2.5 Gbit/s
LiNbO
3
Modulator Driver
GD16076A
General Description
The GD16076A is a 2.5 Gbit/s laser
modulator driver designed for providing a
controllable drive current to an optical
modulator (LiNbO
3
) circuit properly bi-
ased and with 25
Ω
or 50
Ω
characteris-
tic input impedance.
The GD16076A features differential ECL
compatible, wide common mode range
inputs (DIN, NDIN) with loop through ter-
mination capability for optimal input re-
flection coefficient.
The pins OUT and NOUT are open drain
outputs designed for driving an external
load with a characteristic impedance of:
u
25
Ω
(GD16076A-25SLP)
u
50
Ω
(GD16076A-50SLP)
The outputs can sink a current that can
be controlled in the range 40mA - 180mA
by VCIP. The output voltage swing
across an external load may be varied
accordingly.
For the 25
Ω
version the output voltage
swing may be adjusted in the range
1.0 V
P-P
- 4.5 V
P-P
.
For the 50
Ω
version, the output swing
may be adjusted within the range
2.0 V
P-P
- 6.0 V
P-P
(typ.).
The output current may be monitored at
pin SIP.
The part is housed in a high speed 40 pin
leaded Multi Layer Ceramic (MLC)
package.
Features
l
l
l
l
l
2.5 Gbit/s operation.
180 mA maximum modulator current.
ECL compatible differential inputs.
Power dissipation: 1.5 - 2.3 W (typ.)
Available in two versions for:
– 25
Ω
(GD16076A-25SLP)
– 50
Ω
(GD16076A-50SLP)
Housed in a leaded 40 pin MLC
package.
l
Applications
l
Tele Communications systems:
– SDH STM-16
– SONET OC-48
Data Communications.
LiNbO
3
Modulator Driver.
High current laser driver.
High-speed clock buffer.
l
VDD
NOUT OUT
l
l
l
DIN
NDIN
VREF
VCIP
VSIP
Data Sheet Rev. 07
VEE
VEEP VEEP
Functional Details
GD16076A is designed to drive external
loads with a characteristic impedance of:
u
25
Ω
(GD16076A-25SLP)
u
50
Ω
(GD16076A-50SLP).
With DIN high the sink current into OUT
will be high.
In order to avoid reflections, and thereby
to obtain optimum performance, connec-
tions from OUT and/or NOUT must be
made with a transmission line of 25
Ω
(50
Ω),
terminated at the device end into
a matched load. This is because OUT
and NOUT are unterminated open drain
outputs and effectively can be regarded
as modulated current sources.
The termination voltage on an output is
determined by the load impedance Z
L
(25
Ω
or 50
Ω)
and the output current
I
OUT
. The optimum termination voltage
V
CC
is:
V
CC
= V
DD
- 2V + Z
L
× I
OUT
see: “Thermal Considerations” in the sec-
tion
Application Information
below.
The driver output current I
OUT
can be
measured as the voltage drop across a
resistor R
SIP
, internally on the chip, con-
nected in between the VEEP pin and the
SIP pin.
By using an external general purpose
operational amplifier, as illustrated in
Figure 1, the driver output current I
OUT
may be controlled accurately and inde-
pendent of environmental changes.
VREF in Figure 1 should be R
SIP
⋅I
OUT
,
see above. The OP-amp must be able to
drive CIP in the range VEEP - 1.2 V to
VEEP +1.2 V.
R
SIP
is made of two resistors of 4
Ω,
as
shown in Figure on page 1. One of the
resistors are connected to the VEEP pins
4 and 5, and the other to the VEEP pins
16 and 17. If all VEEP are connected,
R
SIP
is 2
Ω.
Each resistor has been
dimensioned to withstand 100 mA.
Therefore if the output current I
OUT
is
> 100 mA the VEEP pins on opposite
sides must be connected externally in
order not to damage the chip.
Notice that the transistors driving OUT
and NOUT are susceptible to breakdown.
Therefore the peak voltage on OUT and
NOUT should never exceed 10.0 V
above VEEP as specified in
Maximum
Ratings.
Application Information
In this section the behaviour of
GD16076A is described when used in
connection with a LiNbO
3
Mach Zender
Interferometer, for simplicity called “the
modulator”. First the modulator and a
transfer function from electrical to optical
signal is derived. Next a SPICE simula-
tion of the application diagram in Figure 1
is shown. In this simulation the modulator
is connected to the output of GD16076A
via a transmission line with a characteris-
tic impedance of 25
Ω.
