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Design Notes

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  Fiber Optic Communication Systems Benefit from Tiny, Low Noise Avalanche Photodiode Bias Supply &mdash DN273
  PDF, 246 Kb, File published: Dec 1, 2001
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  Fiber Optic Communication Systems Benefit from Tiny, Low
  Noise Avalanche Photodiode Bias Supply – Design Note 273
  Michael Negrete
  Avalanche photodiodes (APDs) are the photo detectors of
  choice for long-haul fiber optic communication systems
  because of their high sensitivity and high internal gain. An
  important characteristic of APDs is that their internal gain
  is optimal when there is a high voltage reverse bias (30V
  to 90V) across the APD. Nevertheless, the high gain is all
  for naught if the sensitivity of the APD is compromised
  by a noisy bias supply. The LT1930A, a capacitor-diode tripler and an external
  DAC provide a bias voltage of up to 90V, allowing easy
  temperature compensation (via the DAC) to optimize
  internal gain. By running the IC at a switching frequency
  of 2.2MHz, one can use tiny, low cost capacitors and
  inductors to keep the circuit footprint under 0.5in2. The
  LT1930A’s constant frequency PWM operation keeps
  output noise low and easy to filter. Traditionally, such low noise bias supplies required custom circuits that brought with them another problem:
  large space requirements. Linear Technology’s LT®1930A
  2.2MHz step-up DC/DC converter in a 5-lead SOT-23
  package solves these APD bias voltage problems and
  does so in a compact package suitable for most fiber
  optic applications. Figure 1 shows a high voltage, low noise APD bias supply …

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• Data Conversion > Digital-to-Analog Converters (DAC) > Voltage Output DACs | Unipolar Digital-to-Analog Converters (DAC)