ADN2525

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

input stage (DATAP and DATAN pins) and gained up 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. The

equivalent circuits for MSET, IMODP, and IMODN are shown

in Figure 26 and Figure 27.

VCC

MSET

800

Ω

200

Ω

VCC

Figure 26. Equivalent Circuit of the MSET Pin

VCC

VCC

3.3

Ω

3.3

Ω

25

Ω

IMODP

IMODN

25

Ω

Figure 27. Equivalent Circuit of the IMODP and IMODN Pins

The recommended configuration of the MSET, IMODP, and

IMODN pins is shown in Figure 28. See Table 5 for recom-

mended components.

ADN2525

MSET

VMSET

GND

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

55

0

5

10

15

20

25

30

35

40

45

50

210

200

190

180

170

160

150

140

130

120

110

100

90

80

70

60

MINIMUM

TYPICAL

MAXIMUM

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 ADN2525 (IMODP, 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,

since 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 this pin over supply,

temperature, and modulation current range as shown in

Figure 30. See the Applications Information section for

example headroom calculations.

VIMODP, IMODN

VCC

VCC – 1.1V

VCC + 1.1V

NORMAL OPERATION REGION

Figure 30. Allowable Range for the Voltage at

IMODP and IMODN