Case Studies
Ultrasonic Tissue Characterization
THE PROBLEM
This customer's application is Ultrasonic Tissue Characterization. Their ultrasonic sensors transmit 16 signals. The customer's requirement is to capture 512 points on the 16 channels, average them together, and then store the data in a buffer, with the system running continuously. They would like to be able to update the average buffer and recapture data as fast as possible. They would then like to log the data to disk or process the data and display an image on the screen.
Since the signals coming in from their ultrasonic sensors are very weak, the customer would like to perform signal averaging in order to improve the Signal-to-Noise Ratio. Averaging will reduce the background random noise so that weak signals will become more prominent. (The number of averages increases the Effective Number of Bits [ENOB], so that an 8 bit data acquisition card with averaging can provide the equivalent of 16 or 24 bits, depending on the number of averages performed.)
GAGE'S SOLUTION
There are three options for solving the customer's application requirements:
Option #1: The customer can multiplex the 16 signals to our 12 bit, two-channel CompuScope 6012/PCI card, and use a PCI DSP card to do the averaging and display processing.
The multiplexer should preferably have simultaneous sample and hold, so that all 16 channels would be sampled simultaneously; the data is then frozen and read sequentially, and the next 16-channel data set is sampled. Simultaneous sample and hold thus prevents time skew between channels. In addition, the multiplexer would need to be low-noise and operate at a 30 MHz bandwidth. The PCI bus transfer rate is approximately 45 MSPS, which will increase the customer's throughput significantly.
However, there are several problems with this option. First of all, finding an off-the-shelf multiplexer to satisfy this application may be very difficult (multiplexing 16 channels down to two channels, with 30 MHz bandwidth, low noise, and simultaneous sample and hold). System development could also be very costly and time-consuming. Costs would increase as well with the purchase of a DSP card, DSP software, and special C compilers, in addition to a multiplexer, a CompuScope 6012/PCI card, and system configuration.
Option #2: Ideally, the solution which reduces both the cost and complexity of Option #1 would be a GagePC with a Multi-Card system comprised of eight CompuScope 225 cards. This would allow sampling of 16 parallel channels with a complete turnkey system. All hardware needs are taken care of; the customer need only write simple averaging and display routines.
However, the customer has a very tight budget and cannot afford to purchase an entire GagePC system.
Option #3: Given the customer's budget, we recommend our ETRC Bus Expansion Kit with the Multi-Card CompuScope 225 system (eight cards, with a total of 16 channels). The ETRC Bus Expansion Kit is used to connect the ISA bus in the PC to a passive backplane housed in an expansion chassis with its own power supply, so that the customer will be able to run eight CompuScope cards.
This solution still permits the customer to transfer the required 512 points from 16 channels and then perform an integration on a group of captures. From the CompuScope 225 system, we will have to transfer the data to the PC and let the processor do software averaging. We will have to double buffer to accomplish the integration.
How fast will the customer be able to transfer and integrate each data set?
- CompuScope 225 can transfer data across the ISA bus to the PC host memory at 1 MB/s
- 1 MB/s = 1 µS
- 1 µS * 512 points * 16 channels = 8.2 ms
Therefore, the customer can transfer 512 points from 16 channels to the host memory in 8.2 ms.
Integration time will vary, depending on the processor, but we are assuming the processor can perform an integration on that data in under 2 ms.
We can safely say that with all of the variables involved, there should be a repeat rate of 50 to 100 times a second of the entire cycle of pulse/digitize/integrate. The capture time must be added as well; the sampling rate is an important variable to consider here. Before doing the integration, we can re-arm the cards and shave off some of the re-arm time.
PRODUCT RECOMMENDED
Multi-Card (8) CompuScope 225 system (Master/Slave), with ETRC Bus Expansion Kit
or
GagePC with Multi-Card (8) CompuScope 225 system (Master/Slave)
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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