SLAAEF1 august 2023 AFE11612-SEP

2 Laser and Semiconductor Optical Amplifier Biasing

Laser diodes and semiconductor optical amplifiers (SOAs) require a precision current source and current monitoring to be accurately biased. The AFE11612-SEP has twelve 12-bit DACs that can be configured to be a precision current source with external circuitry. The circuit in Figure 2-1 uses the AFE11612-SEP DAC outputs, external operational amplifiers (op amps), and external MOSFETs to supply a programmable current to a ground-referenced load.

Figure 2-1 Laser Diode Bias Circuit

The first op amp stage uses R_SET and the DAC output voltage (V_DAC) to set the reference current. V_DAC is applied to the noninverting input of OPA1, which sets the high-side of R_SET to the same voltage. This reference current (I_SET) is calculated with Equation 1.

Equation 1.

I_{S E T} = \frac{V_{D A C}}{R_{S E T}}

The second op amp stage is a current-mirror with a gain set by R_A and R_B. The current flowing through R_A is equal to I_SET. This creates a voltage drop across R_A. R_B has the same voltage drop due to the op amp OPA2 feedback forcing the voltage at the low-side of R_B. Thus, with R_A and R_B having the same voltage drop, the current flowing through R_B can be tuned by picking specific resistor values. This current (I_MIR) is the current that is used by the load. Use Equation 2 and Equation 3 to calculate the current gain.

Equation 2.

I_{S E T} \times R_{A} = I_{M I R} \times R_{B}

Equation 3.

C u r r e n t G a i n = \frac{I_{S E T}}{I_{M I R}} = \frac{R_{A}}{R_{B}}

The total gain of the system can be calculated by Equation 4.

Equation 4.

I_{M I R} = \frac{V_{D A C} \times R_{A}}{R_{S E T} \times R_{B}}

For additional information on using voltage DACs to create programmable current sources, see 8-Channel, 16-Bit, 200-mA Current Output, Digital-to-Analog Converter Reference Design and Programmable, Two-Stage, High-Side Current Source Current.

Additionally, the AFE11612-SEP features an ADC with differential inputs which can assist in measuring the voltage differential across R_B to monitor current. The AFE11612-SEP ADC has a range of 5 V, so the P_VDD voltage must be taken into account when using the ADC for measurements. For use in higher voltage applications, an INA or a voltage divider circuit can be used to measure the voltage across R_B, as seen in Figure 2-2.

The AFE11612-SEP also has two remote temperature monitors that trigger an alarm when they detect temperature that exceeds a user-determined value. These can be placed near temperature sensitive components to make sure that the host is informed when the devices are overheating.

Figure 2-2 Laser Diode Bias Circuit with INA