ADN2531ACPZ-R7 datasheet(13/20 Pages) AD

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ADN2531

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The equivalent circuits for the MSET, IMODP, and IMODN pins

are shown in Figure 32 and Figure 33. The two 50 Ω resistors in

Figure 33 represent the active back-termination resistance.

VCC

MSET

400

Ω

200

Ω

VCC

Figure 32. Equivalent Circuit of the MSET Pin

VCC

7.7

Ω

7.7

Ω

IMODP

IMODN

Ω

Figure 33. Equivalent Circuit of the IMODP and IMODN Pins

The recommended configuration of the MSET, IMODP, and

IMODN pins is shown in Figure 34. See Table 6 for recom-

mended components. When the voltage on DATAP is greater

than the voltage on DATAN, the modulation current flows into

the IMODP pin and out of the IMODN pin, generating an

optical Logic 1 level at the TOSA output when the TOSA is

connected as shown in Figure 34.

ADN2531

MSET

VMSET

GND

IMODN

IMODP

FP/DFB

TOSA

Z0 = 25Ω

ZL = 100Ω

Z0 = 25Ω

VCC

IBIAS

Figure 34. 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 35.

100

110

120

130

140

150

160

170

180

190

200

1020304050

DIFFERENTIAL RLOAD (Ω)

MAXIMUM

TYPICAL

MINIMUM

Figure 35. 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 600 Ω resistance of the MSET pin. To be able to drive

80 mA modulation currents through the differential load, the

output stage of the ADN2531 (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 × 50 Ω. This is the case when the

load impedance (RTOSA) is less than 100 Ω differential because

the transmission line characteristic impedance sets the peak-to-

peak amplitude. For the case where RTOSA is greater than 100 Ω,

the single-ended, peak-to-peak amplitude is IMOD × RTOSA ÷ 2.

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 36. See the

Headroom Calculations section for examples of headroom

calculations.

VIMODP, VIMODN

VCC

VCC – 1.1V

VCC + 1.1V

NORMAL OPERATION REGION

Figure 36. Allowable Range for the Voltage at IMODP and IMODN

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