ADN2871 datasheet(15/20 Pages) AD

No. de pieza	ADN2871

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

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Fabricante Electrónico	AD [Analog Devices]

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ADN2871

Rev. 0 | Page 15 of 20

RESISTOR SETPOINT CALIBRATION

In resistor setpoint calibration. PAVREF, ERREF, and RPAV

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 ADN2871

ensures that both PAVSET and ERSET are kept 1.23 V above

GND. The PAVSET and ERSET resistors are given by

SP

AV

PAVSET

R

P

R

×

=

V

2

.

1

(Ω)

MOD

ERSET

I

R

100

V

2

.

1

×

=

(Ω)

where:

RSP = is the optical responsivity (in amperes per watt).

IMOD is the modulation current required.

PAV is the average power required.

IMPD MONITORING

IMPD monitoring can be implemented for voltage setpoint and

resistor setpoint as described next.

Voltage Setpoint

In voltage setpoint calibration, two methods may be used for

IMPD monitoring.

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 may be

connected to RPAV to make this measurement. No decoupling

or filter capacitors should be placed on the RPAV node because

this can disturb the control loop.

VCC

PHOTODIODE

ADN2871

R

1k

Ω

µC ADC

INPUT

PAVSET

RPAV

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

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 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 signifi-

cant signal for the buffered ADC to read, but small enough not

to cause a significant voltage reduction across the IMPD. The

voltage across the sense resistor should 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

LD

PHOTODIODE

µC ADC

DIFFERENTIAL

INPUT

200

Ω

RESISTOR

10pF

PAVSET

ADN2871

Figure 32. Differential Measurement of IMPD Across a Sense Resistor

VCC

LD

PHOTODIODE

µC ADC

INPUT

200

Ω

RESISTOR

PAVSET

ADN2871

Figure 33. Single Measurement of IMPD Across a Sense Resistor

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