ltc5100 Linear Technology Corporation, ltc5100 Datasheet - Page 20

• Manufacturer Part Number: ltc5100
• Description: 3.3v, 3.2gbps Vcsel Driver
• Manufacturer: Linear Technology Corporation

OPERATIO
LTC5100
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, I
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_rng should be chosen to support the total current
required for laser bias and modulation, taking temperature
changes in I
20
I
S
= I
4.5
13.5
B
36
27
18
4.5
Figure 18. Ranges for the Modulation Current
9
0
18
+ I
9
0
I
Figure 17. Ranges for the Source Current
S
I
M
M
(mA)
B
(mA)
and I
Is_rng = 0
Im_rng = 0
U
M
Is_rng = 1
Im_rng = 1
into account.
Is_rng = 2
S
Im_rng = 2
, stays within the guaranteed
Is_rng = 3
Im_rng = 3
MINIMUM IS 1/16
RECOMMENDED
OF FULL SCALE
MINIMUM IS 1/8
RECOMMENDED
OF FULL SCALE
5100 F17
5100 F18
(15)
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, I
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.
LTC5100
Figure 19. High Speed Details of the Modulation Output
MODULATOR
3.2Gbps
C1
M
, stays within the guaranteed cur-
V
DD
M1
V
I
L
L
M
SS
BWA
BWB
MODA
MODB
SRC
14
11
10
TRANSMISSION
I
S
Z
O
LINE
R
50
TYP
T
= R
I
S
10nF
= I
T
C
T
sn5100 5100fs
B
+ I
M
I
B