Current Monitors

The MAX3934 features a bias current monitor output

(BIASMON) and a modulation current monitor output

Ω and 3Ω (±10%)

resistors, respectively. BIASMON and MODMON should

be connected to the inverting input of an operational

amplifier to program the bias and modulation current

(see the Design Procedure section).

Design Procedure

When designing a laser transmitter, the optical output

usually is expressed in terms of average power and

extinction ratio. Table 1 gives relationships helpful in

converting between the optical average power and the

modulation current. These relationships are valid if the

mark density and duty cycle of the optical waveform

are 50%.

Programming the Modulation Current

rent can be calculated based on the laser slope effi-

ciency (

η) using the equations in Table 1.

To program the desired modulation current, connect

the inverting input of an op amp (such as the

MAX4281) to MODMON and connect the output to

MODSET. Connect the positive op-amp voltage supply

graph in the Typical Operating Characteristics to select

modulation current.

Programming the Bias Current

To program the desired laser bias current, connect the

inverting input of an op amp (such as the MAX4281) to

BIASMON and connect the output to BIASSET.

graph in the Typical Operating Characteristics to select

laser bias current.

External Op-Amp Selection

External op amps are required for regulating the bias

and modulation currents. The ability to operate from a

single supply with input common-mode range extending

to the negative supply rail is critical in op-amp selection.

Low bias current and high PSNR are also important. The

op-amp gain bandwidth must be high enough to regu-

late at the power-supply ripple frequency to maintain the

PSNR of the laser driver. Filtering the op-amp output is

recommended (see the Typical Application Circuit). To

maintain stability, the filter capacitor should be smaller

than the op-amp capacitive load specification.

Interfacing with Laser Diodes

Refer to Maxim application note HFAN-2.0: Interfacing

Maxim Laser Drivers with Laser Diodes for detailed

information.

The MAX3934 contains an integrated damping resistor

Ω or 15Ω, depending on part ver-

sion. The modulation output is optimized for driving a

20

10.7Gbps Compact Laser Diode Driver

8

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PARAMETER

SYMBOL

RELATION

Average Power

Extinction Ratio

Optical Power of a “1”

Optical Power of a “0”

Optical Amplitude

Laser Slope Efficiency

Modulation Current

η

Table 1. Optical Power Relations

Note: Assuming a 50% average duty cycle and mark density.

TIME

OPTICAL

POWER

Figure 5. Optical Power Definitions