Data Sheet ADN2848
Rev. B | Page 9 of 12
POWER CONSUMPTION
The ADN2848 die temperature must be kept below 125°C. The
LFCSP_VQ package has an exposed paddle. The exposed
paddle should be connected in such a manner that it is at the
same potential as the ADN2848 ground pins. The θ
JA
for the
package is shown under the Absolute Maximum Ratings. Power
consumption can be calculated using
I
CC
= I
CCMIN
+ 0.3 I
MOD
P = V
CC
× I
CC
+ (I
BIAS
× V
BIAS_PIN
) + I
MOD
(V
MODP_PIN
+ V
MODN_PIN
)/2
T
DIE
= T
AMBIENT
+ θ
JA
× P
Thus, the maximum combination of I
BIAS
+ I
MOD
must be
calculated
where:
I
CCMIN
= 50 mA, the typical value of I
CC
provided on Page 3 with
I
BIAS
= I
MOD
= 0.
T
DIE
= die temperature.
T
AMBIENT
= ambient temperature.
V
BIAS_PIN
= voltage at I
BIAS
pin.
V
MODP_PIN
= average voltage at IMODP pin.
V
MODN_PIN
= average voltage at IMODN pin.
LASER DIODE INTERFACING
Many laser diodes designed for 1.25 Gbps operation are
packaged with an internal resistor to bring the effective
impedance up to 25 Ω in order to minimize transmission line
effects. In high current applications, the voltage drop across this
resistor, combined with the laser diode forward voltage, makes
direct connection between the laser and the driver impractical
in a 3 V system. AC coupling the driver to the laser diode
removes this headroom constraint.
Caution must be used when choosing component values for
ac coupling to ensure that the time constants (L/R and RC, see
Figure 9) are sufficiently long for the data rate and the expected
number of CIDs (consecutive identical digits). Failure to do this
could lead to pattern dependent jitter and vertical eye closure.
For designs with low series resistance, or where external
components become impractical, the ADN2848 supports direct
connection to the laser diode (see Figure 8). In this case, care
must be taken to ensure that the voltage drop across the laser
diode does not violate the minimum compliance voltage on the
IMODP pin.
OPTICAL SUPERVISOR
The PSET and ERSET potentiometers can be replaced with a
dual digital potentiometer, the ADN2850 (see Figure 7). The
ADN2850 provides an accurate digital control for the average
optical power and extinction ratio and ensures excellent
stability over temperature.
CC
V
CC
ADN2848
IMPD
PSET
ERSET
DATAP
DATAN
IDTONE
IMODP
I
BIAS
DATAP
DATAN
IDTONE
SDI
SDO
CLK
CS
DAC1
DAC2
ADN2850
V
CC
Tx
Rx
CLK
CS
02746-007
Figure 7. Application Using the ADN2850 Dual 10-Bit Digital Potentiometer
with Extremely Low Temperature Coefficient as an Optical Supervisor
