LTC5100 datasheet(21/52 Pages) LINER

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LTC5100

sn5100 5100fs

Figure 19 shows how the LTC5100 achieves a low reflec-

tion coefficient. The unavoidable capacitance of the high

speed driver transistor, bond pads and ESD protection

circuitry (C1) is compensated by the inductance of the

bond wires (LBWA and LBWB).

The high speed behavior of the circuit in Figure 19 can be

understood in greater detail by examining the simplified

circuit in Figure 20. In Figure 20 the switched current

source (M1 in Figure 19) launches a current step (1)

toward the termination resistor (2A) and toward the trans-

mission line (2B) connected to the laser. The laser is

typically mismatched to the line impedance and reflects a

portion of the incident wave (3) back toward the MODB

pin. There it encounters an L-C-L structure composed of

the bond wires and driver capacitance. This structure is

carefully designed as a lumped element approximation to

the transmission line impedance. It therefore transmits

wave (3) through the IC package without reflecting energy

back toward the laser. The traveling wave passes through

the chip largely unimpeded (4) and is absorbed by the

matched termination resistor, RT.

The matched termination is provided by the termination

resistor, RT, decoupled by capacitor CT. CT forms an AC

short across the entire frequency range contained in the

modulation data.

The termination resistor, RT, need not be 50Ω. 50Ω is best

for electrical testing because it matches the impedance of

most high frequency instruments. RT can be made smaller,

Ω, for example, to more closely match a laser with low

dynamic impedance or to allow more voltage headroom at

the SRC pin. This may be necessary for lasers that run at

high voltages or high bias currents. RT can be made larger,

Ω for example, to more closely match a laser with high

dynamic impedance or if a narrow, high impedance PC

board trace is needed to connect to the laser.

Figure 21 shows that the high speed modulation current is

confined to the ground system, laser and back termination

network. No high speed current circulates in the power

supply where it could cause radiation and interference

problems.

HIGH SPEED DATA INPUTS

The high speed data inputs, IN+ and IN–, are internally

terminated in 50

Ω and internally AC coupled, eliminating

the need for external termination resistors and AC cou-

pling capacitors. Figure 10 shows the equivalent circuit

for the high speed data pins. By default, the high speed

data inputs are terminated differentially with 100

Ω for

compatibility with LVDS, PECL and similar differential

signaling standards (Cml_en = 0). Alternately, the inputs

can be programmed for 50

Ω single-ended termination to

the power supply for biasing a current mode logic (CML)

driver. To select CML compatibility, program Cml_en to 1.

Although internally AC coupled, the inputs are biased with

high valued resistors (50k equivalent) to VDD(HS)/2, so the

LTC5100 remains compatible with external AC coupling

capacitors. When externally AC coupled, the inputs self-

bias to approximately VDD(HS)/2.

Internal AC coupling gives the LTC5100 rail-to-rail input

common mode capability. The inputs can be driven as

much as 300mV beyond the rail during peak excursions.

The AC coupling circuit is a distributed highpass filter with

OPERATIO

Figure 21. High Speed Current Flow in the Modulation Output

MODA

MODB TRANSMISSION

LINE

ZO = RT

5100 F20

Ω

TYP

LBWA

LBWB

VSS

EXPOSED

PAD

VDD

SRC

MODA

MODB

5100 F21

LTC5100

10nF

NO HIGH

SPEED

CURRENT

Ω

3.2Gbps

MODULATOR

Figure 20. Wave Propagation in the Laser Interconnect

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