Transcription of AN92 - Bias Voltage and Current Sense Circuits for …
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AN92 - Bias Voltage and Current Sense Circuits for …
Application Note 92AN92-1 Bias Voltage and Current Sense Circuits forAvalanche PhotodiodesFeeding and Reading the APDJim Williams, Linear Technology CorporationNovember 2002 INTRODUCTIONA valanche photodiodes (APDs) are widely utilized in laserbased fiberoptic systems to convert optical data intoelectrical form. The APD is usually packaged with a signalconditioning amplifier in a small module. An APD receivermodule and attendant circuitry appears in Figure 1. TheAPD module (figure right) contains the APD and a trans-impedance ( , Current -to- Voltage ) amplifier. An opticalport permits interfacing fiberoptic cable to the APD sphotosensitive portion. The module s compact construc-tion facilitates a direct, low loss connection between theAPD and the amplifier, necessary because of the extremelyhigh speed data rates receiver module needs support circuitry. The APDrequires a relatively high Voltage bias (figure left) tooperate, typically 20V to 90V.
practice, it may be desirable to derive the APD bias voltage regulator’s feedback signal from the indicated point, elimi-nating the 1kΩ shunt resistor’s voltage drop.1 Verifying accuracy involves loading the APD bias line with 100nA to 1mA and noting output agreement.2 DC Coupled Current Monitor Figure 5’s DC coupled current monitor ...
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