ADN2526

Rev. A | Page 12 of 16

VCC

MSET

800Ω

200Ω

VCC

Figure 26. Equivalent Circuit of the MSET Pin

VCC

VCC

3.3Ω

3.3Ω

25Ω

IMODP

IMODN

25Ω

Figure 27. Equivalent IMODP and IMODN Pins, As Seen From Laser Side

The recommended configuration of the MSET, IMODP,

and IMODN pins is shown in Figure 28. See Table 7 for the

recommended components.

ADN2526

MSET

VEE

IMODP

IBIAS

VCC

L

C

TOSA

L

IMODN

VCC

VCC

L

C

L

Figure 28. Recommended Configuration for the MSET, IMODP, and IMODN Pins

The ratio between the voltage applied to the MSET pin and the

differential modulation current available at the IMODP and

IMODN pins is a function of the load resistance value, as shown

in Figure 29.

DIFFERENTIAL LOAD RESISTANCE (Ω)

MINIMUM

MAXIMUM

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

220

210

0

10203040

5060

TYPICAL

Figure 29. MSET Voltage-to-Modulation Current Ratio vs.

Differential Load Resistance

Using the resistance of the TOSA, the user can calculate the

voltage range that should be applied to the MSET pin to generate

the required modulation current range (see the example in the

Applications Information section).

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

drive the 1 kΩ resistance of the MSET pin. To be able to drive

80 mA modulation currents through the differential load, the

output stage of the ADN2526 (the IMODP and IMODN pins)

must be ac-coupled to the load. The voltages at these pins have

a dc component equal to VCC and an ac component with

single-ended, peak-to-peak amplitude of IMOD × 25 Ω. This is

the case even if the load impedance is less than 50 Ω differential,

because the transmission line characteristic impedance sets the

peak-to-peak amplitude. For proper operation of the output stage,

the voltages at the IMODP and IMODN pins must be between

the compliance voltage specifications for these pins over supply,

temperature, and modulation current range, as shown in Figure 30.

See the Applications Information section for examples of

headroom calculations.

IMODP, IMODN

VCC

VCC – 1.1V

VCC + 1.1V

NORMAL OPERATION REGION

Figure 30. Allowable Range for the Voltage at IMODP and IMODN

LOAD MISTERMINATION

Due to its excellent S22 performance, the ADN2526 can drive

differential loads that range from 5 Ω to 50 Ω. In practice, many

TOSAs have differential resistance less than 50 Ω. In this case, with

50 Ω differential transmission lines connecting the ADN2526 to

the load, the load end of the transmission lines are misterminated.

This mistermination leads to signal reflections back to the driver.

The excellent back-termination in the ADN2526 absorbs these

reflections, preventing their reflection back to the load. This

enables excellent optical eye quality to be achieved, even when

the load end of the transmission lines is significantly mistermi-

nated. The connection between the load and the ADN2526 must

be made with 50 Ω differential (25 Ω single-ended) transmission

lines so that the driver end of the transmission lines is properly

terminated.