
IN
≥ 100kHz)
≥ 3.0MHz 0.7V
RMS
(f
IN
≥ 500kHz)
Single 5V 2.0MHz 0.5V
RMS
(f
IN
≥ 400kHz)
Transient Response
2V/DIV
INPUT = 10kHz ± 3V
f
CLK
= 2.5MHz
RATIO = 25:1
INPUT
90%
50%
10%
OUTPUT
t
r
t
d
t
s
1164-7 F06
RISE TIME (t
r
) = ±5%
0.36
f
CUTOFF
SETTLING TIME (t
s
) = ±5%
(TO 1% of OUTPUT)
2
f
CUTOFF
TIME DELAY (t
d
) = GROUP DELAY ≈
(TO 50% OF OUTPUT)
1.15
f
CUTOFF
Figure 6
Table 11. Transient Response of LTC Lowpass Filters
DELAY RISE SETTLING OVER-
TIME* TIME** TIME*** SHOOT
LOWPASS FILTER (SEC) (SEC) (SEC) (%)
LTC1064-3 Bessel 0.50/f
C
0.34/f
C
0.80/f
C
0.5
LTC1164-5 Bessel 0.43/f
C
0.34/f
C
0.85/f
C
0
LTC1164-6 Bessel 0.43/f
C
0.34/f
C
1.15/f
C
1
LTC1264-7 Linear Phase 1.15/f
C
0.36/f
C
2.05/f
C
5
LTC1164-7 Linear Phase 1.20/f
C
0.39/f
C
2.20/f
C
5
LTC1064-7 Linear Phase 1.20/f
C
0.39/f
C
2.20/f
C
5
LTC1164-5 Butterworth 0.80/f
C
0.48/f
C
2.40/f
C
11
LTC1164-6 Elliptic 0.85/f
C
0.54/f
C
4.30/f
C
18
LTC1064-4 Elliptic 0.90/f
C
0.54/f
C
4.50/f
C
20
LTC1064-1 Elliptic 0.85/f
C
0.54/f
C
6.50/f
C
20
* To 50% ±5%, ** 10% to 90% ±5%, *** To 1% ± 0.5%
10µs/DIV
Figure 5
