1

In this circuit designed to measure high frequency light pulses, the 30V high voltage bias input has a direct connection to the inverting input of the op-amp, despite also being connected to the reverse bias PIN diode.

Why (or how) does the 30V bias voltage pass only to the diode and not damage the op-amp input?

enter image description here

analog2525
  • 13
  • 2

1 Answers1

5

It is blocked by the coupling capacitor CC, and even if CC failed shorted, the current is limited by RB1 to less than 300nA.

IC1A can withstand an input current through the protection diodes that is 100,000 times higher than that, according to the datasheet.

enter image description here

So the 300nA will be safely clamped to slightly over the power supply voltage. That's technically outside the stated absolute maximum rating for input voltage, but in fact completely safe.

However if someone were to carelessly short the PD during operation the discharge of CC through the op-amp input could damage it.

Spehro Pefhany
  • 376,485
  • 21
  • 320
  • 842
  • Would the current flowing through the PD when illuminated by the light source cause the discharge of the CC? – analog2525 Oct 29 '20 at 17:14
  • In normal operation the PD provides a tiny current which is balanced by the op-amp output. It would need to conduct 30mA to have danger of damaging the op-amp. Not very likely, barring something very unhealthy happening close by. – Spehro Pefhany Oct 29 '20 at 17:18
  • Thank you! How do you establish the practical maximum value for RB1? – analog2525 Oct 29 '20 at 17:22