Seeking Advice: Labview and Boxcar Integrators

Question

Background:
I currently use two Stanford Research SR250 boxcar integrators to acquire
voltages from two photodiodes (TTL voltages) at a repetition rate of 1 kHz
for a transient absorption experiment. An analog processor then takes the
difference (one is signal and one is reference) before a third boxcar
provides averaging and amplification. The analog output of the third
boxcar is sent to an NI multifunction board and finally monitored with
Labview.
Questions:
I am constructing another system and I am looking for advice on how I
might be able to do all (or most) of the work I described in the
background above with a new multifunction board and Labview (or Labview
RT)...any general comments or recommendati
ons are welcome
Specifically...
I know there are analog inputs, however, I assume the signal has to be
digitized before it can be manipulated (to subtract two analog signals or
divide one analog signal by another) and I want to minimize the amount of
digitization error...are there multifunction boards capable of analog
processing?
Also, will timing of the data acquisition be reproducible without using
Labview RT...along those lines, are there still timing problems with
Labview and Win98...my experience has been that milliseconds are lost due
to the overhead of the operating system...it seems that Labview RT could
solve this problem.
-thank you
Matt

Answer 1

I don't think you will find a standard DAQ board which can perform the gating functions of the SR250. I looked it up and the gate times range from 2 ns to 15 us. The gating suppresses noise and interfering signals during the time the gate is off.
You might be able to simulate this if you are using the long time end, perhps 5-15 us. You could use a board which samples at MHz rates and suppress all the samples except those that occur during the gate time. You would also need to sample the gate signal or trigger signal. All the channels including the trigger/gate channel wold have to be sampled simultaneously (which many boards do not do).
The averaging and differences could be performed in software, provided that the initial digitizing was sufficient to accurately represent your signal. If the signal-to-noise ratio on a single pulse is very poor, you would also need to look at the added quantization noise introduced by the A/D conversion to make sure that that would not degrade a poor signal to the point of unusability.
So can it be done? Maybe, but I doubt that it would be easy. I suggest you keep at least the two front end SR250s.
Lynn