ADN2872ACPZ-R7 datasheet(13/20 Pages) AD

13 / 20 page

ADN2872

Rev. 0 | Page 13 of 20

RESISTOR SETPOINT CALIBRATION

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

pins must all be tied to VCC. 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 27. The ADN2872

ensures that both PAVSET and ERSET are kept 1.2 V above

GND. The PAVSET and ERSET resistors are given by:

PAVSET





(Ω)

MPD

ERSET











(Ω)

where:

PAV (mW) is the average power required.

RSP (A/W) is the monitor photodiode responsivity.

PCW (mW) is the dc optical power specified on the laser data

sheet.

IMPD_CW (mA) is the MPD current at that specified PCW.

ER is the desired extinction ratio (ER = P1/P0).

IMPD MONITORING

IMPD monitoring can be implemented for voltage setpoint and

resistor setpoint as follows.

Voltage Setpoint

In voltage setpoint calibration, the following methods can 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 28) 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 con-

nected 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

ADN2872

Ω

PAVSET

RPAV

MICROCONVERTER

ADC

INPUT

Figure 28. Single Measurement of IMPD at 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 29, 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 30.

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 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

PHOTODIODE

MICROCONVERTER

ADC

DIFFERENTIAL

INPUT

200

Ω

10pF

PAVSET

ADN2872

Figure 29. Differential Measurement of IMPD Across a Sense Resistor

VCC

PHOTODIODE

MICROCONVERTER

ADC

INPUT

PAVSET

ADN2872

200

Ω

Figure 30. Single Measurement of IMPD Across a Sense Resistor

Resistor Setpoint

In resistor setpoint calibration, the current through the resistor

from PAVSET to ground is the IMPD current. The recommended

method for measuring the IMPD current is to place a small

resistor in series with the PAVSET resistor (or potentiometer)

and measure the voltage across this resistor, as shown in Figure 31.

The IMPD current is then equal to this voltage divided by the

value of the resistor used. In resistor setpoint, PAVSET is held to

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

be selected so that the voltage across the sense resistor does not

exceed 250 mV.

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