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ADN2873
(Rev.:RevB)

3.3 V, 50 Mbps to 4.25 Gbps, Single-Loop, Laser Diode Driver

Analog Devices 

Data Sheet

ADN2873

RESISTOR SETPOINT CALIBRATION

In resistor setpoint calibration, the PAVREF, ERREF, and RPAV

pins must all be tied to VCC. The average power and extinction

ratio can be set using the PAVSET and ERSET pins, respectively.

A resistor is placed between the pin and GND to set the current

flowing in each pin, as shown in Figure 30. The ADN2873

ensures that both PAVSET and ERSET are kept 1.23 V above

GND. The PAVSET and ERSET resistors are given by

RPAVSET

=

1.2 V

PAV × RSP

RERSET = 1.2 V × 100

IMOD

where:

PAV is the average power required (mW).RSP is the optical

responsivity (in mA/mW).

IMOD is the modulation current required (mA).

Power-On Sequence in Resistor Setpoint Mode

(kΩ)

(kΩ)

Note that during power-up, the ADN2873 starts an initial process

sequence that allows 25 ms before enabling the device alarms.

The resistors connected to the PAVSET and ERSET pins must

be stable within 20 ms after turning on the power supply. The

ADN2873 alarm may kick in and assert FAIL, provided the

PAVSET and ERSET resistors are stabilized 20 ms after turning

on the power supply.

IMPD MONITORING

IMPD monitoring can be implemented for voltage setpoint and

resistor setpoint as described in the following sections.

Voltage Setpoint

In voltage setpoint calibration, two methods can be used for

IMPD monitoring: measuring voltage at RPAV and measuring

IMPD across a sense resistor.

Method 1: Measuring Voltage at RPAV

The IMPD current is equal to the voltage at RPAV divided by the

value of RPAV (see Figure 31) as long as the laser is on and is

being controlled by the control loop. This method does not

provide a valid IMPD reading when the laser is in shutdown or fail

mode. A microconverter-buffered ADC input can be connected to

RPAV to make this measurement. No decoupling or filter capaci-

tors must be placed on the RPAV node because this can disturb

the control loop.

VCC

PHOTODIODE

MICROCONVERTER

ADC

INPUT

PAVSET

ADN2873

RPAV

R

1kΩ

Method 2: Measuring IMPD Across a Sense Resistor

The second method has the advantage of providing a valid IMPD

reading at all times but has the disadvantage of requiring a

differential measurement across a sense resistor directly in

series with the IMPD. As shown in Figure 32, a small resistor, Rx,

is placed in series with the IMPD. If the laser used in the design

has a pinout where the monitor photodiode cathode and the

lasers anode are not connected, a sense resistor, Rx, can be placed

in series with the photodiode cathode and VCC, as shown in

Figure 33. When choosing the value of the resistor, the user must

take into account the expected IMPD value in normal operation.

The resistor must be large enough to make a significant signal

for the buffered ADC to read, but small enough not to cause a

significant voltage reduction across the photodiode. The voltage

across the sense resistor must not exceed 250 mV when the laser

is in normal operation. It is recommended that a 10 pF capacitor

be placed in parallel with the sense resistor.

VCC

PHOTODIODE

LD

MICROCONVERTER

ADC DIFFERENTIAL

INPUT

200Ω

Rx

PAVSET

ADN2873

10pF

Figure 32. Differential Measurement of IMPD Across a Sense Resistor

VCC VCC

200Ω

Rx

MICROCONVERTER

LD

ADC

INPUT

PHOTODIODE

PAVSET

ADN2873

Figure 33. Single Measurement of IMPD Across a Sense Resistor

Resistor Setpoint

In resistor setpoint calibration, the current through the resistor

from PAVSET to GND is the IMPD current. The recommended

method for measuring the IMPD current is to place a small resis-

tor in series with the PAVSET resistor (or potentiometer) and

measure the voltage across this resistor, as shown in Figure 34. The

IMPD current is then equal to this voltage divided by the value of

resistor used. In resistor setpoint calibration, PAVSET is held to

1.2 V nominal; it is recommended that the sense resistor be

selected so that the voltage across the sense resistor does not

exceed 250 mV.

Figure 31. Single Measurement of IMPD at RPAV in Voltage Setpoint Mode

Rev. B | Page 15 of 20

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