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I am designing an APD preamp and I need to switch between two bias voltages to the APD 25V and 46V. So I am looking for a SPDT mux that can switch between 25V and 46V in less than 300ns. For clarification:

  • APD is Avalanche Photo Diode.
  • Current needed : 30mA max.
SamGibson
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giora eytan
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  • You should explain what an APD is… also your 25-46V signal to be switched: how much current? restriction of impedance or distortion of various kind? – Lorenzo Marcantonio Mar 12 '21 at 09:00
  • Why use an SPDT mux? There are more convenient ways as far as I can tell. – Andy aka Mar 12 '21 at 09:03
  • APD is avalanche photo diode. Need 30mA max current. – giora eytan Mar 12 '21 at 09:12
  • Just for understanding: Why do you need to switch within 300ns? Do you need to cycle bias supplies continuously at MHz frequencies? – DLnd Mar 12 '21 at 10:12
  • can you use a diode for the 25V input and a transistor to switch the 46V – Jasen Слава Україні Mar 12 '21 at 10:16
  • great idea, @Jasen. I was going to suggest two FETs with inverted drives, but yours is less material.However, it is not going to work if fast turn-off of the 46V is required (depending on the total bias current drawn and APD capacitance). At 10pF and 30mA discharge is only a few ns, but if the bias current is lower or your capacitance is higher you will quickly exceed 300ns. A quick search shows typical capacitances between 1pF and 1nF, so that might well be a concern even at 30mA. – DLnd Mar 12 '21 at 11:49
  • Andy aka,what is your proposal? – giora eytan Mar 12 '21 at 21:24

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I think what you want is power multiplexer. There are dedicated ICs for this task, but it might be hard to find one to work at 46 volts (https://www.ti.com/power-management/power-switches/power-muxes/overview.html)

Example of custom power mux circuit that may work with some optimization copied idea from other post (Switching between two power sources).

Delay 48V to 26V

Schematic

Ernesto
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