EVAL-AD5235EBZ Analog Devices Inc, EVAL-AD5235EBZ Datasheet - Page 28

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EVAL-AD5235EBZ

Manufacturer Part Number
EVAL-AD5235EBZ
Description
BOARD EVALUATION FOR AD5235
Manufacturer
Analog Devices Inc
Datasheet

Specifications of EVAL-AD5235EBZ

Main Purpose
Digital Potentiometer
Utilized Ic / Part
AD5235
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Secondary Attributes
-
Embedded
-
Primary Attributes
-
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.
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.
CLK
SDI
CS
CONTROL
AD5235
Figure 58. Optical Supervisory System
EEMEM
EEMEM
RDAC1
RDAC2
DATAP
DATAN
CLKN
CLKP
B2
W1
A2
W2
A1
B1
PSET
ERSET
ADN2841
IMODP
IMPD
IBIAS
V
CC
V
CC
Rev. D | Page 28 of 32
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.
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
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 R
taper characteristic of the circuit behaves.
The equation is approximated as
Users should also be aware of the need for tolerance matching
as well as for temperature coefficient matching of the components.
Figure 59. Reduce Resistance by Half with Linear Adjustment Characteristics
Figure 61. Resistor Scaling with Pseudo Log Adjustment Characteristics
V
R
W
E
QUIVALENT
(
D
)
=
Figure 60. Lowering the Nominal Resistance
R3
(
R
=
+
AB
D
R
//
AB
×
A1
B1
R2
R
//
R2
D
AB
A1
B1
R2
)
W1
×
+
R
×
R1
R3
51
A
B
AB
1024
W1
,
D
V
200
A2
B2
0
+
DD
W
R
51
AB
×
+
,
, the more the pseudo log
W2
V
200
1024
DD
×
R
(16)
(17)

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