DAQ Boards and Speed

The speed of DAQ systems depends on:

• Throughput
• Aliasing
• Acquisition Time
• Conversion Time
• Transfer Time
• Overlap

Speed may be one of the most important factors when one is selecting data-acquisition components. In high-speed applications, such as destructive testing, the sample rate (aka "throughput") is the most important factor. Therefore, data-acquisition boards used in such applications must accommodate. Many DAQ board manufacturers specify throughput in samples per second: mega-samples per second (MS/s) or kilo-samples per second (kS/s). So, e.g., this board from National Instruments provides 500 kS/s analog-to-digital throughput and can sample 500,000 times per second, transferring the resulting data to system memory.

Some DAQ boards use more than one ADC (analog-to-digital converter or A/D), so the throughput is the sum of each ADC's throughput.

Earlier we discussed the importance of having the sample frequency at least twice as fast as the input signal's highest frequency component (see this article). it's important to understand this principle if you either have signals with very high-frequency components or you don't know the signal's frequency. In the latter case, many users sample at the highest frequency they can or remove the high-frequency components by using a low-pass filter (an anti-aliasing filter may be built-in in some DAQ boards). One DAQ manufacturer claims that boards using delta-sigma ADCs are very effective "anti-aliasing filters" because of their inherent high sampling speed and built-in DSP (digital signal processing).

If analog input circuits share a common ADC, the number of input channels can effectively reduce throughput. e.g., if you want to sample eight input channels at 60 kS/s each, you need a DAQ system with a throughput of at least 480 kS/s (8 times 60 kS/s).

The following three things determine ADC throughput:

• conversion time -- time required to convert analog to digital
• acquisition time -- time required by analog circuits (such as multiplexer, amplifier and sample/hold -- to accurately acquire signal
• transfer time -- time required to transfer data from board to memory

Usually, a DAQ board first acquires a signal and then converts it. Throughput can be calculated by the adding the conversion time and the acquisition time. High-speed boards can increase throughput by overlapping the acquisition time on one sample with analog-to-digital conversion time of the previous sample. Essentially, then, the ADC manipulates two signals at once.

Finally, the issue of amplification can affect throughput. This can become significant when high gains are needed from low-levels input signals (e.g., 0.02 - 0.10 Volts). Like any analog amplifier, the bandwidth (speed) of the board's programmable gain amplifier decreases as gain increases. Because of this limitation, many boards uniformly increase acquisition time for all gains, thereby reducing throughput from, say, 60 kS/s to 3 kS/s.

The table below shows the relationship between Resolution, Input Ranges, and Bit Rates