Datasheet AD5235 (Analog Devices) - 28

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AD5235 Data Sheet OPTICAL TRANSMITTER CALIBRATION WITH RESISTANCE SCALING ADN2841
The AD5235 offers 25 kΩ or 250 kΩ nominal resistance. When The AD5235, together with the multirate 2.7 Gbps laser diode users need lower resistance but must maintain the number of driver, ADN2841, forms an optical supervisory system in which adjustment steps, they can parallel multiple devices. For example, the dual digital potentiometers can be used to set the laser average Figure 59 shows a simple scheme of paralleling two channels of optical power and extinction ratio (see Figure 58). The AD5235 RDACs. To adjust half the resistance linearly per step, program is particularly suited for the optical parameter settings because both RDACs concurrently with the same settings. of its high resolution and superior temperature coefficient characteristics.
A1 A2 B1 W1 W2 B2 VCC VCC
058 02816- Figure 59. Reduce Resistance by Half with Linear Adjustment Characteristics
IMPD AD5235
In voltage divider mode, by paralleling a discrete resistor, as
ADN2841
shown in Figure 60, a proportionately lower voltage appears at
A1
Terminal A to Terminal B. This translates into a finer degree of
CS W1 EEMEM PSET IMODP
precision because the step size at Terminal W is smaller. The
B1 CLK CONTROL RDAC1 IBIAS
voltage can be found as
SDI A2
(R //R2) D
W2
AB
EEMEM
( ) = × ×
ERSET
W V D DD V (16)
B2
R3 + R //R2 1024
P
AB
P A AN RDAC2 KN K AT AT V CL CL D D DD CLKN R3
057
CLKP DATAP
02816-
DATAN A R2 R1
Figure 58. Optical Supervisory System
W B
The ADN2841 is a 2.7 Gbps laser diode driver that uses a 059 unique control algorithm to manage the average power and
0
02816- extinction ratio of the laser after its initial factory calibration. Figure 60. Lowering the Nominal Resistance The ADN2841 stabilizes the data transmission of the laser by Figure 59 and Figure 60 show that the digital potentiometers continuously monitoring its optical power and correcting the change steps linearly. Alternatively, pseudo log taper adjustment variations caused by temperature and the degradation of the is usual y preferred in applications such as audio control. Figure 61 laser over time. In the ADN2841, the IMPD monitors the laser shows another type of resistance scaling. In this configuration, diode current. Through its dual-loop power and extinction the smal er the R2 with respect to R ratio control calibrated by the dual RDACs of the AD5235, the AB, the more the pseudo log taper characteristic of the circuit behaves. internal driver controls the bias current, IBIAS, and consequently the average power. It also regulates the modulation current,
A1
IMODP, by changing the modulation current linearly with slope
W1 B1 R
efficiency. Therefore, any changes in the laser threshold current 060 or slope efficiency are compensated for. As a result, the optical 02816- supervisory system minimizes the laser characterization efforts Figure 61. Resistor Scaling with Pseudo Log Adjustment Characteristics and, therefore, enables designers to apply comparable lasers The equation is approximated as from multiple sources. D × R + , 51 200 R AB = Q E UIVALENT (17) D × R + , 51 200 +1024 × AB R Users should also be aware of the need for tolerance matching as wel as for temperature coefficient matching of the components. Rev. F | Page 28 of 32 Document Outline Features Applications General Description Functional Block Diagram Revision History Specifications Electrical Characteristics—25 kΩ, 250 kΩ Versions Interface Timing and EEMEM Reliability Characteristics—25 kΩ, 250 kΩ Versions Timing Diagrams Absolute Maximum Ratings ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Test Circuits Theory of Operation Scratchpad and EEMEM Programming Basic Operation EEMEM Protection Digital Input and Output Configuration Serial Data Interface Daisy-Chain Operation Terminal Voltage Operating Range Power-Up Sequence Layout and Power Supply Bypassing Advanced Control Modes Linear Increment and Decrement Instructions Logarithmic Taper Mode Adjustment Using to Re-Execute a Previous Command Using Additional Internal Nonvolatile EEMEM Calculating Actual End-to-End Terminal Resistance RDAC Structure Programming the Variable Resistor Rheostat Operation Programming the Potentiometer Divider Voltage Output Operation Programming Examples EVAL-AD5235SDZ Evaluation Kit Applications Information Bipolar Operation from Dual Supplies Gain Control Compensation High Voltage Operation DAC Bipolar Programmable Gain Amplifier 10-Bit Bipolar DAC Programmable Voltage Source with Boosted Output Programmable Current Source Programmable Bidirectional Current Source Programmable Low-Pass Filter Programmable Oscillator Optical Transmitter Calibration with ADN2841 Resistance Scaling Resistance Tolerance, Drift, and Temperature Coefficient Mismatch Considerations RDAC Circuit Simulation Model Outline Dimensions Ordering Guide