Plug-in data acquisition boards: Speed vs throughput

Throughput, or the speed at which an A/D board can acquire data, is always a consideration in data acquisition systems. There is, however, some confusion in the throughput figures quoted by board manufacturers when relating the performance of a particular board. Often these figures relate to the maximum data acquisition rate and can depend on the particular method of storing values into memory. Herein lies the real key to the throughput of data acquisition boards.

Strictly speaking, the throughput specification of an A/D board indicates the total number of analog signal input samples that can be converted to their digital equivalents per second.

As it's usual for several analog input circuits to share a common A/D converter, the number of input channels in use also affects throughput. Therefore, the sample rate of each channel is the total throughput divided by the number of channels sampled.

Maximum throughput / channel = Total throughput / # Channels used e.g., if you wish to sample four channels at 50 kHz each, you need an A/D board with a throughput of at least 200 kHz (four inputs X 50 KHz / input).

A/D board conversion throughput is determined by:

• Acquisition time: the time needed by the signal conditioning and acquisition circuitry (multiplexer, amplifier, filter and sample / hold), to obtain and present an accurate analog input signal to the A/D converter.

• Conversion time: the time needed to perform the actual A/D conversion and have the digital output available in a register or buffer to be transferred to memory.

Here the speed (determined by the type of A/D conversion process or combination of processes) is paramount. High speed, high quality A/Ds with low drift and requiring less calibration will obviously increase the cost of a data acquisition board.

Total throughput is also determined by:

• Transfer time: the time required to transfer data to and /or from the data acquisition board to memory, where software can determine if it's to be displayed and /or transferred to a permanent storage location. The transfer rate of the data acquisition system, is the slowest of either the board throughput, the data transfer rate or, if relevant, the storage rate.

In most data acquisition systems the storage rate is not significant, since the amount of data involved is small enough to allow it all to be stored in memory, left there, displayed if necessary, and stored later. The transfer rate of the link from memory to permanent data storage is therefore not important.

Where the volume of data acquired is greater than the memory available to it, and particularly when the required throughput is very high, then alternative methods of storing the data must be found.

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