To keep the bias and modulation currents in compli-

ance, the following constraint on the total current must

be made for 50

Ω EAM modules:

Ω≥ 1.55V

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 extend-

ing to the negative supply rail is critical in the op amp

selection. Low bias current and high PSRR also are

important. Bias current to the inverting input passes

through a 5k

Ω resistor. This could add an error to the

voltage produced by the modulation and bias current-

sense resistors. The op amp gain bandwidth must be

high enough to regulate at the power-supply ripple fre-

Pulse-Width Control Setup

Two methods of control are possible when pulse predis-

tortion is desired to minimize distortion at the receiver.

The pulse width can be set with a 2k

Ω potentiometer (or

equivalent fixed resistors); or applying a voltage, with

Table 1 for the desired effect of the pulse-width setting.

I

V

BIAS

BIASREF

=

75

.

Ω

10.7Gbps EAM Driver

_______________________________________________________________________________________

9

Table 1. Pulse-Width Control

2k

Ω

PWC+

PWC-

50

Ω

50

Ω

DATA-

1mA

1mA

DATA+

CLK-

CLK+

50

Ω

50

Ω

50

Ω

50

Ω

50

Ω

50

Ω

D

Q

0

MUX

1

PWC

5k

Ω

5k

Ω

BIASMON

BIASSET

MODSET

EAM

50

Ω

50

Ω

MODN1

MOD

MOD2

MOD1

MODN2

BIAS

L

100

Ω

100

Ω

75

Ω

75

Ω

MODEN

RTEN

Figure 3. MAX3935EGJ Functional Diagram

PULSE

WIDTH

RP, RN for

RP + RN = 2k

Ω

100%

1k

Ω or Open

>100%

RP > RN

<100%

RP < RN