ADN2531

Rev. 0 | Page 14 of 20

LOAD MISTERMINATION

Due to its excellent S22 performance, the ADN2531 can drive

differential loads that range from 5 Ω to 140 Ω. In practice, many

TOSAs have differential resistance not equal to 100 Ω. In this

case, with 100 Ω differential transmission lines connecting the

ADN2531 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 ADN2531

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 ADN2531

must be made with 100 Ω differential (50 Ω single-ended)

transmission lines so that the driver end of the transmission

lines is properly terminated.

⎟

⎠

⎞

⎜

⎝

⎛

×

+

⎟

⎠

⎞

⎜

⎝

⎛

+

×

=

A

V

V

V

I

V

V

P

BSET

IBIAS

SUPPLY

MSET

CC

/

10

8

.

5

To ensure long-term reliable operation, the ADN2531 junction

temperature must not exceed 150°C, as specified in Table 3. For

improved heat dissipation, the module case can be used as a

heat sink, as shown in Figure 38.

DIE

PACKAGE

THERMALCOMPOUND

MODULE CASE

PCB

VIAS

COPPER PLANE

THERMOCOUPLES

CROSSPOINT ADJUST

The crossing level in the output electrical eye diagram can be

adjusted between 35% and 65% using the crosspoint adjust (CPA)

control input. This can be used to compensate for asymmetry in

the laser response and to optimize the optical eye mask margin.

The CPA input is a voltage-control input, and a plot of eye cross-

point vs. CPA control voltage is shown in Figure 14 and Figure 15

in the Typical Performance Characteristics section. The equivalent

circuit for the CPA pin is shown in Figure 37. To disable the

crosspoint adjust function and set the eye crossing to 50%, the

Figure 38. Typical Optical Module Structure

A compact optical module is a complex thermal environment, and

calculations of device junction temperature using the junction-to-

accurate results. The following equation, derived from the

model in Figure 39, can be used to estimate the IC junction

temperature:

(

)

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

×

=

7k

Ω

CPA

7k

Ω

7k

Ω

wher

he temperature at the top of the package in °C.

.

tts.

tion to the top of

ermal resistance from the IC junction to the

e:

is t

is the temperature at the package exposed paddle in °C

is the IC junction temperature in °C.

P

is the ADN2531 power dissipation in wa

the package.

exposed paddle of the package.

Figure 37. Equivalent Circuit for CPA Pin

POWER CONSUMPTION

The power dissipated by the ADN2531 is given by

P

θ

J-TOP

θ

J-PAD

BIAS

IBIAS

SUPPLY

MSET

CC

I

V

I

V

V

P

×

+

⎟

⎠

⎞

⎜

⎝

⎛

+

×

=

8

.

5

where:

Figure 39. Electrical Model for Thermal Calc ations

perature

are given in Table 2.

ul

PAD

can be determined by measuring the tem

at points inside the module, as shown in Figure 38. The thermo-

couples should be positioned to obtain an accurate measurement of