3 EML Negative Bias Voltage

Optical networking applications that use an electro-absorption modulated laser (EML) require a negative voltage to bias the integrated electro-absorption modulator (EAM). The AFE11612-SEP has an internal 2.5V reference that scales the DAC output range from 0V to 5V. The DAC output can be converted to a negative voltage through the use of an inverting op amp circuit that is powered by a negative power supply. The circuit in Figure 3-1 uses the op amp OPA4H199-SEP to offset and scale the 5V output range to 0V to -5V. The feedback resistor ratio can be selected for an increased or decreased output range. The plot in Figure 3-2 shows the OPA output voltage vs the input DAC voltage.