The Importance of Simultaneous Sampling in Data Acquisition Systems
Usually, when we want to choose a data acquisition system, we look at three main specifications. The sample rate, the bit resolution and the number of channels. While these specifications are very important, there is another characteristic that we tend to forget – simultaneous sampling.
The simplest multi-channel DAQ architecture consists of a multiplexer (MUX), an amplifier (AMP) and an analog to digital converter (ADC). All the input channels are connected to the MUX, which sends each channel through the amplifier to the ADC to be converted into digital data. This means that each channel is sampled at a different time, which results in a time delay between contiguous samples. A DAQ system which overcomes this time delay problem is called a Simultaneous DAQ System.
There are two common simultaneous sampling architectures. One that uses a sample-hold mechanism (SSH) and one that uses multiple analog to digital converters (ADCs). The SSH circuit has two modes, sample and hold. In sample mode, it tracks the input analog voltage. In hold mode, it holds the current voltage at a constant value. This is usually done by a switch and a capacitor. The SSH simultaneous sampling architecture places a SSH circuit before each channel is inserted into the multiplexer, and sets their mode to sample and hold at the same time. The result is that each channel is sampled separately, but the values of the digitized data are from the same point in time – as if they were sampled simultaneously. While this architecture does solve the time delay problem, it has several disadvantages. The first one is that the sample rate of the ADC must divide between the number of input channels. This means that if you have an ADC that samples at up to 1 Mhz, and you have 10 input channels, then the max sample rate per channel is actually 100 khz (assuming that you are using all the channels). In reality, the sample rate is even lower (by about 30%), due to latency of the SSH circuit and the MUX.
Another architecture is one that uses multiple ADCs. One for every channel. This architecture is also much less complex. There is no need for SSH switching and a MUX. Each input channel is connected to its own amplifier and ADC. This architecture achieves the higher sample rates per channel and better dynamic accuracy. That is why the multiple ADC approach is known to be the best architecture for simultaneous data acquisition systems.
So why don’t all data acquisition systems use the multiple ADC architecture? Well, the answer is, as you must have guessed, the cost. The ADC is one of the most expensive item in the DAQ system, and using one for every channel increases the cost considerably. But sometimes it is worth it to pay the extra buck. Especially when there is a need to take advantage of all the channels that the DAQ system has to offer.
Sometimes you come across a data acquisition system that uses the multiple ADC architecture, and costs the same as one that uses the SSH architecture. Go to www.usb-daq-boards.com and check out their new usbdaq-225-18 board. It has 18 input channels at up to 225 khz samples per second. It uses the multiple ADC architecture, but costs the same as a similar device which uses the SSH architecture.
Mike Rothman is an Electrical Engineer whose specialty is usb data acquisition boards
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