Finally some ther-
mal properties of GD16076A are
considered.
The Mach Zender
Interferometer.
The modulator is typically made as
shown in Figure 2. Its function is to inten-
sity modulate the incoming unmodulated
light.
In optical data transmission systems, cur-
rent modulation of the light directly in the
semiconductor laser, causes the optical
signal from the laser to have chirp (fre-
quency fluctuations of the light fre-
quency). This degrades system
performance.
Modulator
VCC
25
Optical
In
25
VCC
VDD NOUT
OUT
Optical
Out
50
Data
50
NData
DIN
NDIN
VCIP 50
50
50
100nF
VSIP
1k
VTT = -2V
VEE
-5V
VEEP
-5V -5V
VEEP
VREF
Figure 1.
Application Diagram
Data Sheet Rev. 07
GD16076A
Page 2
Optical Input
Optical Wave Guides
in LINbO3
Active Area
Electrical
Signal Input
d
t
Segmented
Transmission
Line
V
n+1
; L
n+1
L
n
+
V
n
-
VCC
Delay = nd
t
/ v
I
Assuming first that the velocity of light is
much higher than the electrical signals
propagation velocity, the voltage that any
light wave actually sees, will be the aver-
age of the voltages along the transmis-
sion lines, taking into account that the
transmission line represents a loss. Now
this voltage can be used as V
ACT
in the
above formula. In reality the velocity of
light v
l
is approximately c/2.2 for LiNbO
3
,
whereas the electrical signal’s propaga-
tion velocity is approximately
c
c
=
113 3.4
.
where c is the velocity of light in open air.
This means that instead of just averaging
the voltage across the active area, the
voltage that the optical wave actually
sees is a function of time.
This was modeled by splitting up the ac-
tive area into 10 parts, see Figure 2. The
optical wave present at L
n
at time t
n
was
present at L
n+1
at time t
n
- d
t
/ v
l
, where d
t
is the distance from L
n
to Ln+1. The ef-
fective voltage V
ACT
exposed to the light
wave entering the active area at time t -
causing the optical refractive index of the
LiNbO
3
to change, and thereby changing
the velocity of the light - therefore can be
expressed as:
1
N
∑
V
n
(
t
−
ndt
/
V
I
)
N
+
1
n
=
where V
n
(t) is the voltage at L
n
.
V
ACT
=
(2)
V
n
v
sumn
Figure 2.
Optical Modulator with Segmented Transmission Line.
The incoming light to the modulator is an
unmodulated continuous light wave, and
therefore does not suffer from chirp. The
signal treatment within the modulator is
linear, see below. Consequently the
modulated light signal does not suffer
from chirp.
High speed optical communication
systems using a Mach Zender Interfer-
ometer therefore has better system per-
formance than systems using direct
current modulation.
and transmission line characteristics),
and fitting the component values to mea-
surements.
At low modulation rates the relation be-
tween the voltage applied to the modula-
tor and the relative light p on the output
can be described as:
(1
+
cos(
π
(
V
ACT
+
V
OFF
) /
V
Ñ
))
(1)
2
Where:
V
ACT
is the voltage applied to the active
region
V
OFF
is an offset voltage (material de-
pendent)
V
Ñ
is the voltage difference between the
applied voltages that causes fully on and
fully off light on the output respectively.
p
=
From (1) some important features of the
modulator can bee derived. When
(V
ACT
+ V
OFF
)/V
Ñ
= 1 then p = 0.
If (V
ACT
+ V
OFF
)/V
Ñ
becomes slightly
larger than 1 or smaller than 1, p still ap-
proximates 0 very closely. 20 % over (un-
der) shoot causes p to be only 0.1. This
is due to the sine transformation in (1).
Therefore the modulator effectively acts
as a pulse shaper on the voltage V
ACT
defined above and attenuates any small
over and/or undershoot in the electrical
signal.
However if the over and/or undershoot in
the electrical signal becomes larger than
approximately 1/3 V
Ñ
there will be no lim-
iting effect. Instead two pulses will be
created on the optical output. An over-
shoot of 50 % causes p to be 0.5. I.e. in-
stead of only one optical output pulse a
second pulse has been created. There-
fore it is important to ensure that the ring-
ing on the electrical signal is less than
approximately 20%.
