epends on the output load). Figure 26 illustrates the load dependence of the CTR radiation response for an optocoupler operated with a forward current of 4 mA. It also demonstrates that the common practice of testing at a fixed VCE of 5 V with no circuit loading significantly over-predicts the CTR degradation, although such data can be used for worst case estimates. Data such as these suggest the mitigation of CTR degradation by operation of the optocoupler at the highest drive current possible while minimizing VCE to obtain the desired I for the application [Reed98]. In this case the optocoupler is driven into saturation. However, the trade-off between reliability concerns for LED operation in this mode must be considered. Also, the forward current selected may impact the rate of defect annealing observed in some LEDs, as discussed in the previous section. Figure 25 CTR measurement setup showing the input voltage and resistance (V1 and R1), and output voltage (Vo). Two independent power supplies are used to sweep V- and Vo. Changes in V1 alter the LED drive current. VCE is the difference between Vo and the measured voltage drop across the load resistance (RL), which can be varied. After [Reed98]. IV-94 Vce/Vceo load=1 kohm Vce/Vceo load=2.7 ohm CTR/CTRo load=1 kohm CTR/CTRo Load=2.7 ohm CTR/CTRo no load Vce=5V 0.0E+00 2.0E+10 4.0E+10 6.0E+10 8.0E+10 1.0E+11 1.2E+11 Fluence (p/cm²) Figure 26 Plot of the normalized CTR (solid lines) and normalized VCE (dashed lines) for optocoupler operated with a forward current of 4 mA and irradiated with 195 MeV protons. In the case of the 2.7 S load (with VCE remaining at 0.3 V), the degradation is almost twice that observed for the 1 kO load where the degradation is mitigated by an increase in VCE. (Vo was set SO that the initial VCE = 0.3 V for both loads.) After [Reed98]. To the extent that laboratory testing has shown the CTR response to be dominated by displacement damage to the internal LED, the analysis tools presented in sections 2.3 and 2.4 can be used together with a