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Waveform digitizers

What’s a Waveform Digitizer?

Waveform Digitizers are electronic devices that are used to measure high-speed transient and continuous electrical voltages. The measured voltage signals may be derived from an electronic system or sensors. Sensors are electrical devices that convert one type of physical phenomenon (such as temperature, heat, distance) or light to voltage for measurement purposes. Digitizers are available that sample as high as 4 GigaSamples/s although Strategic Test specializes in products that are focused on sample rates between 1 kS/s and 500 MegaSamples/s, which means up to 500 million readings per second. A Waveform Digitizer consists of five main sections:

  • A measurement section comprising of one or more Analog-to-Digital Converters (ADCs), pre-amplifiers and impedance matching components. Impedance matching and pre-amplification allows the input signal to be conditioned to best match the voltage range of the ADC so that the maximum signal resolution can be measured. The ADC converts the applied voltage signal into a binary quantization level that records the voltage level of the signal at that instant in time. This data is then transferred to the Digitizer memory.
  • A memory section used to temporarily store the received quantization data. Depending on the functionality of the Digitizer, the device can transfer the data to the host PC at the end of the measurement (we call this the Standard Mode of operation) or the data can be continuously transferred to the host PC while the measurement continues (we call this the FIFO or Streaming Mode).
  • A bus section that is the electrical interface to the host computer. Its purpose is to transfer commands to the Digitizer for measurement setup and control, and to transfer the recorded data to the host PC.
  • A clock section that generates a clock signal that determines the sample rate of the ADC(s), provides internal bus synchronization and refreshes the onboard memory devices.
  • A control section. This is used to control the previous four sections and transfer the data from onboard memory to the bus.
Waveform Digitizers, Digital Storage Oscilloscopes and Transient Recorders

These same five functional sections can also be found in many other measurement instruments including Data Acquisition cards, Oscilloscopes and Transient Recorders. Not surprisingly this leads to some confusion as what is one persons Digitizer is another person’s Transient Recorder or Oscilloscope card. In this website we shall refer to these devices as Digitizers / Transient Recorders because two keywords are better than one for internet search engines ;-) These devices are available as standalone instruments and as electronic cards that can be integrated with standard PC’s or cards that require an external industrial chassis based on the PXI, CompactPCI, VXI, VME or CAMAC bus standards. Strategic Test supplies instrument cards based on the PCI Express, PCI-X, PXI and CompactPCI standards.

Compared to Data Acquisition Systems

The main difference between low-cost data acquisition cards or systems is that these sample at much slower speeds and normally use one ADC and pre-amplifier section for up to 16 analog input channels. A multiplexer switches from one channel to the next so that the ADC can record the signals on one input channel after the other. This technique means that the signals of all channels are not recorded at exactly the same time, but slightly delayed. This means that the recorded signal have time-skew errors or also called phase errors. When the incoming signals have a slow frequency, such as temperature readings from a thermocouple sensor, these errors can usually be ignored as the multiplexer switching speed is much faster that the used sample rate – so the delay between channel readings is very small in comparison. However, at higher sample rates this delay can be significant. The maximum sample rate for the fastest data acquisition systems is in the region of 1 MSample/s. The slowest Transient Recorders and Digitizers tends to start at 100 kS/s and go up to 4 GSamples/s. Oscilloscopes are available from 10 MSamples/s to 40 GSamples/s and even higher. Digitizers / Transient Recorders have one ADC and pre-amplifier per input channel. In order to ensure that all channels are measured simultaneously (without time-skew errors), the onboard clock signal is fed to all ADC’s in parallel. The addition of a pre-amplifier to each ADC means that the user is able to select the best input voltage range to match the signal on that channel and so perform a measurement to the highest resolution of the ADC.

What about multiple instrument card synchronization?

While all signals being fed into the instrument are recorded simultaneously, what happens if two or more instruments are required to record more signals? PXI cards have the natural advantage that clock and trigger synchronization signals are contained on the bus for multiple card synchronization. However, the PCI Express, PCI-X and CompactPCI buses were not originally designed for instrumentation applications and so don’t have this feature. Strategic Test solves this problem with the Star Hub option. This consists of a small daughterboard that is added to one of the PCIe, PCI-X or cPCI cards, which feeds the master clock and trigger synchronization signals to all the other cards via small flat cables. Star Hub enables up to 16 cards to be perfectly synchronized in one PC or chassis, while the System Star Hub option that is only available on the PCIe and PCI-X cards can also synchronize up to 17 Star Hub enabled PC’s equipped with up to 271 instrument cards. One other differentiator between these instruments and most data acquisition devices is that these instruments also have local memory as the speed that data is recorded often means it can’t be transferred over the PCI, PXI or cPCI bus fast enough to the host PC. For example, measuring two channels at 100 MSamples per second would generate 200 MBytes/s data at 8-bit ADC resolution and 400 MBytes/s data at ADC resolutions of 12, 14 or 16 bits.

What Are the Advantages of PCI Express, PCI, PXI or CompactPCI Cards over Standalone Instruments?

If you need the highest sample rates possible, then only standalone oscilloscopes have the capability to measure at rates above 4 GSamples/s. Below this speed modular instruments based on one of the standard bus formats will usually be more cost-effective or the preferred choice when flexibility, memory depth, number of channels and system size are determining factors. For example, consider configuring a mixed-signal system consisting of a Digitiser with 4 channels and an Arbitrary Waveform Generator also having 4 channels. Strategic test could supply two PCIe cards that were synchronized with the Star Hub option that would fit in any low-cost good performance PC that can also be used to analysis and report the measured data. The alternative standalone system would consist of two large chassis that may have synchronization difficulties and would most likely cost more. In addition, these would be using older embedded processors that offer no possibility to upgrade and have a fixed use.