The above formula works well at low
modulation rates. However at high speed
data rates the formula does not describe
the function precisely, because the volt-
age actually travels as a wave along the
active part.
Characterisation of
The Modulator
When the light enters the modulator it is
split into two branches, as shown in
Figure 2. On the output it is combined
again.
The propagation delay of the light wave
can be adjusted through one (or both) of
the branches by applying a voltage to the
substrate near the optical wave-guide.
This is because the refractive index of
the material changes proportional to the
voltage applied to the substrate and be-
cause the velocity of light is proportional
to the refractive index. Changing the re-
fractive index in one branch therefore
gives a tuneable delay variation between
the two branches. Thereby the light can
be combined in phase, making the light
pass through to the output without atten-
uation, or in counter phase, thereby turn-
ing off the light.
Typically the electrical data signal for a
high-speed modulator is connected into
the modulator via a transmission line,
traveling along one of the optical
branches. On the output the transmission
line is terminated in order to obtain a
good input impedance.
An equivalent diagram for the modulator
has been derived. The diagram was
made from the physical components of
the modulator (input pin, bonding wires
The above formulas (1) and (2) were
used together with the electrical equiva-
lent diagram for the modulator to make
SPICE simulations of the behaviour of
GD16076A connected to an optical
modulator as shown in Figure 1.
Data Sheet Rev. 07
GD16076A
Page 3
Simulations
In Figure 3 a simulation of the application
diagram in Figure 1 is shown. The modu-
lation current is 180 mA. The simulation
shows the relative optical output power p
as defined above. As shown the optical
output has little ringing. Rise and fall
times are below 90 ps.
In conclusion the above simulations have
shown that GD16076A is capable of
driving a Mach Zender Interferometer
with a 180 mA drive current into 25
Ω
at
2.5 Gbit/s.
1.0
180 mA Modulation Current into 25
Ω
Temperature 75.0
o
C
0.5
Thermal Considerations
As shown in Figure 1 both the modulator
and the unused NOUT outputs are termi-
nated to V
CC
. In order to reduce the chip
power consumption it is important that
V
CC
is kept as low as possible. If e.g. the
output current is 180 mA into 25
Ω
the
voltage swing will be 4.5 V.
If V
CC
= 4.5 V the power consumption P
OC
due to output current will be:
P
OC
=
(V
CC
-V
EEP
-180mA × 25Ω) × 180mA = 900mW
(3)
0
0s
0.5ns
1.0ns
1.5ns
2.0ns
2.5ns
3.0ns
3.5ns
Figure 3.
Simulation of the Optical Output of the Modulator
If instead V
CC
= 2.5 V is used the power
consumption will be only 540 mW.
Approximately 360 mW of P
OC
is con-
sumed by the current source and current
sense resistors, see Figure on page 1 re-
gardless of V
CC
. Therefore the power
consumption in the output FET’s are re-
duced from 540 mW (270 mW each out-
put FET) down to 180 mW (90 mW each
output FET).
Therefore it is recommended to use the
lowest possible value for V
CC
, which does
not sacrifice the performance of the mod-
ulator driver. The open drain outputs of
GD16076A works down to V
DD
- 2 V.
With 180 mA output current into 25 W
this means that V
CC
can be as low as
2.5 V.
Data Sheet Rev. 07
GD16076A
Page 4
Pin List
Mnemonic:
DIN
NDIN
OUT
NOUT
(OUT)
(NOUT)
VCIP
VSIP
VDD
Pin No.:
32, 32
29, 30
9, 10
11, 12
(9)
(12)
6, 15
2, 19
3, 7, 8, 13, 14, 18,
21, 23, 24, 25, 26,
28, 33, 35, 36, 37,
40
(10,11)
VEE
VEEP
VREF
1, 20, 22, 27, 34,
39
4, 5, 16, 17
38
PWR
PWR
Analogue IN
BIAS IN
Analogue OUT
PWR
Pin Type:
ECL IN
OPEN DRAIN
Description:
Loop through’ data inputs.
Data Outputs for 25Ω version GD16076A-25SLP.
(Data Outputs for 50Ω version GD16076A-50SLP).
Driver Current control input.
Driver Current sense output.
Ground. Common for both 25Ω and 50Ω versions.
(Only 50Ω version GD16076A-50SLP)
Negative Supply.
Negative Supply.
For normal operation leave open.
Data Sheet Rev. 07
GD16076A
Page 5
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