ADN2526

Rev. A | Page 11 of 16

2Ω

2kΩ

100Ω

IBIAS

VCC

VCC

Figure 21. Equivalent Circuit of the IBIAS Pin

VCC

IBMON

VCC

100Ω

500Ω

VCC

Figure 22. Equivalent Circuit of the IBMON Pin

The recommended configuration for BSET, IBIAS, and IBMON

is shown in Figure 23.

ADN2526

BSET

GND

IBMON

IBMON

IBIAS

TO LASER CATHODE

L

R

1kΩ

IBIAS

Figure 23. Recommended Configuration for the BSET, IBIAS, and IBMON Pins

The circuit used to drive the BSET voltage must be able to drive

the 1 kΩ input resistance of the BSET pin. For proper operation

of the bias current source, the voltage at the IBIAS pin must be

between the compliance voltage specifications for this pin over

supply, temperature, and bias current range (see Table 1). The

maximum compliance voltage is specified for only two bias

current levels (10 mA and 100 mA), but it can be calculated for

any bias current by

(1)

See the Applications Information section for examples of

headroom calculations.

The function of the inductor, L, is to isolate the capacitance of

the IBIAS output from the high frequency signal path. For

recommended components, see Table 7.

AUTOMATIC LASER SHUTDOWN (ALS)

The ALS pin is a digital input that enables/disables both the bias

and modulation currents, depending on the logic state applied,

as shown in Table 5.

Table 5. ALS Functions

ALS Logic State

IBIAS and IMOD

High

Disabled

Low

Enabled

Floating

Enabled

The ALS pin is compatible with 3.3 V CMOS and LVTTL logic

levels. Its equivalent circuit is shown in Figure 24.

VCC

ALS

VCC

100Ω

40kΩ

2kΩ

Figure 24. Equivalent Circuit of the ALS Pin

MODULATION CURRENT

The modulation current can be controlled by applying a dc

voltage to the MSET pin. This voltage is converted into a dc

current by using a voltage-to-current converter using an

operational amplifier and a bipolar transistor, as shown in

Figure 25.

50Ω

200Ω

800Ω

MSET

GND

IMODP

IMODN

ADN2526

VCC

FROM INPUT STAGE

IMOD

Figure 25. Generation of Modulation Current on the ADN2526

This dc current is switched by the data signal applied to the

input stage (DATAP and DATAN pins) and amplified by the

output stage to generate the differential modulation current at

the IMODP and IMODN pins.

The output stage also generates the active back-termination,

which provides proper transmission line termination. Active

back-termination uses feedback around an active circuit to

synthesize a broadband termination resistance. This provides

excellent transmission line termination, while dissipating less

power than a traditional resistor passive back-termination.

A small portion of the modulation current flows in the virtual

50 Ω active back-termination resistor. All of the preset IMOD

modulation current, the range specified in Table 1, flows into

the external load. The equivalent circuits for MSET, IMODP, and

IMODN are shown in Figure 26 and Figure 27. The two 25 Ω

resistors in Figure 27 are not actual resistors. They represent the

active back-termination resistance.