Feeding USB-6343 digital out to 50 Ohm photon counter

[KrisJa]

Hi all,

I am building a confocal fluorescence microscope. Long story short, photons are recorded using a Time Correlated Single Photon Counter (TCSPC). This TCSPC is capable of time stamping each individual photon.

Since the detector is a point detector, I "scan" a laser beam over the sample using galvo mirrors to image it and I correlate the position of my laser beam with the photon data.

Laser scanning is achieved by generating a wave form on an NI-USB6343 (specs in link) AO channel. At the start of each wave form period I also generate a pulse on one of the digital outputs of the NI device. This digital out is recorded by the TCSPC on a dedicated channel and serves as a reference to reconstruct the images. For this, the TCSPC has a dedicated channel.

The issue:

I can do programming and chemistry but have zero knowledge on electronics.

APDs, TCSPCs etc... all operate on 50 Ohms (no idea why). My NI box is not particularly designed for that it seems and any 5V triggers generated by it are not registered by the TCSPC.

Connecting the digital out of the NI box to a scope using a 50 Ohm BNC cable with a 50 Ohm terminator on a T-splitter, I can indeed see that my pulses are not 5 V anymore but merely 1.5 to 2 V. All other aspects of my pulses (rise time) seem to correspond to the TTL spec.

I have attached the electrical specs of all hardware involved, being the TCSPC, the APD photon detector (whose pulse are picked up correctly) and the NI hardware.

Since the TCSPC costs more than a very nice car, I'd like to get some feedback on the type of circuitry that might help to adapt the output of the NI box to the requirements of the TCSPC without the risk of damaging either.

I would also be helped by potential suppliers of such equipment or perhaps companies where I could order custom circuits to achieve my goal. I have zero knowledge of electronics let alone soldering skills but I really need to solve this issue :s

Any help/feedback would be appreciated!

[JakeA]

Hi KrisJa,

As I understand it, you have three pieces of equipment which you would like to connect up together. I've read over the specifications you attached, and I noticed that the maximum input voltage for the TCSPC is just 1.5 volts (or 1500 millivolts) while the NI USB 6343 you have connected up to the TCSPC is capable of generating a digital output of 5 volts. If this information is correct, it would appear that the highest voltage levels that the TCSPC can handle are much lower than the USB 6343 can generate, and that the voltages you measured are at or above the damage level for this piece of equipment. If the TCSPC is still functioning as normal, I would assume that the device has some sort of built in voltage protection which would limit the input voltage to the device. I would recommend that you alter your DO voltage from the USB 6343 to 1.5 volts or lower. If the TCSPC has already been damaged then I don't think there is anything I can do to help you, I'm afraid, as I can only offer support for National Instruments hardware and software.

I hope this information is helpful to you, if you have any more questions please don't hesitate to ask.

[Henrik_Volkers]

Seems that the trigger input works with a negative pulse , triggering on the falling edge. And I assume you create a positve going edge with your NI device.

To create a negative going pulse you can try to put a small capacitor (1nF)  between your DIO and the trigger in. And you need to invert the logic of your triggerpulse to active low (the output is usually on the high level and is going down to GND for the trigger pulse.

Assuming an source resistance of about 50 Ohm (from the spec diagram page 😎 and a slew rate of 500ns (blue trace) a quick simulation show a 'nice'  negative pulse (green trace) .

The size (in Farad not in meter 😉 ) of the capacitor and the slew rate of the DIO define the peak voltage level to your sync input.  You can fit the value with your scope terminated with 50 Ohm.

A more protective way is shown below

the diodes (say 1N4148)  protect your input to about +700mV to -1400mV. AND you have a shorter delay.....

All fits between a male and female BNC connector with  solid stable copper ground wire  holding them together 😉

 (edit: If -700mV is fine for the trigger (seems so), you can use one diode instead of the two in series) 

DO that at own risk !    Ask the owner of the instrument and try to get a e-engineer(student) if you don't know where the hot end of the solder iron is 😄