ADN2815 Data Sheet
Rev. C | Page 20 of 24
Transmission Lines
Use of 50 Ω transmission lines is required for all high frequency
input and output signals to minimize reflections: PIN, NIN,
CLKOUTP, CLKOUTN, DATAOUTP and DATAOUTN (also
REFCLKP and REFCLKN, if a high frequency reference clock is
used, such as 155 MHz). It is also necessary for the PIN/NIN
input traces to be matched in length, and the CLKOUTP/
CLKOUTN and DATAOUTP/DATAOUTN output traces to be
matched in length to avoid skew between the differential traces.
The high speed inputs, PIN and NIN, are internally terminated
with 50 Ω to an internal reference voltage (see Figure 20).
A 0.1 μF is recommended between VREF, Pin 3, and GND to
provide an ac ground for the inputs.
As with any high speed mixed-signal design, take care to keep
all high speed digital traces away from sensitive analog nodes.
04952-0-026
C
IN
C
IN
50
0.1F
50
3k
NIN
PIN
ADN2815
2.5V
VREF
50
50
TIA
CC
Figure 20. ADN2815 AC-Coupled Input Configuration
Soldering Guidelines for Lead Frame Chip Scale Package
The lands on the 32-lead LFCSP are rectangular. The printed
circuit board (PCB) pad for these should be 0.1 mm longer than
the package land length and 0.05 mm wider than the package
land width. The land should be centered on the pad. This
ensures that the solder joint size is maximized. The bottom of
the chip scale package has a central exposed pad. The pad on
the PCB should be at least as large as this exposed pad. The user
must connect the exposed pad to VEE using plugged vias so
that solder does not leak through the vias during reflow. This
ensures a solid connection from the exposed pad to VEE.
Choosing AC Coupling Capacitors
AC coupling capacitors at the input (PIN, NIN) and output
(DATAOUTP, DATAOUTN) of the ADN2815 must be chosen
such that the device works properly over the full range of data
rates used in the application. When choosing the capacitors, the
time constant formed with the two 50 Ω resistors in the signal
path must be considered. When a large number of consecutive
identical digits (CIDs) are applied, the capacitor voltage can
droop due to baseline wander (see Figure 21), causing pattern-
dependent jitter (PDJ).
The user must determine how much droop is tolerable and
choose an ac coupling capacitor based on that amount of droop.
The amount of PDJ can then be approximated based on the
capacitor selection. The actual capacitor value selection can
require some trade-offs between droop and PDJ.
For example, assuming 2% droop can be tolerated, then the
maximum differential droop is 4%. Normalizing to V p-p:
Droop = Δ V = 0.04 V = 0.5 V p-p (1 − e
–t/τ
); therefore, τ = 12t
where:
τ is the RC time constant (C is the ac coupling capacitor, R =
100 Ω seen by C).
t is the total discharge time, which is equal to nT.
n is the number of CIDs.
T is the bit period.
The capacitor value can then be calculated by combining the
equations for τ and t:
C = 12 nT/R
Once the capacitor value is selected, the PDJ can be
approximated as
6.0/15.0
r
nT/RC
pspp
etPDJ
where:
PDJ
pspp
is the amount of pattern-dependent jitter allowed;
< 0.01 UI p-p typical.
t
r
is the rise time, which is equal to 0.22/BW,
where BW ~ 0.7 (bit rate).
Note that this expression for t
r
is accurate only for the inputs.
The output rise time for the ADN2815 is ~100 ps, regardless of
data rate.
