ADN2525

Rev. 0 | Page 12 of 16

LOAD MIS-TERMINATION

Due to its excellent S22 performance, the ADN2525 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

ADN2525 to the load, the load end of the transmission lines are

mis-terminated. This mis-termination leads to signal reflections

back to the driver. The excellent back-termination in the

ADN2525 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 mis-terminated. The connection between the load

and the ADN2525 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.

POWER CONSUMPTION

The power dissipated by the ADN2525 is given by

IBIAS

V

I

V

VCC

P

IBIAS

SUPPLY

MSET

×

+

⎟

⎠

⎞

⎜

⎝

⎛

+

×

=

5

.

13

where:

VCC is the power supply voltage.

IBIAS is the bias current generated by the ADN2525.

IMODP, and IMODN pins of the ADN2525 when

IBIAS = IMOD = 0 expressed in amps (see Table 1).

IBIAS

BSET

SUPPLY

MSET

V

V

I

V

VCC

P

×

+

⎟

⎠

⎞

⎜

⎝

⎛

+

×

=

10

5

.

13

To ensure long-term reliable operation, the junction tempera-

ture of the ADN2525 must not exceed 125°C, as specified in

Table 2. For improved heat dissipation, the module’s case can be

used as heat sink as shown in Figure 31. A compact optical

module is a complex thermal environment, and calculations of

ambient thermal resistance) do not yield accurate results.

TPAD

DIE

PACKAGE

THERMAL COMPOUND

MODULE CASE

PCB

VIAS

COPPER PLANE

THERMO-COUPLE

Figure 31. Typical Optical Module Structure

The following procedure can be used to estimate the IC

junction temperature:

P = Power dissipation in W.

pad.

P

θ

J-TOP

θ

J-PAD

Figure 32. Electrical Model for Thermal Calculations

at points inside the module as shown in Figure 31. The thermo-

couples should be positioned to obtain an accurate measurement

of the package top and paddle temperatures. Using the model

shown in Figure 32, the junction temperature can be calculated

using the following formula:

(

)

TOP

J

PAD

J

TOP

J

PAD

PAD

J

TOP

TOP

J

PAD

J

J

T

T

P

T

−

−

−

−

−

−

θ

+

θ

θ

×

+

θ

×

+

θ

×

θ

×

=

dissipated by the ADN2525.