Hello, I'm trying to find a suitable DAQ device to relace a PCI-6220 for an analog voltage application. The present application is a classroom experiment to measure the lifetime of cosmic ray muons. We use a Time-to-Amplitude converter (TAC), which outputs a signal between 0 - 10 V whose size is proportional to the time difference between muon arrival (start signal) and muon decay (stop signal). The TAC also produces a TTL signal which we use to trigger the 6220 to sample the TAC's output voltage. The muons arrive and decay at random times, so we can't sample at regular time intervals. An oscilloscope screen shot of the TAC output is attached, showing the analog output (blue) is about 2 usec in width, and the TTL signal (pink) is about 0.6 usec.
A sample VI that works with the 6220, and screenshot is attached. Here the VI is being tested with a NIM pulse generator configured such that the TAC is outputing pulses of 1.28, 2.62, 3.94, 5.22, 6.54, 7.84, 9.16 V. On the VI, I used AI0, RSE, number of samples=1. For sample clock I used PFI0, 20 kHz, falling edge. For trigger, I used source= PFI0, rising edge. I run for a few minutes, and everything works as expected, as shown in the histogram.
Our department bought new computers that don't have PCI slots, so I have to figure out a new DAQ solution. I tried a USB-6001 but it didn't work. See the attached screenshot of the VI front panel. I used the same settings when possible, AI0, RSE, samples=1, trigger source=PFI0, rising edge. However this device doesn't support using a PFI as a sample clock, so I used the onboard clock, again set to 20 KHz. I believe that what I'm seeing in the histogram, is that the onboard clock ticks aren't correlated to the Rising or Falling edges of the TTL signal on PFI0 at all, so it essentially randomly sampling the input voltage. The ticks are 50 usec apart, trying to sample pulses that are 2 usec in length, arriving randomly. Whereas, with the PCI-6220 the leading edge of the TTL "wakes up" the board, and the actual sampling is done 0.6 usec later on the trailing edge of the TTL.
So my questions are:
1. are there any USB DAQ devices that support using one of the PFI's as a clock source?
2. If not, are there other DAQ devices I could use that run on a bus other than PCI?
3. If not, does anyone have any suggestions on changing the VI to solve this problem?
