Hi Svenja, I see you picked up on the posted I started sometime ago. I never had time to follow up on the problem so I'm interested that you've found a work around. How did you connect an external supply to the card?
Just to confirm, you are hoping to gate your APD (SPCM from perkin elmer or excelitas) with your NI board or you are having problems with the +5V pin on the board?
Thanks for letting me know I understood your question correctly. What I figured out is the 6602 board can't sink enough current to pull the SPCM gate to zero. If I recall the 6602 can sink something like 30mA but the SPCM needs to be connected to something that can sink closer to 100mA. If you take a voltmeter and connect one lead to the pin in the SPCM gate BNC and the other to ground you'll see 5V. There is an internal pull up resistor inside the SPCM that holds the gate at high (5V) so you can use the unit when nothing is connected. When you connect something to the SPCM gate it has to be able to dissipate, or sink, enough current to pull the gate connection to zero volts. The solution, as the people at Excelitas told me, is to make a small buffer circuit. I'm attaching the PDF they sent me to this post. I'm also attaching a PDF with the circuit I used; I used a 2N7000 N-Channel transistor and a 1 mega ohm pull down resistor. These two components worked well for what I was doing and we're very inexpensive. I'm sure there are a lot of people who know a lot more about electronics on these forms than me so if anyone wants to jump in with additional advice I'm sure we'd all benefit.
