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AD5235 View Datasheet(PDF) - Analog Devices

Part NameDescriptionManufacturer
AD5235 Nonvolatile Memory, Dual 1024-Position Digital Potentiometer ADI
Analog Devices ADI
AD5235 Datasheet PDF : 32 Pages
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AD5235
OPTICAL TRANSMITTER CALIBRATION WITH
ADN2841
The AD5235, together with the multirate 2.7 Gbps laser diode
driver, ADN2841, forms an optical supervisory system in which
the dual digital potentiometers can be used to set the laser average
optical power and extinction ratio (see Figure 58). The AD5235
is particularly suited for the optical parameter settings because
of its high resolution and superior temperature coefficient
characteristics.
VCC
VCC
AD5235
CS
CLK
SDI
CONTROL
EEMEM
RDAC1
EEMEM
RDAC2
IMPD
ADN2841
A1
W1 PSET IMODP
B1
IBIAS
A2
W2
ERSET
B2
CLKN
CLKP
DATAP
DATAN
Figure 58. Optical Supervisory System
The ADN2841 is a 2.7 Gbps laser diode driver that uses a
unique control algorithm to manage the average power and
extinction ratio of the laser after its initial factory calibration.
The ADN2841 stabilizes the data transmission of the laser by
continuously monitoring its optical power and correcting the
variations caused by temperature and the degradation of the
laser over time. In the ADN2841, the IMPD monitors the laser
diode current. Through its dual-loop power and extinction
ratio control calibrated by the dual RDACs of the AD5235, the
internal driver controls the bias current, IBIAS, and consequently
the average power. It also regulates the modulation current,
IMODP, by changing the modulation current linearly with slope
efficiency. Therefore, any changes in the laser threshold current
or slope efficiency are compensated for. As a result, the optical
supervisory system minimizes the laser characterization efforts
and, therefore, enables designers to apply comparable lasers
from multiple sources.
RESISTANCE SCALING
The AD5235 offers 25 kΩ or 250 kΩ nominal resistance. When
users need lower resistance but must maintain the number of
adjustment steps, they can parallel multiple devices. For example,
Figure 59 shows a simple scheme of paralleling two channels of
RDACs. To adjust half the resistance linearly per step, program
both RDACs concurrently with the same settings.
A1
B1 W1
A2
B2 W2
Figure 59. Reduce Resistance by Half with Linear Adjustment Characteristics
In voltage divider mode, by paralleling a discrete resistor, as
shown in Figure 60, a proportionately lower voltage appears at
Terminal A to Terminal B. This translates into a finer degree of
precision because the step size at Terminal W is smaller. The
voltage can be found as
VW (D)
=
(RAB // R2)
R3 + RAB // R2
×
D
1024
×
VDD
(16)
VDD
R3
A
R2 R1
W
B
0
Figure 60. Lowering the Nominal Resistance
Figure 59 and Figure 60 show that the digital potentiometers
change steps linearly. Alternatively, pseudo log taper adjustment
is usually preferred in applications such as audio control. Figure 61
shows another type of resistance scaling. In this configuration,
the smaller the R2 with respect to RAB, the more the pseudo log
taper characteristic of the circuit behaves.
A1
W1
B1
R
Figure 61. Resistor Scaling with Pseudo Log Adjustment Characteristics
The equation is approximated as
REQUIVALENT
=
D × RAB + 51, 200
D × RAB + 51, 200 + 1024 × R
(17)
Users should also be aware of the need for tolerance matching
as well as for temperature coefficient matching of the components.
Rev. E | Page 28 of 32
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