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Choosing a modular waveform digitizer

05 Mar 2014  | Arthur Pini, Greg Tate, Oliver Rovini

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Bandwidth: The required bandwidth of a digitizer depends on the nature of the waveforms you are measuring. For sine waves, a bandwidth of greater than twice the maximum frequency is generally adequate. If the waveform is pulse-like with fast transitions, we suggest that you use a bandwidth of five times the frequency of the pulse waveform. That lets you capture up to the signal's fifth harmonic.

Sample Rate: The sampling theorem states that to avoid aliasing, the sample rate of a digitizer needs to be more than twice the highest frequency component in the signal being acquired. Sampling at just twice the highest frequency component isn't enough to accurately reproduce fast edges in time-domain signals. Accurate digitizing of a signal requires the digitizer's sample rate be at least three to four times the required bandwidth.

Resolution and Dynamic Range: Resolution determines the dynamic range of the digitizer. The dynamic range is the ratio of the highest to the lowest signal level a digitizer can handle. Applications that involve dynamic signals (signals with both large and small voltage components) need a high-resolution instrument. As a first-order estimate of the required dynamic range divide the highest signal level by the smallest signal level that is expected. As an example, consider a full scale range of 1V and the desired minimum detectable signal level of 100uV. The ratio is 10,000:1 or 80 dB. With 6 dB per bit, this requires 13.3 bits of resolution in an ideal case with no additive noise, so a 14 bit digitizer would be required. Note that there is a trade-off between resolution and maximum sample rate. Higher resolution comes at the price of lower maximum sample rate.

Acquisition Memory Length: Determines the longest record length the digitizer can accommodate in a single acquisition. It also affects the sample rate on any given record duration. Record length is equal to the sample period times the acquisition memory length. For a given digitizer record length, the greater the memory length the higher the sample rate that can be used without overflowing the memory.

Triggering: Triggers synchronise data acquisition with external events. Effective use of a digitizer requires great flexibility in device triggering. Simple edge triggers based on the slope and signal level are pretty standard on most digitizers. Many offer window triggering as well. Trigger sources include the acquisition channels and multiple external trigger inputs. For maximum trigger flexibility these inputs along with a rearm capability can be combined logically to produce advanced trigger states.

Number of channels and Synchronisation: Each modular digitizer has a specific number of channels per card. Using multiple cards can increase the total number of channels. In order to maintain synchronisation, multiple cards need to be synchronised so that they share common triggers and a common clock. For example, up to eight digitizers such as those shown in figure 1 can be linked with a Star-Hub module with no phase delay between channels.

Acquisition modes: Digitizers generally offer a number of different acquisition modes. Some digitizers offer ring buffer mode (similar to an oscilloscope operation), FIFO or streaming mode, multiple recording (segment mode), gated sampling, and a multiple time base (ABA mode), which combines slow continuous recording with a fast acquisition on trigger events. These multiple acquisition modes feature a fast re-arm time. In the case of the digitizer in figure 1 it is as short as 40 sample periods (i.e. 80 ns at 500MSamples/s). These different acquisition modes let you configure the digitizer to best use the acquisition memory.

Digital Data Transfer:One of the key advantages of digitizers is the ability to rapidly stream data to a computer for further analysis and archiving. The Spectrum digitizers, in FIFO mode (streaming mode), are designed for continuous data transfer between the digitizer buffer memory and PC memory. Utilising a PCI Express x8 Gen 2 interface streaming speed is up to 3.4GB/s.

Form factor: Modern digitizers come in a variety of different form factors and standards. PCIe is popular, typically used when size is critical and the digitizer needs to go inside the PC. PXI and PXIe digitizers are also common in production test systems. When building large automated testing systems, with a number of different test instruments, choosing a common form factor generally makes the integration task much easier.

Software support: Digitizers should include driver software that supports your operating system and programming language. It is advantageous to have drivers that are common to multiple digitizer models so that changing a board type doesn't require major updates to your software.

If you intend to use third party software such as LabVIEW or MATLAB then drivers and examples should be supplied.

While most digitizers are controlled by user-written software, you often need manufacturer-supplied software for direct control of the hardware for system integration and hardware verification. Spectrum offers such a program called SBench 6 shown in figure 2. You can tailor software to match your requirements from single channel to hundreds of channels handling synchronisation and of multiple modules. The software also offers powerful measurement, analysis, and signal processing functions. In addition to showing the acquired sine wave(s) both cursor and parameter measurements are being displayed. The program includes the ability to do basic waveform math and advance calculations such as histograms and FFTs (fast Fourier transforms).

Figure 2: An example of a typical two channel digitizer acquisition with basic measurements and the FFT of one of the acquired channels.

Under computer control, modular digitizers let you capture and display waveforms. When you only need a few instruments, PCIe digitizers let you install them in desktop computers, which minimises the footprint of an automated test station. Storing data for offline analysis lets you evaluate performance of systems and their components.

About the authors
Arthur Pini, Greg Tate and Oliver Rovini contributed this article.

To download the PDF version of this article, click here.

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