ADN2526ACPZ-R2 AD [Analog Devices], ADN2526ACPZ-R2 Datasheet - Page 12

• Manufacturer Part Number: ADN2526ACPZ-R2
• Description: 11.3 Gbps Active Back-Termination, Differential Laser Diode Driver
• Manufacturer: AD [Analog Devices]

ADN2526
The recommended configuration of the MSET, IMODP,
and IMODN pins is shown in Figure 28. See Table 7 for the
recommended components.
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.
Figure 28. Recommended Configuration for the MSET, IMODP, and IMODN Pins
V
Figure 27. Equivalent IMODP and IMODN Pins, As Seen From Laser Side
MSET
220
210
200
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
Figure 29. MSET Voltage-to-Modulation Current Ratio vs.
0
MSET
Figure 26. Equivalent Circuit of the MSET Pin
MSET
ADN2526
VCC
10
VEE
25Ω
DIFFERENTIAL LOAD RESISTANCE (Ω)
Differential Load Resistance
IMODP
IMODN
IBIAS
IMODN
VCC
20
3.3Ω
MINIMUM
MAXIMUM
TYPICAL
800Ω
200Ω
Z
Z
0
0
3.3Ω
= 25Ω
= 25Ω
30
IMODP
VCC
VCC
VCC
L
L
25Ω
C
C
40
VCC
L
L
VCC
Z
Z
0
0
= 25Ω
= 25Ω
50
TOSA
60
Rev. A | Page 12 of 16
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.
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.
Figure 30. Allowable Range for the Voltage at IMODP and IMODN
VCC + 1.1V
VCC – 1.1V
VCC
IMODP, IMODN
NORMAL OPERATION REGION