ADN2530 Analog Devices, Inc., ADN2530 Datasheet - Page 11

• Manufacturer Part Number: ADN2530
• Description: 11.3 Gbps, Active Back-termination, Differential Vcsel Driver
• Manufacturer: Analog Devices, Inc.

The equivalent circuits of the BSET, IBIAS, and IBMON pins
are shown in Figure 25 to Figure 27.
The recommended configuration for BSET, IBIAS, and IBMON
is shown in Figure 28.
The circuit used to drive the BSET voltage must be able to drive
the 1 kΩ input resistance of the BSET pin. For proper operation
of the bias current source, the voltage at the IBIAS pin must be
between the compliance voltage specifications for this pin over
supply, temperature, and bias current range (see Table 1). The
maximum compliance voltage is specified for only two bias
current levels (2 mA and 25 mA), but it can be calculated for
any bias current by
Figure 28. Recommended Configuration for BSET, IBIAS, and IBMON Pins
V
COMPLIANCE
VBSET
BSET
Figure 27. Equivalent Circuit of the IBMON Pin
Figure 26. Equivalent Circuit of the IBIAS Pin
Figure 25. Equivalent Circuit of the BSET Pin
(V) = VCC (V) − 0.75 − 22 × IBIAS (A)
VCC
VCC
VCC
BSET
ADN2530
IBIAS
VCC
GND
800Ω
200Ω
10Ω
100Ω
IBMON
IBIAS
IBMON
VCC
TO LASER CATHODE
VCC
500Ω
100Ω
VCC
2kΩ
L
R
750Ω
IBMON
IBIAS
Rev. A | Page 11 of 20
See the Headroom Calculations section for examples.
The function of Inductor L is to isolate the capacitance of the
IBIAS output from the high frequency signal path. For
recommended components, see Table 6.
AUTOMATIC LASER SHUTDOWN (ALS)
The ALS pin is a digital input that enables/disables both the bias
and modulation currents, depending on the logic state applied,
as shown in Table 5.
Table 5.
ALS Logic State
High
Low
Floating
The ALS pin is compatible with 3.3 V CMOS and TTL logic
levels. Its equivalent circuit is shown in Figure 29.
MODULATION CURRENT
The modulation current can be controlled by applying a dc
voltage to the MSET pin. This voltage is converted into a dc
current by using a voltage-to-current converter that uses an
operational amplifier and a bipolar transistor, as shown in
Figure 30.
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.
MSET
Figure 30. Generation of Modulation Current on the ADN2530
FROM CPA STAGE
800Ω
200Ω
Figure 29. Equivalent Circuit of the ALS Pin
ALS
GND
VCC
35kΩ
VCC
100Ω
IBIAS and IMOD
Disabled
Enabled
Enabled
VCC
2kΩ
ADN2530
100Ω
IMOD
ADN2530
IMODP
IMODN