Datasheet

LTC6603
16
6603fa
APPLICATIONS INFORMATION
The following graphs show a few of the possible lowpass
fi lters.
Gain and Group Delay vs Frequency
(2.5MHz Lowpass Response)
Gain and Group Delay vs Frequency
(650kHz Lowpass Response)
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
R
BIAS
(Pin 4) and avoid routing noisy signals near R
BIAS
.
Use a ground plane connected to Pin 14 and the Exposed
Pad (Pin 25).
FREQUENCY (Hz)
GAIN (dB)
GROUP DELAY (µs)
6603 G17
0
–20
–100
–80
–60
–40
–120
1.2
1.0
0.2
0.4
0.6
0.8
0
100k 10M1M
GAIN
GROUP DELAY
FREQUENCY (Hz)
GAIN (dB)
GROUP DELAY (µs)
6603 G18
0
–60
–40
–20
–80
1
2
3
4
0
100k 1M
GAIN
GROUP DELAY
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 R
BIAS
pin. The circuit in Figure 6
sets the clock frequency by using a programmable current
source and in the expression for f
CLK
, the resistor R
BIAS
is replaced by the ratio of 1.17V/I
CONTROL
. Because the
voltage of the R
BIAS
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
R
BIAS
pin is:
V
RBIAS
•k
2
N
•R1
the equivalent R
BIAS
is:
2
N
•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 confi gured as
a VCO. A voltage source is connected in series with the
R
BIAS
resistor. The clock frequency, f
CLK
, will vary with
V
CONTROL
. Again, this circuit decouples the relationship
between the current out of the R
BIAS
pin and the voltage
of the R
BIAS
pin; the frequency accuracy will be degraded.
The clock frequency, however, will increase monotonically
with decreasing V
CONTROL
.
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 R
BIAS
resistor is still necessary.
The value of R
BIAS
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 R
BIAS
resistance of no more