AD805
The gain of the loop integrator is small for high jitter frequen-
cies, so that larger phase differences between the phase detector
inputs are needed to make the internal loop control voltage big
enough to tune the range of the VCPS. Large phase errors at
high jitter frequencies cannot be tolerated. In this region, the
gain of the loop integrator determines the jitter accommodation.
Since the gain of the loop integrator declines linearly with
frequency, jitter accommodation decreases with increasing jitter
frequency. At the highest frequencies, the loop gain is very small
and little tuning of the VCPS can be expected. In this case, jitter
accommodation is determined by the eye opening of the input
data, the static phase error and the residual loop jitter. The jitter
accommodation is roughly 0.5 UI in this region. The corner
frequency between the declining slope and the flat region is the
3 MHz closed-loop bandwidth of the AD805’s internal
delay-locked loop.
APPLICATIONS
155.52 MBPS CLOCK RECOVERY AND DATA RETIMING
USING AT&T 157-TYPE VHF VOLTAGE-CONTROLLED
CRYSTAL OSCILLATOR
The AD805 design can be used with any VCXO circuit that has
a gain of roughly 1 ϫ 106 rad/volt-sec, a frequency pull range of
at least ±50 ppm, a positive slope (a greater VCXO control
voltage corresponds to a greater output frequency) and a
modulation bandwidth of 500 kHz. These VCXO parameters
contribute to overall circuit low frequency jitter tolerance and
jitter transfer.
The output jitter of the overall circuit is largely determined by
the output jitter of the VCXO. The AD805 adds little jitter
since it just buffers the VCXO frequency output, adding
distortion (duty cycle distortion) of only ±0.5%.
Overall circuit jitter bandwidth is determined by the slope of the
VCXO output frequency vs. control voltage curve. A greater
slope corresponds to a greater jitter bandwidth.
USING THE AD805
Ground Planes
Use of two ground planes, an analog ground plane and a digital
ground plane, is recommended. This will isolate noise that may
be on the digital ground plane from the analog ground plane.
Figure 12 shows a schematic of the AD805 in a 155.52 Mbps
clock recovery and data retiming application with an AT&T
157-Type VCXO (see insert). Figures 15 and 16 show typical
jitter tolerance and jitter transfer curves for the circuit.
Power Supply Connections
Power supply decoupling should take place as close to the IC as
possible. This will keep noise that may be on a power supply
from affecting circuit performance.
Note that the 157-Type VCXO control voltage bandwidth
(modulation bandwidth) varies with respect to control voltage
from 80 kHz to 500 kHz. The low value of this modulation
bandwidth causes some jitter peaking when used with the
AD805. The limited modulation bandwidth introduces excess
phase in the frequency control loop through the VCXO. This
causes the frequency control loop to become less damped. Jitter
peaking of 1 dB or 2 dB results in the jitter transfer function.
The compensation network on the VCXO control voltage
between the AD805 and the 157-Type VCXO shown in Figure
12, effectively reduces the high frequency loop gain through the
frequency control loop. The addition of this compensation
network eliminates jitter peaking. The compensation network
1 kΩ resistor works with the AD805 VCXO CONTROL 1 kΩ
output impedance to halve the loop crossover frequency. This
avoids excess phase caused by the limited modulation band-
width of the 157-Type VCXO.
Use of a 10 µF tantalum capacitor between VEE and ground is
recommended.
Use of 0.1 µF ceramic capacitors between IC power supply or
substrate pins and either analog or digital ground is recom-
mended. Refer to schematic, Figure 12, for advised connections.
The ceramic capacitors should be placed as close to the IC pins
as possible.
Connections from VEE to load resistors for DATAIN, DATAOUT,
CLKIN, and CLKOUT signals should be individual, not daisy
chained. This will avoid crosstalk on these signals.
Transmission Lines
Use of 50 Ω transmission lines are recommended for DATAIN,
DATAOUT, CLKIN, and CLKOUT signals.
Terminations
Termination resistors should be used for DATAIN, CLKIN,
DATAOUT, and CLKOUT signals. Metal, thick film, 1%
tolerance resistors are recommended. Termination resistors for
the DATAIN and CLKIN signals should be placed as close as
possible to the DATAIN and CLKIN pins.
Input Buffer
Use of an input buffer, such as a 10H116 Line Receiver IC, is
suggested for an application where the DATAIN signals do not
come directly from an ECL gate, or where noise immunity on
the DATAIN signals is an issue.
REV. 0
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