LTC5100EUF datasheet(20/52 Pages) LINER

20 / 52 page

LTC5100

sn5100 5100fs

Figure 17 depicts the current ranges for the source cur-

rent. The guaranteed full scale is 6mA per range. The

minimum operating level should be limited to 1/16 of full

scale to avoid the coarse relative quantization seen in any

ADC or DAC when operated at low levels. The source

range, Is_rng, should be selected as low as possible such

that the source current, IS, stays within the guaranteed

current limits over temperature, considering the laser

temperature characteristics. From Equation 1 we can see

that the source current is the sum of the laser bias and the

average modulation currents:

IS = IB+ IM

(15)

Is_rng should be chosen to support the total current

required for laser bias and modulation, taking temperature

changes in IB and IM into account.

Figure 18 depicts the current ranges for the average

modulation current. This is the average modulation cur-

rent at the MODA and MODB pins of the chip (recall that the

MODA and MODB pins are connected on-chip). The peak-

to-peak modulation at the pins of the chip is twice the

average. Guaranteed full scale is 3mA average or 6mA pp

per range. The minimum operating level should be limited

to 1/8 of full scale to preserve the quality of the eye

diagram. Operating below 1/8 full scale causes increased

overshoot and undershoot. The modulation range, Im_rng,

should be selected as low as possible such that the

modulation current, IM, stays within the guaranteed cur-

rent limits over temperature. The modulation current

varies over temperature to compensate the loss in slope

efficiency typical of most VCSELs. Therefore, the choice of

Im_rng should take temperature changes into account.

High Speed Aspects of the Modulation Output

The LTC5100 modulation output presents a resistive drive

impedance with very low reflection coefficient. This output

design suppresses ringing and reflections to maintain the

quality of the eye diagrams in spite of laser impedance

variations. The reflection coefficient is sufficiently low that

the LTC5100 can drive the laser over an arbitrary length of

transmission line, as shown in Figure 19. A well designed

transmission line stretching the entire length of a typical

transceiver module goes virtually unnoticed in this sys-

tem. The only practical limitation on interconnect length to

the laser is high frequency line loss.

OPERATIO

Figure 17. Ranges for the Source Current

Figure 18. Ranges for the Modulation Current

Figure 19. High Speed Details of the Modulation Output

Is_rng = 0

5100 F17

4.5

Is_rng = 1

RECOMMENDED

MINIMUM IS 1/16

OF FULL SCALE

Is_rng = 2

Is_rng = 3

IS (mA)

4.5

Im_rng = 0

5100 F18

13.5

Im_rng = 1

RECOMMENDED

MINIMUM IS 1/8

OF FULL SCALE

Im_rng = 2

Im_rng = 3

IM (mA)

VSS

LBWB

IS = IB + IM

ZO = RT

VDD

SRC

MODA

MODB

LTC5100

Ω

TYP

10nF

3.2Gbps

MODULATOR

TRANSMISSION

LINE

LBWA

LTC5100EUF Datasheet(PDF) 20 Page - Linear Technology

LTC5100EUF Datasheet(HTML) 20 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

Datasheet Download

Link URL