Waveform Digitizers in Automatic Test Systems

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Fully functional digital electronic oscilloscope

Electronic oscilloscopes are widely used instruments by engineers in laboratories, factories and on-site. In fact, electronic oscilloscopes are also the products with the largest sales volume and the highest sales amount among electronic test and measurement instruments. In the late 1930s and early 1940s, driven by the rapid development of the television broadcasting and radar ranging market, analog electronic oscilloscopes were basically finalized and divided into four parts: vertical amplification, horizontal scanning, trigger synchronization and oscilloscope (CRT) display. The real-time bandwidth of analog electronic oscilloscopes reached a peak of 1000MHz in the 1970s. With the emergence of digital technology and integrated circuits, analog electronic oscilloscopes dominated by vacuum tubes and broadband amplifier circuits were gradually replaced by digital electronic oscilloscopes from the 1980s. With the explosive development of information technology and digital communication markets, the real-time bandwidth of digital electronic oscilloscopes exceeded 1GHz after the 1990s. In the 2010s of the 21st century, digital electronic oscilloscopes also made a leap forward, with real-time bandwidth exceeding 10GHz and equivalent sampling bandwidth reaching 100GHz.

The circuit structure of a digital oscilloscope is simpler than that of an analog oscilloscope. It mainly consists of four parts: analog/digital converter (ADC), waveform storage/processor, digital/analog converter (DAC) and liquid crystal (LCD) waveform display. Analog oscilloscopes need to have broadband response from the front end of signal input to the back end of waveform display, but digital oscilloscopes only need the front-end analog/digital converter to have the same broadband response as the input signal, and then the frequency response of various circuits is reduced accordingly. According to the sampling principle, under the best conditions, the sampling frequency is equal to twice the highest frequency of the input analog signal. After filtering and DAC processing, the ADC output digital information can reproduce the waveform of the input signal. Obviously, the clock frequency of the DAC can be much lower than the sampling frequency of the ADC. In addition, in order to reduce the aliasing signal caused by signal filtering and processing, the actual sampling frequency used by the ADC of the digital oscilloscope is 4 times the highest frequency of the analog input signal instead of 2 times.

At present, the highest level of ADC sampling frequency reaches 20GHz and resolution is 8 bits. If two ADCs with a sampling frequency of 20GHz are superimposed on the time axis, the equivalent ADC function of a sampling frequency of 40GHz and a resolution of 8 bits will be obtained. In other words, with the help of an ADC with a sampling frequency of 20GHz, a bandwidth of 10GHz can be achieved, but the resolution is only 8 bits. If it is allowed to reduce the sampling rate of the ADC, it is not difficult to improve the resolution of the ADC. For example, an ADC with a sampling rate of 1MHz can achieve a resolution of 28 bits. Digital electronic oscilloscopes with a real-time bandwidth of more than 100MHz completely use 8-bit resolution. In order to improve the resolution, multiple samples can be averaged, but the measurement time will also increase accordingly. Digital electronic oscilloscopes with a real-time bandwidth of less than 100MHz can provide products with resolutions of 8 bits, 10 bits, and more than 16 bits.

Compact Waveform Digitizer

From the above introduction, we can see that the digital electronic oscilloscope is a fully functional benchtop instrument with strong visibility, interactivity, signal integrity, hardware-defined measurement functions, a good user interface, and the highest real-time bandwidth. It is suitable for electronic product development, evaluation, measurement, and troubleshooting applications.

However, in the product line of electronic components and equipment, it is often necessary to increase the measurement speed, obtain all the data of electrical indicators in the shortest measurement time, and ensure that the product is launched on the market in time. In this case, the waveform digitizer came into being. In essence, the waveform digitizer is a simplified version of the digital electronic oscilloscope. The waveform digitizer only retains the ADC front end and data storage/calculator, and omits the DAC and LCD display at the back end. In other words, retaining high speed and discarding low speed, simplifying the process makes the waveform digitizer more suitable for the application of automatic test systems.

Circuit Configuration of Waveform Digitizer

The circuit block diagram of the waveform digitizer is shown in Figure 1. It can be seen that the waveform digitizer as a module focuses on the digitization of the input analog signal. The circuit structure is very simple. The front end is the ADC chip, followed by the digital memory and the on-board digital signal processor to perform arithmetic operations and waveform analysis on the signal, and finally transmit it to the data acquisition controller of the automatic test system via the high-speed bus. Compared with the digital electronic oscilloscope, since the waveform digitizer only retains the high-speed front-end ADC circuit, it directly uses the high-speed peripheral bus of the PC to transmit digital data, such as PCI and PCI Express buses, which have higher transmission speeds than GPIB, VXI, LXI and other instrument general buses. In addition, the on-board digital signal processor of the waveform digitizer promptly analyzes and processes the data after analog/digital conversion, and then hands it over to the PC of the data acquisition subsystem for background calculation.

