Designing Versatile COTS Data Acquisition Systems

Designing a versatile data acquisition (DAQ) system begins by comparing features of the signal of interest to the capabilities of current state-of-the-art A/D converters. Specifically, signal bandwidth and maximum signal frequency are compared to the A/D maximum sampling rate and bandwidth. The specifications in Table 1 are typical of current A/Ds.

A fundamental principle of DAQ relates the minimum allowable sampling rate (the Nyquist sampling rate) to the input signal bandwidth. In order to uniquely distinguish each frequency component of the signal of interest, the sampling rate must be at least twice (in practice 3-4 times) the signal bandwidth. From Table 1 we see that theoretically, if the signal bandwidth is less than 80MHz, we can sample with a 16-bit converter at 160 MSPS. If the signal bandwidth is less than 250 MHz we can use a 12-bit A/D, and if it’s less than 1500 MHz we must use an 8-bit A/D.

The first trade-off in DAQ is apparent as an increase in signal bandwidth typically necessitates a reduction in converter resolution and SNR. The higher the sampling rate of an A/D converter, the more difficult it becomes to convert each sample with high precision. An efficient and versatile DAQ system, therefore, allows a choice of several different A/D converter front ends so that resolution (SNR) can be maximized without changing the “back-end” (post A/D) processing, allowing redeployment of the DAQ system in a new application while leveraging existing embedded processing algorithms and host software.

In the simplest DAQ system (Figure 1, top), the input signal is converted to baseband and the A/D converter samples at least as fast as the Nyquist sampling rate. That is, we have at least two (and preferably 3-4) samples per cycle of the maximum signal frequency. A low-pass filter (LPF) is used before the A/D to prevent aliases by limiting the signal bandwidth to less than half the sampling rate.