LTC6957-2_15 Datasheet, PDF (22/36 Pages) Linear Technology – Low Phase Noise, Dual Output Buffer/Driver/ Logic Converter

LTC6957-1/LTC6957-2/
LTC6957-3/LTC6957-4
Applications Information
Figure 6 shows a suggested CMOS to LTC6957 interface.
The transmission line shown is the PCB trace and the
component values are for a characteristic impedance of
50Ω, though they could be scaled up or down for other
values of Z0. The R1/R2 divider at the CMOS output cuts
the Thevenin voltage in half when the Z_OUT of the driver is
included. More importantly, it drives the transmission line
with a Thevenin driving resistance of 50Ω, matching the
Z0 of the line. On the other end of the line, a 50Ω load is
presented, minimizing reflections. This results in a second
2:1 attenuation in voltage, so the LTC6957 input will be
approximately 800mV_P-P with 3V CMOS; 1.25V_P-P with 5V
and 600mV_P-P with 2.5V. All of these levels are less than
the maximum input swing of 2V_P-P yet with clean edges
and fast slew rates should be able to realize the full phase
noise performance of the LTC6957.
CMOS
R1
75Ω
R_OUT ≈ 25Ω
Z0 = 50Ω
R2
100Ω
+
50Ω LTC6957
–
phase noise spectrum related to the other signals pro-
cessed in the driver.
Input Resistors
The LTC6957 input resistors, seen in Figure 1, are present
at all times, including during shutdown. Although they
constitute a large portion of the shutdown current, this
behavior prevents the shutdown and wake-up cycling of
the LTC6957 from “kicking back” into prior stages, which
could create large transients that could take a while to
settle. Particularly in the common case of AC-coupling
where the coupling cap charge is preserved.
Input Filtering
The LTC6957 includes input filtering with three narrowband
settings in addition to the full bandwidth limitation of the
circuit design.
Table 1
FILTA
Low
High
FILTB
Low
Low
BANDWIDTH
1200MHz (Full Bandwidth)
500MHz (–3dB)
6957 F06
Low
High
160MHz (–3dB)
Figure 6. CMOS Input
The various capacitors are for AC-coupling and should
have Z << 50Ω at the operating frequency. The capacitors
allow the LTC6957 to set its own DC input bias level, and
reduce the DC current drain, which at 12.4mA (for the
case of a driver powered from 3.3V) is significant. This
current drain can be reduced (with some potential for a
noise penalty) by increasing the attenuation at the R1/
R2 network, taking care to keep the Thevenin impedance
equal to the Z0 of the trace.
When using CMOS logic, it is important to consider how
all of the output drivers, in the same IC, are being used.
For best performance, the entire IC should be devoted to
driving the LTC6957, or if other gates in the same pack-
age must be put to use, they should only carry the same
timing signal (such as for fan-out) or be multiplexed in
time so that only one timing signal is being processed at
a time, such as for multiplexing selective shutdowns of
different segments of a system. Otherwise performance
is likely to suffer with spurs or other interference in the
High
High
50MHz (–3dB)
For slow slewing signals (i.e., <100MHz sine wave signals)
substantially lower phase noise can be achieved by using
this feature. Bandwidth limiting is useful because it limits
the impact of all of the spectral energy that will alias down
to (on top of) the fundamental frequency.
The best filter setting to use for a given application will
depend on the clock frequency, amplitude, and waveform
shape, with the single biggest determinant being the slew
rate at the input of the LTC6957. Any amplifier noise will
add phase noise inversely proportional to its input slew
rate, just from the dV/dt changing voltage noise to time
base noise. But a fast slew rate may not be possible with
other design constraints, such as the use of sine waves
for EMI/RFI reasons, signal losses, etc. A limiting ampli-
fier such as the LTC6957 should have enough bandwidth
to preserve the slew rate of the input. But any additional
bandwidth will provide no improvement in phase noise
due to slew rate preservation, while incurring a phase
noise penalty from noise aliasing.
6957f
22
For more information www.linear.com/LTC6957-1