The data flow of the waveform digitizer is shown in Figure 2. Analog instruments do not have physical panels and display screens, but only virtual panels. Therefore, the working status of the waveform digitizer circuits at all levels is instructed through the graphical interface, including the input signal conditioning and signal acquisition method at the front end. The input signal is converted into a data stream by the ADC, that is, it is temporarily stored by the memory and the digital signal processing is performed. The memory performs continuous digital acquisition of the data stream and stores a large number of waveforms. At the same time, the digital signal processor performs mathematical operations and parameter analysis on the waveform. The results are sent to the background computer of the automatic test equipment subsystem through the high-speed peripheral bus for measurement result processing. It can be seen that the waveform digitizer is a software-defined measuring instrument. The software plays a key role and provides measurement engineering and technical personnel with a platform with multi-channel signal input, large-scale data processing, short measurement time, small footprint, convenient maintenance, and low measurement cost. The comparison between digital electronic oscilloscopes and waveform digitizers is shown in Table 1.

Table 1 Comparison between digital electronic oscilloscope and waveform digitizer

Both test and measurement instrument companies and automatic test system companies have waveform digitizers, and the real-time bandwidth of their products covers low frequency to radio frequency, with a variety of models. It is worth noting that waveform digitizer suppliers such as National Instruments (NI), which uses the self-developed PXI instrument bus platform with Lab VIEW graphical programming software to form a data acquisition system that plays an important role in automatic test systems and automated measurements. Another company, ZTEC Instruments, is a professional analog digitizer supplier. Its feature is that it can supply both PC industry standard analog and measurement instrument standard bus modules. The highest-performance waveform digitizer uses an ADC chip with a sampling rate of 4GS/S, a resolution of 8 bits, and a real-time bandwidth of 1GHz.

Since the circuit structure of waveform digitizer simulation is not complicated, there are many types of ADC chips supplied in the market, especially information technology and mobile communication products, which need to process video signals, thus promoting the development and production of video ADC chips. Well-known Texas Instruments (TI) and Analog Devices (ADI) have a variety of ADC series chips available, with a very high cost performance. If we need a dedicated waveform digitizer simulation when designing an automatic test system. In addition to purchasing ready-made products, you can also consider designing it yourself. In particular, ADC chips with a sampling rate of 200MS/S, a resolution of 16 bits, and a real-time bandwidth of 100MHz are widely used. A large number of products are needed in wireless communications, digital cameras, mobile phones, radars, medical imaging, data acquisition, test and measurement, and other fields. Taking the ADS548X series ADC chip of Texas Instruments as an example, it has the following characteristics: [page]

Maximum sampling rate 200MS/S, bandwidth 100MHz

16-bit resolution, full-scale background noise 78dB

Dynamic range without spurious frequencies 95dBc

On-chip high impedance differential buffer input amplifier

High-efficiency low voltage differential signal (LVDS) digital output

Supply voltage +5V and +3V, power dissipation 70mW

64-pin QFN-64 package, footprint 9×9mm

Temperature range -40 to 85°C

The structural block diagram of the ADS548X series ADC chip is shown in Figure 3. It can be seen from the figure that the input signals INP and INM are amplified by the buffer input stage and then sampled by the sampling clock of the 16-bit ADC analog/digital converter. After digital correction and shaping, the sample values ​​are sent to 8 groups of low-voltage differential amplifiers and output to the post-stage memory through data lines D0, D2 and D14.

The ADS548X chip also has auxiliary function blocks such as power regulation circuit, reference voltage, timing control, and working mode control. As the front-end chip of the waveform digitizer, the ADS548X is an ADC parallel analog/digital converter with high integration and performance. With the back-end buffer memory and digital signal processor, and the interface bus of the signal acquisition system, a complete waveform digitizer module can be formed. Texas Instruments also provides an evaluation board for the ADS548X chip. As the industry's leading digital signal processor supplier, it has a variety of DSP chips for engineering designers to choose from, or with the help of the corresponding DSP development kit, the software design process of the module can be simplified to achieve rapid design and evaluation of the waveform digitizer.

The appearance of several waveform digitizers is shown in Figure 4.

Figure 1 Waveform digitizer circuit block diagram

Figure 2 Data flow of waveform digitizer

Figure 3 ADS548 series ADC chip structure block diagram

Figure 4 Several waveform digitizers

Reference address:Waveform Digitizers in Automatic Test Systems

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