AN-637
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com
INTRODUCTION
This application note describes the 32-lead ADN2848 laser
diode driver Rev. A1 evaluation kit. The evaluation kit is
a demonstration board that provides ac-coupled optical
evaluation of the ADN2848. This document describes how
to congure the board in order to operate this part 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
BOARD DESCRIPTION
The ADN2848 is a 3 V dual-loop 50 Mbps to 1.25 Gbps laser
diode driver. To use the board in an optical conguration,
a suitable laser diode driver must be soldered onto the
board. The monitor photodiode, MPD, current is fed into
the ADN2848 to control the average power and extinction
ratio. The ADN2848 uses automatic power control, APC, to
maintain a constant average power over time and tem-
perature. The ADN2848 also uses closed-loop extinction
ratio control to allow optimum setting of extinction ratio
for every device. This board is congured for lasers in mini-
DIL packages only. LEDs for Power Supply, DEGRADE, and
FAIL are made available for monitoring purposes. Power
to the ADN2848 evaluation circuitry is only –3.3 V.
QUICK START FOR OPTICAL OPERATION
To ensure proper operation in the optical conguration,
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
connecting K4 to B. If the clock inputs are not used, or
if 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)
ADN2848 AC-Coupled Optical Evaluation Kit
By Mark Murphy, Ferenc Barany, and Michael O'Flanagan
and power up the board by applying –3.3 V to the POWER
input SMA, J3. If Jumper K1 is not connected, the user
should make the power supply sufciently negative to
ensure that the 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
ADN2848 may be observed using a digital communi-
cations analyzer that has an optical input channel with
the required bandwidth.
7. The bias and modulation currents can also be monitored
by observing IBMON and IMMON, respectively. IBMON
and IMMON are both a 1:100 ratio of I
BIAS
and IMOD. 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. With the power supply turned off, adjust Potentiometers
R20 (ERSET) and R21 (PSET) to approximately 20 k.
b. With the evaluation board powered on and the data
signal switching, the user can reduce the value of
Potentiometer R21 to establish the desired average
optical power.
c.
Potentiometer R20 can then be reduced in value to
increase the modulation current, and thus increase the
extinction ratio. The bias and modulation currents can
be monitored using IBMON and IMMON.
When adjusting the extinction ratio, the user should allow
adequate time for the eye to settle. The allowable resistance
range at the power set input (PSET), the Extinction Ratio
Set Input (ERSET), and the Alarm Set (ASET) is between
1 k and 25 k. Resistors R31 through R33 ensure that the
resistance at these nodes never falls below the minimum
allowable value. If the node resistances increase above 25 k,
the ADN2848 may not operate within its specications.
REV. 0
