Case Studies
Time Resolved Spectroscopy of Protein Crystals
THE PROBLEM
The customer is involved in Time Resolved Spectroscopy of protein crystals. This involves shooting a laser onto the protein crystals and measuring the pulses generated by the crystals (as they are projected out of the sample) and their decay over time.
Due to the very noisy nature of the return signals, it is essential to average many pulses in order to recover the pulse shape and measure the decay.
The decay takes place over many tens of milliseconds, so it is necessary to acquire very long records (in the order of 4 million points).
The repeat rate of the firing of the laser is less than 1 Hz. The application requires the digitizer to acquire the long record, transfer it to PC, and average it all within one second.
A POSSIBLE SOLUTION VIA GPIB
One possible solution to this problem is to use a standalone digital oscilloscope with deep memory to acquire the data. However, these oscilloscopes perform 8 bit digitization and will require many more averages to extract the pulses of interest than if one were to use a 12 bit digitizer.
The very slow transfer rates of GPIB (100 kB/sec) will also mean that this averaging will take 100 to 1000 times longer than with Gage PCI bus CompuScopes, thereby limiting the repeat rate of the measurement.
Finally, the cost of these deep memory DSOs will be much higher than a 12 bit PCI bus CompuScope-based solution from Gage.
GAGE'S SOLUTION
The solution is a 12 bit CompuScope 6012/PCI A/D card with 4M of on-board memory. For a turn-key system that will work straight out of the box, we suggest that the customer houses the CompuScope in Gage's Instrument Grade PC - the GagePC 580. Finally, Gage can provide the customer with the averaging software needed to process the data.
Since the signals are acquired by the 12 bit CompuScope 6012/PCI, the number of averages that must be performed is a great deal smaller than would be the case for 8 bit DSOs.
The CompuScope-based solution is very simple:
- Capture the entire decay in the on-board memory.
- Transfer it to PC memory using bus mastering, which provides transfer rates of 100 MB/s.
- Re-arm the CompuScope board to start looking for the next trigger.
- Perform averaging of the captured data using the Pentium processor.
- Repeat the procedure.
The software which does this already exists and is being used by another Gage customer, who purchased a system configuration virtually identical to the one proposed here.
One option suggested by the customer was to use a CompuScope with shorter on-board memory and to acquire the decay in two steps: first, measure the early part of the decay at a high sample rate and store the averaged file on disk; second, measure the decay at a much slower sample rate and store that file on disk; finally, combine the two data files to produce a complete picture of the decay.
The only drawback of this method is that the customer would have to write software to combine the two files. The cost of a higher memory board may end up being much less than the cost of software development.
PRODUCT RECOMMENDED
12 bit CompuScope 6012/PCI A/D card with 4M of on-board memory
GagePC 580 - Gage's Instrument Grade PC
Gage averaging software tailored to the customer's application
This application note is provided "as is" without any warranties of any kind, either expressed or implied, including but not limited to the implied warranties of merchantability or fitness for a particular purpose. Gage Applied Technologies further does not warrant the accuracy and completeness of the material contained herein. Gage Applied Technologies may make changes to this material, or to the products described in it, at any time without notice.
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