How can I adjust the threshold voltage of edge counting?

[Siyoung]

Hi all,

I'm using PCI-6602 with BNC-2121 for photon counting purpose.

I connect the input to PFI39 channel, nothing is connected to BNC-2121 terminal without this.

After some test, it is clear that my 6602 only count the event which has higher voltage value than 800mV. It has no response(counting) with input signal below 800mV. It only get the count when input voltage increases over 800mV.

But you know, voltage from photomultiplier is usually lower than 800mV. It is impossible to make PMT voltage higher than 800mV for all of my samples.

So I need to find out the way to adjust this kind of 'threshold voltage for counting'.

Thank you for reading.

[KateB]

6602 threshold is not configurable. You may build comparator circuit using opamp to make it TTL compatible votage level.

[Siyoung]

So all PCI6602's threshold voltage are set to 800mV?

But input voltage range is from -0.3V~5V. I think this means that PCI6602 can count input voltage signal within this range.

Threre is no specific information about this in the catalog&data sheet. The maxiumum value for input low voltage is 800mV <-- This is the only information that related with my problem, I think.

[johnsold]

This is in the Specifications document:

This is standard TTL/CMOS logic circuit compatibility. Most of the counters on NI DAQ boards have this kind of logic level input specifications.  The -0.3 to +5 V range in simply the range of voltages which can be applied without damage to the input circuits.  To get a count transition the input voltage must go from the Input Low voltage to the Input High voltage, or the opposite, depending on which edge is being counted. Any signal which does not move from one input level to the other does not get counted.

If you want to count a signal which is not a standard logic signal (such as your PMT signal), then you need some signal conditioning between the sensor and the counter.  Although KateB recommended a comparator built from an op amp, I suggest that a device designed to work as a comparator may be better. Some op amps are slow to come out of saturation and may limit the count rate or the minimum pulse width you will be able to detect.

Lynn