Analog Devices ADN2841 Application Note

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 •
ADN2841/ADN2847 DC-Coupled Optical Evaluation Kit
INTRODUCTION
This application note describes the 32-lead ADN2841/
ADN2847 laser diode driver Rev. A1 evaluation kit. The
evaluation kit is a demonstration board that provides a
dc-coupled optical evaluation of the ADN2841 or ADN2847.
This document describes how to configure the board in
order to operate these parts optically. The document
contains the following information:
• Board description
• Quick start for optical operation
• Description of board settings
• Component list
• Schematic of board
• Board layout information
• Silkscreen image of board
• Optical eyes
BOARD DESCRIPTION
The ADN2841 is a 5 V dual-loop 50 Mbps to 2.7 Gbps laser
diode driver. The ADN2847 is a 3 V dual-loop 50 Mbps to
3.3 Gbps laser diode driver. To use the board in an opti-
cal configuration, a suitable laser diode must be soldered
onto the board. The monitor photodiode (MPD) current
is fed into the ADN2841/ADN2847 to control the average
power and extinction ratio. The ADN2841 and ADN2847
use automatic power control (APC) to maintain a constant
average power over time and temperature. The ADN2841
and ADN2847 use closed-loop extinction ratio control to
allow optimum setting of extinction ratio for every device.
This board is configured for lasers in mini-DIL packages
only. LEDs for power supply, DEGRADE, and FAIL are made
available for monitoring purposes. Power to the ADN2841
evaluation circuitry is –5 V only. Power to the ADN2847
evaluation circuitry is –3.3 V only.
QUICK START FOR OPTICAL OPERATION
To ensure proper operation in the optical configuration,
verify the following:
1. Jumpers K3 and K4 are connected to A; Jumper K2 is
connected to B.
2. If the input data is clocked, it is necessary to enable
the clock select pin (CLKSEL). CLKSEL is enabled by
REV. 0
By Mark Murphy, Ferenc Barany, and Michael O'Flanagan
APPLICATION NOTE
connecting K4 to B. If the clock inputs are not used, or
the input data is not latched, connect K4 to A.
3. The power supply is diode protected to ensure the
device is not damaged if a positive power supply is
accidently connected. The user may connect Jumper
K1 (short circuit) and power up the board by applying
–5 V to the power input SMA, J3, if the ADN2841 is the
device to be evaluated, or –3.3 V to the power input
SMA, J3, if the ADN2847 is the device being evaluated.
If Jumper K1 is not connected, the user should make
the power supply sufficiently negative to ensure that
the ADN2841 supply is –5 V or the ADN2847 supply is
–3.3 V. The actual DUT supply can be measured at the
anode of D1.
4. Apply a differential signal, typically 500 mV, to J6 and
J7 (DATAN and DATAP). Single-ended operation may
result in a degraded eye.
5. If the clock select pin is enabled by K4, apply a differen-
tial clock signal, typically 500 mV, to J4 and J5 (CLKN
and CLKP). If the clock pin is not enabled, a clock signal
should not be connected.
6. The optical eye and switching characteristics of the
ADN2841/ADN2847 may be observed using a digital
communications analyzer that has an optical input
channel with the required bandwidth.
7. The bias and modulation currents can also be moni-
tored by observing IBMON and IMMON
IBMON and IMMON are both a 1:100 ratio of IBIAS and
IMON. Both are terminated with resistors and so can
be viewed at Test Points T3 and T4 using a voltmeter
or oscilloscope.
8. To establish the desired average power and extinction
ratio, the user should follow this procedure.
a. Withthepowersupplyturnedoff,adjustPotentiometers
R20 (ERSET) and R21 (PSET) to approximately 20 k.
b. Withtheevaluationboardpoweredonandthedatasignal
switching,theusercanreducethevalueofpotentiometer
R21 to establish the desired average optical power.
c. Potentiometer R20 can then be reduced in value to
increase the modulation current, and hence increase
the extinction ratio.The bias current will decrease as the
modulation current increases.The bias and modulation
currents can be monitored using IBMON and IMMON.
AN-631
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respectively.
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