ADN2872ACPZ-R7 datasheet(14/20 Pages) AD

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ADN2872

Rev. 0 | Page 14 of 20

VCC

PHOTODIODE

ADN2872

PAVSET

MICROCONVERTER

ADC

INPUT

Figure 31. Single Measurement of IMPD Across a

Sense Resistor in Resistor Setpoint IMPD Monitoring

LOOP BANDWIDTH SELECTION

To ensure that the ADN2872 control loops have sufficient

bandwidth, the average power loop capacitor (PAVCAP) and

the extinction ratio loop capacitor (ERCAP) are calculated

using the laser slope efficiency and the average power required.

For resistor setpoint control,

PAVCAP





6

(Farad)

PAVCAP

ERCAP 

(Farad)

For voltage setpoint control,

PAVCAP





6

(Farad)

PAVCAP

ERCAP 

(Farad)

where:

PAV (mW) is the average power required.

LI (mW/mA) is the typical slope efficiency at 25°C of a batch of

lasers that are used in a design.

The preceding capacitor estimation formulas are used to obtain

a centered value for the particular type of laser that is used in a

design and average power setting. Laser LI can vary by a factor

of 7 between different physical lasers of the same type and across

temperature without the need to recalculate the PAVCAP and

ERCAP values. In the ac coupling configuration, LI can be

calculated as

MOD





(mW/mA)

where

P1 is the optical power (mW) at the one level, and P0 is

the optical power (mW) at the zero level.

These capacitors are placed between the PAVCAP and ERCAP

pins and ground. It is important that these capacitors are low

leakage multilayer ceramics with an insulation resistance

greater than 100 GΩ or a time constant of 1000 sec, whichever

is less. The capacitor tolerance can be ±30% from the calculated

value to the available off-the-shelf value, including the capacitor’s

own tolerance.

POWER CONSUMPTION

The ADN2872 die temperature must be kept below 125°C. The

LFCSP package has an exposed paddle that should be connected

such that it is at the same potential as the ADN2872 ground pins.

Power consumption can be calculated as:

ICC = ICC min + 0.3 IMOD

P = VCC × ICC + (IBIAS × VBIAS_PIN) + IMOD (VMODP_PIN +

VMODN_PIN)/2

TDIE = TAMBIENT + θJA × P

where:

ICC min is 30 mA, the typical value of ICC provided in Table 1

with IBIAS = IMOD = 0.

TDIE is the die temperature.

TAMBIENT is the ambient temperature.

VBIAS_PIN is the voltage at the IBIAS pin.

VMODP_PIN is the voltage at the IMODP pin.

VMODN_PIN is the voltage at the IMODN pin.

Thus, the maximum combination of IBIAS + IMOD must be

calculated.

AUTOMATIC LASER SHUTDOWN (Tx_DISABLE)

ALS (Tx_DISABLE) is an input that is used to shut down the

transmitter optical output. The ALS pin is pulled up internally

with a 6 kΩ resistor and conforms to SFP MSA specifications.

When ALS is logic high or open, both the bias and modulation

currents are turned off.

BIAS AND MODULATION MONITOR CURRENTS

IBMON and IMMON are current-controlled current sources

that mirror a ratio of the bias and modulation current. The

monitor bias current, IBMON, and the monitor modulation

current, IMMON, should both be connected to ground through

a resistor to provide a voltage proportional to the bias current

and modulation current, respectively. When using a micro-

controller, the voltage developed across these resistors can be

connected to two of the ADC channels, making available a

digital representation of the bias and modulation current.

IBIAS PIN

ADN2872 has one on-chip, 800 Ωpull-up resistor. The current

sink from this resistor is VIBIAS dependent.

IBIAS





(mA)

where

VIBIAS is the voltage measured at the IBIAS pin after setup

of one laser bias current, IBIAS. Usually, when set up, a maximum

laser bias current of 100 mA results in a

VIBIAS of about 1.2 V. In

a worst-case scenario,

VCC = 3.6 V, VIBIAS = 1.2 V, and IUP ≤ 3 mA.

This on-chip resistor helps to damp out the low frequency

oscillation observed from some inexpensive lasers. If the on-

chip resistance does not provide enough damping, one external

RZ may be necessary (see Figure 32 and Figure 33).

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