LTC6603 Ver la hoja de datos (PDF) - Linear Technology
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LTC6603 Datasheet PDF : 24 Pages
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LTC6603
APPLICATIONS INFORMATION
The following graphs show a few of the possible lowpass
filters.
Gain and Group Delay vs Frequency
(2.5MHz Lowpass Response)
0
1.2
GAIN
–20
1.0
–40
0.8
–60
0.6
GROUP DELAY
–80
0.4
–100
0.2
–120
100k
1M
FREQUENCY (Hz)
0
10M
6603 G17
Gain and Group Delay vs Frequency
(650kHz Lowpass Response)
0
4
GAIN
–20
3
–40 GROUP DELAY
2
–60
1
–80
100k
1M
FREQUENCY (Hz)
0
6603 G18
The oscillator is sensitive to transients on the positive
supply. The IC should be soldered to the PC board and
the PCB layout should include a 0.1μF ceramic capacitor
between V+A (Pin 2) and ground, as close as possible to
the IC to minimize inductance. The PCB layout should also
include an additional 0.1μF ceramic capacitor between
V+D (Pin 16) and ground. Avoid parasitic capacitance on
RBIAS (Pin 4) and avoid routing noisy signals near RBIAS.
Use a ground plane connected to Pin 14 and the Exposed
Pad (Pin 25).
16
Alternative Methods of Setting the Clock Frequency of
the LTC6603
The oscillator may be programmed by any method that
sinks a current out of the RBIAS pin. The circuit in Figure 6
sets the clock frequency by using a programmable current
source and in the expression for fCLK, the resistor RBIAS
is replaced by the ratio of 1.17V/ICONTROL. Because the
voltage of the RBIAS pin is approximately 1.17V ±5%, the
Figure 6 circuit is less accurate than if a resistor controls
the clock frequency.
In this circuit, the LTC2621 (a 12-bit DAC) is daisy chained
with the LTC6603. Because the sinking current from the
RBIAS pin is
VRBIAS • k
2N • R1
the equivalent RBIAS is
2N • R1
k,
where k is the binary DAC input code and N is the resolu-
tion. Figure 7 shows some of the frequency responses
that can be obtained using this circuit.
Figure 8 shows the LTC6603’s oscillator configured as
a VCO. A voltage source is connected in series with the
RBIAS resistor. The clock frequency, fCLK, will vary with
VCONTROL. Again, this circuit decouples the relationship
between the current out of the RBIAS pin and the voltage
of the RBIAS pin; the frequency accuracy will be degraded.
The clock frequency, however, will increase monotonically
with decreasing VCONTROL.
Operation Using an External Clock
The LTC6603 may be clocked by an external oscillator
for tighter bandwidth control by pulling CLKCNTL (Pin 5)
to ground and driving a clock into CLKIO (Pin 15). If an
external clock is used, the RBIAS resistor is still necessary.
The value of RBIAS must be no larger than the value that
would be required for using the internal oscillator. For
example, a 100k resistor would program the internal oscil-
lator for 24.705MHz, so an external oscillator frequency of
24.705MHz would require an RBIAS resistance of no more
6603f
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