Multifunction DAQ
Card for analgue input + timing
Apologies in advance for the fairly long post - I'm trying to chose a card for a specific application, and would appreciate any input!
I have three signals; A, B and C. A and B are digital signals, C is analogue. A pulse on A is 0 – 5 V and lasts for 1 ms, a pulse on B is 0 – 4 V and lasts for ~350 µs, and signals on C have a range of ~-200 to 800 mV. The time pulses appear on channel B after a pulse on channel A is random.
We want to do two things – measure the time of each pulse of signal B after the single pulse on signal A, and each time there is a pulse on signal B measure signal C for a certain amount of time. We will then have a series of times after T0 (corresponding to a pulse on channel A) which are T1, T2 and T3 in the above diagram, and associated with each of those the analogue voltages read from signal C for a time after (the blue periods on the above diagram). If another pulse on B arrives while we are still capturing signal C (or doing associated data processing), then we ignore it. We do all this for ~500 ms after the pulse on signal A, then we reset and wait for the next pulse on signal A.
Therefore we end up with a number of datasets recorded from signal C with the time after the pulse of signal A for each of them:
Time after T0 Voltage signal C
T1 (e.g. 0.2 ms) 0.1, 0.12, 0.12, 0.11, 0.10, 0.05 etc
T2 (e.g. 20 ms) 0.1, 0.12, 0.12, 0.11, 0.10, 0.05 etc
T3 (e.g. 65 ms) 0.1, 0.12, 0.12, 0.11, 0.10, 0.05 etc
The measurement of times T1, T2, T3 etc should be accurate to 1 µs, and signal C should have minimum 12 but preferably 14 bit resolution, and a minimum of 2 MS/s temporal resolution.
There are a number of ways I can think to do this:
a) As described above – The pulse on channel A is T0. This triggers a timer that is used to give the time after T0 of each pulse on channel B. Each pulse on channel B also triggers a short period of analogue acquisition (~300 µs). This happens for each pulse on channel B for e.g. 500 ms and then stops and waits for the next pulse on channel A. The danger I see here is that if another pulse arrives on channel B whilst we are still dealing with previous data from channel C, we don’t want to record it. This is because what we want is a series of times after T0 for channel B, along with the respective data set from channel C for each one. I don’t want to end up with more timed pulses from channel B than datasets from channel C as then I don’t know which dataset goes with which timed pulse.
b) We record both channel B and C continuously for ~500 ms as analogue inputs, triggered by channel A. We can then search through the channel B data to find when it changed from low to high (and therefore when we have a pulse), and then use this set of pulse times to cut the required corresponding datasets out of the channel C data. Downsides I can see is that this might be slow – especially the searching through the channel B data to find the timing of the pulses. Also, is there a large enough buffer on the card to record continuously for ~500 ms on two channels at high sampling rate.
c) We record channel C continuously for ~500 ms, triggered by channel A. We use a timer triggered by channel A to get the timing of pulses on channel B. We then use this timing information to extract the required parts of channel C for each event on channel B.
I’ve had a look at the NI cards, and the PCI-6115 or PCI-6132/6133 would seem to fit – would these cards be suitable for this application? I'm leaning towards the PCI-6132/6133 for the 14 bit AI resolution. Or does anyone have better ideas on how to go about this?
Thanks very much,
Alex
