Case Studies
Kinetics Of Protein Dynamics
The customer is measuring kinetics of protein dynamics with a time scale of between a few nanoseconds and about a hundred milliseconds. The customer's two response signals occur after a few-nanosecond-pulse excitation that repeats at a rate of 1-10Hz. This requires a bandwidth from DC to a few hundred MHz, sampling speed of about 1-2GS/s, and processing time for one acquisition that is less than 1-0.1secs. An analog signal of about 10-30mV comes from a Photo-Multiplier Tube (PMT) or a photo-diode with a trans-impedance pre-amplifier.
The customer is currently using a couple of Tektronix scopes (TDS754A and TDS420A, with 50K and 60K points of memory, respectively) to effectively cover the whole capture time range. The sampling rate usually corresponds to 0.5ns/point, i.e. 2GS/s. The Tektronix scopes are able to keep up with a 1Hz pulse repetition rate without missing triggers. The customer wants to move to a higher performance embedded solution that is capable of keeping up with the 10Hz repetition rate.
GAGE'S SOLUTION
The CompuScope 82G is able to sample at the required 2GS/s in single-channel mode. Models are available with 2, 4 or 8 Megasamples of acquisition memory, more than enough to fulfill the requirement.
Previously, the customer achieved repetitive signal averaging using the built-in signal averaging functions in his TDS oscilloscopes. The disadvantage of this set-up was that occasionally a strong, randomly occurring surge pulse would occur, compromising the integrity of the averaged signal. By contrast, the CS82G data are transferred to host PC memory for numerical averaging. This allows "intelligent averaging", wherein surge signals may be identified and rejected before being summed into the averaging buffer. Therefore, only signals that improve the signal-to-noise ratio are processed. The CS82G's ability to reject data before processing proved to be a powerful feature for the customer's application.
As an example of the achievable rate of repetitive captures, we consider the acquisition of 50,000 Samples on two channels at 2 GS/s using two CS82G's in a Master/Slave configuration. The current PCI bus transfer speed for the CS82G is over 3 MB per second. Consequently, the download time to PC RAM for 2 x 50,000 one Byte Samples would be:
PCI transfer time = 100,000 B / 3 MB/s = 33 milliseconds
The 25 m sec capture time and the short trigger re-arm time are insignificant compared to this transfer time. Consequently, a signal repetition frequency of about 30 Hz (> 1/33 ms) can be achieved by the CS82G hardware. Strictly speaking, this rate applies to a good PCI bus operating under a single-tasking operating system like MS-DOS. In a tight-loop in a C application under MS Windows, however, this rate is reduced by a rate of 2 or 3. The rate still far exceeds the 1 Hz repeat rate available using GPIB transfer and exceeds the 10 Hz requirement.
This benchmark has been repeated on a 2.5-fold faster machine (i.e. a PentiumII/450MHz) and we observed a six-fold improvement in processing time.
PRODUCT RECOMMENDED
Instrument Mainframe 2020A
2 x CompuScope 82G-2M
Master Multi-Card Upgrade for CS82G
Slave Multi-Card Upgrade for CS82G
CompuScope SDK for C/C++ for Windows
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.
print this
email this
