话音和DAC

• Linearity: The relationship between the digital code and resulting control current for the Pot is a 1/R relationship. This is a linear relationship for the DAC. The linear design has 2 primary advantages:
  1. 1. The DAC linear design has a very regular and predictable step size, so it is easy to calculate if the design can be calibrated to the appropriate end-target accuracy. As discussed above, the pot design can have an undesired step-size problem, due to the 1/R relationship, hence the design might not be able to hit the end accuracy.
  2. 2. The control algorithm used to adjust the output current based on measuring the module output power can be much simpler to design and optimize for throughput using a linear control loop software algorithm.
• Density/Board Size: In some designs, the board layout can actually be made smaller with a DAC. Since typically, there are other control parameters requiring a DAC on the board, a DAC with 2 extra channels (for the laser driver modulation depth and bias current control currents) can be chosen. Hence, the function can be fulfilled by another IC that is already required for another part of the design, rather than adding a new IC just for this purpose.
• Resolution: Pots with resolution of greater than 8 bits are not commonly available. DACs can be found with a wide range of resolutions.
• Range optimization: The full-scale voltage range of the DAC can correspond directly to the range of currents desired by the choice of the resistor. If the laser driver available programming range needs to change, with the DAC approach, the resistor is all that is necessary to change; with the POT approach, the POT may need to be changed to a different POT because the resistance value is embedded in the pot.
• Bipolar ease of implementation: If the laser driver must run from a -5V voltage rail, there are fewer pots from which to choose. Using a bipolar DAC, or similar voltage translation technique allows an alternative.
• Unit to Unit Consistency: Most digital pots have large (30% or more) error in the resistance value. DACs resistors can have errors significantly below 1%, depending on the voltage reference and resistors used. This error is trimmed out in the calibration process. However, the error needs to be taken into account in the error budget/calibration range of the calibration algorithm. Also, if it is desired to understand the distribution of required voltages to understand the control of the calibration process, the data is better if the controls are more consistent.

Conclusion