LTC5100 datasheet(49/52 Pages) LINER

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LTC5100

sn5100 5100fs

APPLICATIO S I FOR ATIO

Figure 31 and Figure 32 show the schematic and layout of

a minimum reflection coefficient, minimum peaking solu-

tion. Two capacitors, C1 and C2 are used to further reduce

the inductance in the termination network. C2 has two vias

to the ground plane.

TEMPERATURE COMPENSATION

The LTC5100 has first and second order digital tempera-

ture compensation for the laser bias current, laser modu-

lation current, and monitor diode current. Recall that in

constant current control mode, the LTC5100 provides

direct temperature compensation of the laser bias current

and the laser modulation current. In automatic power

control mode, the laser bias current is under closed-loop

control and the LTC5100 provides temperature compen-

sation for the monitor diode current and the laser modu-

lation current. The simplest procedure for determining the

temperature coefficients (TC1 and TC2 in Equation 12,

Equation 18, Equation 23, and Equation 29) is as follows:

• Select a nominal or representative laser diode and

assemble it into a transceiver module with the LTC5100.

• Set all temperature coefficients to zero.

• Place the transceiver module in a temperature chamber

and find the values of Ib_nom, Im_nom, and Imd_nom

that give constant average optical power and extinction

ratio at several temperature points.

• Record the LTC5100’s temperature reading, T_int, at

each temperature point.

• Select a convenient value for T_nom, the nominal tem-

perature. (It is customary, but not mandatory, to use

°C for the nominal temperature.)

• Find the best values of TC1 and TC2 by fitting the

quadratic temperature compensation formula (Equa-

tion 12) to the experimental values of Ib_nom, Im_nom,

Imd_nom, and T_int.

To configure the LTC5100 for normal operation, set the

nominal current to the value found at the nominal tem-

perature. Set TC1 and TC2 to the values determined by the

best fit of the data. For standalone operation, store these

values in the EEPROM. For microprocessor operation,

store the values in the microprocessor’s internal non-

volatile memory or in another source of nonvolatile memory

and load them into the LTC5100 after power-up.

The above procedure not only corrects for the laser

temperature drift, but also corrects the small temperature

drift found in the LTC5100’s internal references.

DEMONSTRATION BOARD

Figure 33 shows the schematic of the DC499 demonstra-

tion board. Details of the use of this demo board and

accompanying software can be found in the DC499 demo

board manual. Figure 34 shows the layout of the demo

board and Table 31 gives the bill of materials for the demo

board.

The core applications circuit for the LTC5100 VCSEL

driver appears inside the box in Figure 33. This is the

complete circuit for an optical transceiver module, includ-

ing power supply filtering. It consists of the LTC5100 with

EEPROM for storing setup parameters, L1 and C3 for

power supply filtering, and R1, C1, and C2 for terminating

the 50

Ω modulation output. The circuitry outside the box

in Figure 33 is for support of the demonstration. 5V power

enters through 2-pin connector P2 and is regulated by U3

to 3.3V to power the LTC5100. Connector P1 sends 5V

power and serial control signals to another board, allow-

ing a personal computer to control the LTC5100. U4

produces 1.8VDC to bias the modulation output for elec-

trical eye measurements.

High speed data enters the LTC5100 through SMA con-

nectors J1 and J2. The LTC5100 high speed inputs are

internally AC coupled with rail-to-rail common mode input

voltage range. The input signal swing can go as much as

300mV above VDD or below VSS without degrading perfor-

mance or causing excessive current flow. The high speed

inputs may be AC coupled, in which case the common

mode voltage floats to mid-supply or 1.65V nominally.

A common cathode VCSEL can be attached to the demo

board via SMA connector J3. R1 establishes a precision,

low reflection coefficient 50

Ω modulation drive. By main-

taining a wide band microwave quality 50

Ω path, the

length of the connection to the laser can be arbitrarily long.

The LTC5100 generates 20% to 80% transition times of

60ps (80ps 10% to 90%), corresponding to an instanta-

neous transition filtered by a 4.4GHz Gaussian lowpass

filter. At these speeds the primary limitation on line length

is high frequency loss. For high grade, low loss laboratory

cabling with silver coated center conductor and foamed

PTFE dielectric, a practical limit is about 30cm.

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