Datasheet

LTC1066-1
11
10661fa
APPLICATIONS INFORMATION
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AC PERFORMANCE
AC (Passband) Gain
The passband gain of the LTC1066-1 is equal to the
passband gain of the internal switched-capacitor lowpass
filter, and it is measured at f = 0.25f
CUTOFF
. Unlike conven-
tional monolithic filters, the LTC1066-1 starts with an
absolutely perfect 0dB DC gain and phases into an “imper-
fect” AC passband gain, typically ± 0.1dB.
The filter’s low passband ripple, typically 0.05dB, is mea-
sured with respect to the AC passband gain.
The LTC1066-1 DC stabilizing loop slightly warps the
filter’s passband performance if the – 3dB frequency of the
feedback passive elements (1/2πR
F
C
F
) is more than the
Figure 4. Passband Behavior
FREQUENCY (Hz)
10
GAIN (dB)
100 1k 10k 20k
1066-1 F04
1.00
0.75
0.50
0.25
0
–0.25
–0.50
–0.75
–1.00
T
A
= 25°C
f
CLK
/f
C
= 50:1
R
F
= 20k,
C
F
= 1µF
CURVE D: f
CUTOFF
= 20kHz = 2500 ×
2πR
F
C
F
1
CURVE C: f
CUTOFF
= 5kHz = 625 ×
2πR
F
C
F
1
CURVE B: f
CUTOFF
= 2kHz = 250 ×
2πR
F
C
F
1
CURVE A: f
CUTOFF
= 1kHz = 125 ×
2πR
F
C
F
1
A B CD
cutoff frequency of the internal switched-capacitor filter
divided by 250. The LTC1066-1 clock tunability directly
relates to the above constraint. Figure 4 illustrates the
passband behavior of the LTC1066-1 and it demonstrates
the clock tunability of the device. A typical LTC1066-1
device was used to trace all four curves of Figure 4. Curve
D, for instance, has nearly zero ripple and 0.04dB passband
gain. Curve D’s 20kHz cutoff is much higher than the 8Hz
cutoff frequency of the R
F
C
F
feedback network, so its
passband is free from any additional error due to R
F
C
F
feedback elements. Curve B illustrates the passband error
when the 1MHz clock of curve D is lowered to 100kHz. A
0.1dB error is added to the filter’s original AC gain of
0.04dB.