LTC6603 Linear Technology, LTC6603 Datasheet - Page 21

• Manufacturer Part Number: LTC6603
• Description: Dual Adjustable Lowpass Filter
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
results in four matched fi lter channels, all synchronized to
the same clock. The master has its CLKCNTL pin pulled
to V+
slave has its CLKCNTL pin pulled to ground, confi guring its
CLKIO pin as an input. Note that in order to synchronize the
two fi lters, the clock frequency must not be buffered. This
requires that the fi lters be close together on the PC board.
If the clock is buffered, the fi lters would have matching
bandwidths, but would not be synchronized.
Output Drive
The fi lter outputs can drive 1k and/or 50pF loads connected
to AC ground with a 0.5V to 2.5V signal (corresponding
to a 4V
connected between +OUTA and –OUTA or +OUTB and
–OUTB) the outputs can produce a 4V
and/or 25pF . For smaller signal amplitudes, the outputs can
drive correspondingly larger loads. For larger capacitive
loads, an external 50Ω series resistor is recommended
for each output.
Clock Feedthrough
Clock feedthrough is defi ned as the RMS value of the clock
frequency and its harmonics that are present at the fi lter’s
output. The clock feedthrough is measured with +INA and
–INA (or +INB, –INB) tied to V
board layout and the power supply decoupling. The clock
feedthrough can be reduced with a simple RC post fi lter.
Decoupling Capacitors
The LTC6603 uses sampling techniques, therefore its
performance is sensitive to supply noise. 0.1μF ceramic
decoupling capacitors must be connected from V+
and V+
A ground plane should be used. Noisy signals should be
isolated from the fi lter’s input pins. In addition, a 0.1μF
decoupling capacitor at Pin 20 is recommended since this
pin receives clocked current injection.
Aliasing
Aliasing is an inherent phenomenon of sampled data fi lters.
Signifi cant aliasing only occurs when the frequency of the
input signal approaches the sampling frequency or mul-
D
, confi guring its CLKIO pin as an output, while the
D
P-P
(Pin 16) to ground with leads as short as possible.
differential signal). For differential loads (loads
OCM
and depends on the PC
P-P
signal across 2k
A
(Pin 2)
tiples of the sampling frequency. The ratio of the LTC6603
input sampling frequency to the clock frequency, f
determined by the state of control bits LPF1 and LPF0.
Table 6 shows the possible input sampling frequencies for
a clock frequency of 80MHz. The input sampling frequency
is proportional to the clock frequency. For example, if the
clock frequency is lowered from 80MHz to 40MHz, the
input sampling frequency will be lowered by half. Input
signals with frequencies near the input sampling frequency
will be aliased to the passband of the fi lter and appear at
the output unattenuated.
Table 6. Input Sampling Frequency (f
A simple LC anti-aliasing fi lter is recommended at the
fi lter inputs to attenuate frequencies near the input sam-
pling frequency that will be aliased to the passband. For
example, if the clock frequency is set to 80MHz and the
cutoff frequency of the fi lter is set to its maximum (LPF1
= ‘1’), the lowest frequency that would be aliased to the
passband would be f
= 157.5MHz. The LTC6603 fi lter inputs should be driven
by a low impedance output (<100Ω).
Wideband Noise
The wideband noise of the fi lter is the RMS value of the
device’s output noise spectral density. The wideband noise
is nearly independent of the value of the clock frequency
and excludes the clock feedthrough. Most of the wideband
noise is concentrated in the fi lter passband and cannot be
removed with post fi ltering.
Power Supply Current
The power supply current depends on the state of the
lowpass cutoff frequency controls (LPF1, LPF0) and the
value of R
the middle cutoff frequency (LPF1 = ‘0’, LPF0 = ‘1’), the
supply current is reduced by about 23% relative to the
supply current for the higher bandwidth setting. Pro-
LPF1
0
0
1
1
BIAS
. When the LTC6603 is programmed for
LPF0
0
1
0
1
CLK
– f
CUTOFF
Input Sampling Frequency (MHz)
CLK
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, i.e. 160MHz – 2.5MHz
= 80MHz)
LTC6603
160
160
20
40
21
CLK
6603f
, is