08 February 2012

App Note 92

Bias voltage and current sense circuits for avalanche photodiodes: Feeding and reading the APD. 32 pages.

More fiber-optic-laser support circuits (see App Notes 89 and 90) are discussed here, in particular, the design of voltage-biasing supplies and current-monitoring circuits for avalanche photodiodes (APD). These circuits are used on the receiving end of an optical communications system.

Monitoring supply current is hard, especially in a high-voltage, single-supply application. Jim works through several approaches, starting with a direct instrumentation amplifier in Figures 2 and 3, both of which are impractical. His first real solution is the AC-coupled lock-in amplifier in Figure 4, using an LTC1043 and a charge pump to create a negative power-supply voltage (remember Figure 4 in App Note 3?). A DC-coupled amplifier is shown in Figure 5, with an amplifier floating up at the high-voltage rail.

Several high-voltage power supplies are presented, with output-noise measurements. A programmable flyback (with voltage-tripler) APD voltage supply is shown in Figure 6. A careful measurement of the 200-microvolt output noise in shown in Figure 7. Figure 8 combines the features of Figures 5 and 6. Figure 9 implements the features of Figure 8 using a transformer instead of a voltage-tripler circuit for the high-voltage output.

Figure 11 is a simple inductor-based design, but it requires a cascode on the LT1946, which gives Jim an excuse to reference the 1939 paper by Hickman and Hunt again (see reference 11). Figure 13 and 15 produce the low-noise supply voltage using the LT1533 controller (which was discussed, at length, in App Note 70). Again, the pedantry in footnote 11 makes me very happy,
Noise contains no regularly occurring or coherent components. As such, switching regulator output "noise" is a misnomer. Unfortunately, undesired switching related components in the regulated output are almost always referred to as "noise." Accordingly, although technically incorrect, this publication treats all undesired output signals as "noise."
Figures 16 and 18 use a crazy flying-capacitor scheme, reminiscent of Figure 23 in App Note 3, this time with optically driven switches. Figures 19 and 20 discuss digital schemes for output-current monitoring.

The various circuits are summarized in the table in Figure 21, along with Jim's usual love for summary tables,
Figure 21's chart is an attempt to summarize the circuits presented, although such brevity breeds oversimplification. As such, although the chart reviews salient features, there is no substitute for a thorough investigation of any particular application’s requirements.

Appendix A discusses two schemes for deriving the feedback voltage on the high-voltage supply, while minimizing the induced error in the current measurement. Appendix B discusses a few reasons why vintage oscilloscopes are so prized in his laboratory.

Appendix C reprints Appendix C from App Note 70. (He is getting quote a bit of mileage out of this Appendix, but it makes good sense: once you've written the definitive treatise on noise measurement, you might as well use it!)

Appendix D discusses the LT1150 chopper-stabilized amplifier, which has a clock-output pin that can be used to charge-pump a negative supply voltage in single-supply applications. "The circuit provides a simple way to obtain output swing to zero volts, permitting a true "live at zero" output."

Appendix E lists a few protection circuits for the expensive APD module, including a voltage clamp, a current limiter, and a bias-voltage crowbar.

The app note ends with a cartoon, betraying some obvious frustration with coworkers, perhaps? "Someday, when I don't have to make anything work, I'm gonna wear a tie too."

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