LTC1066-1
13
10661fa
ALIASED OUTPUT (dB)
0
–26
–85
INPUT FREQUENCY
1066-1 F07
f
CLK
– f
C
2f
CLK
– f
C
2f
CLK
– 4f
C
f
CLK
– 4f
C
f
CLK
+ 4f
C
2f
CLK
+ 4f
C
f
CLK
+ f
C
2f
CLK
+ f
C
f
CLK
2f
CLK
ALIASED OUTPUT (dB)
0
–60
–80
INPUT FREQUENCY
1066-1 F06
f
CLK
– f
C
2f
CLK
– f
C
2f
CLK
– 2.3f
C
2f
CLK
+ 2.3f
C
f
CLK
+ f
C
2f
CLK
+ f
C
f
CLK
2f
CLK
APPLICATIONS INFORMATION
WUU
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Aliasing
In a sampled-data system the sampling theorem says that
if an input signal has any frequency components greater
than one half the sampling frequency, aliasing errors will
appear at the output. In practice, aliasing is not always a
serious problem. High order switched-capacitor lowpass
filters are inherently band limited and significant aliasing
occurs only for input signals centered around the clock
frequency and its multiples.
Figure 6 shows the LTC1066-1 aliasing response when
operated with a clock-to-cutoff frequency ratio of 50:1.
With a 50:1 ratio LTC1066-1 samples its input twice
during one clock period and the sampling frequency is
equal to two times the clock frequency.
The figure also shows the maximum aliased output gener-
ated for inputs in the range of 2f
CLK
±f
C
. For instance, if the
LTC1066-1 is programmed to produce a cutoff frequency
of 20kHz with 1MHz clock, a 10mV, 1.02MHz input signal
will cause a 10µV aliased signal at 20kHz. This signal will
be buried in the noise. Maximum aliasing will occur only
for input signals in the narrow range of 2MHz ±20kHz or
multiples of 2MHz.
Figure 7 shows the LTC1066-1 aliased response when
operated with a clock-to-cutoff frequency ratio of 100:1
(linear phase response with pin 8 to ground).
Figure 6. Aliasing vs Frequency
f
CLK
/f
C
= 50:1 (Pin 8 to V
+
)
Clock is a 50% Duty Cycle Square Wave
Figure 7. Aliasing vs Frequency
f
CLK
/f
C
= 100:1 (Pin 8 to Ground)
Clock is a 50% Duty Cycle Square Wave
