Datasheet

LTC6603
22
6603fa
APPLICATIONS INFORMATION
the LTC6603 for the lowest cutoff frequency (LPF1 = 0,
LFP0 = 0) reduces the supply current by about 60%. Power
supply current vs. cutoff frequency for various bandwidth
settings is shown in the Typical Performance Characteris-
ticst section. The LTC6603 can be programmed through
the serial interface to enter into a low power shutdown
mode. The power supply current during shutdown is less
than 235µA.
Supply Current vs Noise Trade-Off
The passband of the LTC6603 is determined by the master
clock frequency (which is set by R
BIAS
when the internal
oscillator is used), LPF1 and LPF0. The LTC6603 is op-
timized for use with R
BIAS
having a value between 200k
and 30.9k to set the internal oscillation frequency from
12.36MHz to 80MHz. The lowpass corner frequency is
proportional to the clock frequency (internal or external).
Figure 14. f
CLK
vs Filter Cutoff Frequencies Figure 15. Supply Current vs Filter Cutoff Frequency
Table 7. Total Input Referred Integrated Noise Voltage (Passband Gain = 24dB)
LPF1 LPF0 NOISE VOLTAGE
0 0 –81dBm
0 1 –80dBm
1 X –76dBm
To extend the fi lter’s operational frequency range, the
master clock is divided down before reaching the fi lter.
LPF1 and LPF0 set the division ratio of the lowpass clock.
Figure 14 shows the possible cutoff frequencies versus
f
CLK
, LPF1 and LPF0. Overlapping frequency ranges allow
more than one possible choice of bandwidth settings for
some cutoff frequencies. Figure 15 shows supply current
as a function of the fi lter cutoff frequency, LPF1 and LPF0.
Note that the higher bandwidth setting always gives the
minimum supply current for a given cutoff frequency. The
input referred integrated noise voltage for a passband
gain of 24dB is shown in Table 7. Note that the noise is
higher for the higher bandwidth settings. This creates a
tradeoff between supply current and noise. For a given
cutoff frequency, using the highest possible bandwidth
setting gives the minimum supply current at the expense
of higher noise.
FILTER CUTOFF FREQUENCY (Hz)
f
CLK
(MHz)
6603 F14
100
10
10k 1M 10M100k
LPF1 = 1
LPF1 = 0
LPF0 = 1
LPF1 = 0
LPF0 = 0
FILTER CUTOFF FREQUENCY (Hz)
SUPPLY CURRENT (mA)
6603 F15
180
160
140
120
100
80
60
40
20
0
10k 1M 10M100k
LPF1 = 1
LPF1 = 0
LPF0 = 1
LPF1 = 0
LPF0 = 0
T
A
= 25°C
V
S
= 3V
CLKCNTL PIN FLOATING
GAIN = 0dB