High Frequency ADCs: Significance And Applications

As part of your enrollment in the Analog Electronics unit, you are required to submit a case study report on the following topic. Refer to unit plan on blackboard for further details of due date and submission process.

Topic: “High frequency electronics: signals, sensors and electronic interfaces to ADC”.

Include in your discussion: electronic circuitry and characteristics, and discuss application examples.

Attached is an example case study submitted by a MEng student. He prepared it in the format of a research paper.
It is not necessary to prepare a case study in the format of a research paper.

The report may be six printed pages in length and should not be reproduced without modification from any source.

Types of ADCs

ADC is becoming more and more popular in the high-frequency applications. These areas require faster signal processing capabilities which are wider range and noise floor are one of the most important factors in high frequency domain. Earlier, external down conversion systems were used as an intermediate system between the received RF signal and the processed output signal. This made the systems bulkier, costly and required greater power intake. This is solved by the use of high frequency ADCs. This process of digitization is employed by the ADCs. The incoming signals are directly down converted to the digitized output signal. This increases the performance as well as the bandwidth. Now more signal can be analyzed by the Fourier transformer systems at once. Greater the resolution, greater is the accuracy and applicability when there is flat frequency response, low full-scale voltage of the input and greater bandwidth of the input signal. This is helpful in the areas of aviation industries, Earth substations to catch the incoming radio frequency signals from the satellites, radar, surveillance systems, navigation systems, military and many more. This way multiple channel systems which processes the incoming intermediate frequency signals is eliminated by the use of high frequency ADCs making the overall system more flexible and thus increases the performance and the maintenance cost is also lowered. This gives an edge over the traditional systems.

Today is either used in almost every device because they have to interact with the environment which is analog in nature. The choice of the right ADC determines the performance of the system. There are many types of ADCs such as flash, pipeline, dual slope, Delta Sigma and successive approximation. They are used depending upon the application (Lightwave Staff, 2014). Flash ADC and SAR ADC are used wherever speed is required. Flash ADCs offer a sampling frequency of 10 Giga samples per second which is the highest of all the other ADCs. SAR ADC’s offer a little less sampling frequency than the flash ADCs of 10 Mega samples per second (Frenzel, 2016). ADC is used in RF sampling at first and provides dynamic range and a wider bandwidth. They can directly convert the input signal to its digital form which makes it very flexible. Modern telecommunication systems like 4G LTE as well as the future 5G systems will require faster data processing to meet the demands of the growing population. These use with multiple carrier waves with orthogonal frequency division multiplexing and quadrature amplitude modulation. So is desired that these large bandwidth signals, which can be of high or low amplitude, to be processed accurately and at a faster rate. This is possible using high frequency ADCs that work in the range of gigabytes (Reeder, 2019), (Kincaid, 2018), (Keim, 2017). Figure 1 shows a circuit of a flash ADC. It works on the conventional principle of comparing the input analog voltage with the reference voltage. The input voltage is digitized or converted into many digital levels which are 2ⁿ -1 in numbers. Each level corresponds to one comparator therefore 2ⁿ -1 comparators are used. In figure 1, a simple 2-bit flash ADC is used that includes three operational amplifiers which serves as three comparators. All the non-inverting ends are connected to the input voltage while each inverting pin is connected to a set of reference voltages of the magnitude V/2, V/4 and 3V/4 which can be easily obtained with the help of a voltage divider circuit of the input voltage V volts. The output of any operational amplifier is a logic 1 only when the inverting signal is less than the non- inverting signal while it is a logic 0 only when the non-inverting signal is less than the inverting signal in magnitude (Electronics-tutorial, 2018).

Application of High Frequency ADCs

The main components are the ADC, clock for sampling the signals and an amplifier or balun. In this the AC signal which acts as an input, is ready to the ADC via coupling with the help of a balun. The balun converts the signal differentially and this signal passes to the ADC input through the DC blocks which prevents any DC component to pass through it, allowing only the AC component. The signal reaching the ADC is AC coupled and the ADC is configured for the signal through a control signal. The systems does not require any DC component to be present hence there is no need for passing the DC by the balun. This system serves as a wideband system consisting of many channels that have narrow bandwidth. The tendency to pass the incoming high-frequency AC coupled signal through these channels is determined by the balun. It has to match the phase and amplitude efficiently. Poor performance of the balun develops herbal distortions because the input signal to the ADC are not perfectly differential signals and hence the output signal from the ADC is not up to the mark. This configuration as shown in the figure 2. The typical performance depends on three factors namely, ENOB (Effective Number of Bits), SNR (Signal to Noise Ratio) and SFDR (Spurious-Free Dynamic Range) (Jones, et al., 2012), (Marjorie, 2010), (Browne, 2008), (Frenzel, Fast ADC Facilitates Direct RF Sampling at Higher Frequencies, 2017).

The input to the ADC is passed an amplifier in DC coupled mode. The amplifier converts the signal to a differential signal and this signal acts as an input to the ADC. ADC has a common mode input voltage signal which drives the common mode input of the amplifier, this ensures the monitoring the common mode input voltage of the ADC along with temperature and other agents. This configuration is needed in oscilloscopes because that is erupted of the signal is to be measured and therefore the whole setup is DC coupled. Signal to noise ratio is one of the important factors that it reminds the performance of the system. The noise can be reduced by using a low pass filter between the amplifier and the ADC. Thus this increases the performance and limits the bandwidth of the input signal to the desired range. The configurations shown in the figure 3. The typical performance depends on three factors namely, ENOB (Effective Number of Bits), SNR (Signal to Noise Ratio) and SFDR (Spurious-Free Dynamic Range).

Performance factors of High Frequency ADCs

This configuration is similar to the AC coupled configuration as used in the communication systems, the common mode input pin of the ADC is grounded and therefore does not arise the convert input of the amplifier used. The DC component is still blocked by the amplifier and provides the ADC with a differential AC signal. The passing of the amplifier is not possible as we have seen earlier because the common mode input is grounded therefore an external biasing is required. This configuration is used in LIDAR requires AC. This application does not require a wideband bandwidth of signal. The necessary gain provided by the amplifier used. The noise enters into this system and hence harmonic distortions. This affects the overall performance of the system. The configuration is shown in the figure 4. The typical performance depends on three factors namely, ENOB (Effective Number of Bits), SNR (Signal to Noise Ratio) and SFDR (Spurious-Free Dynamic Range).

High frequency ADCs serves an important purpose in real-time measurement systems. In particle accelerators, the subatomic particles travel at the speed of light and therefore the measurements becomes critical as well as the time between the two consecutive measurements should be infinitesimally less. This can be easily achieved using high-frequency AC and Flash ADCs. Higher the resolution of the ADC used, greater is the performance and accuracy. The traditional acoustic spectrometers measure the spectral lines emitted from the zone and carbon dioxide molecules in the atmosphere. These spectral lines are prone to the variation in temperature and pressure therefore needs frequent and fast data measurements (Mccaney, 2016). Unlike the traditional systems, high-frequency ADCs serves best under the instable conditions for the temperature and pressure as well as gives better and fast results. The precision and the faster data capturing capabilities is achieved using Flash ADCs (Keysight Technologies, 2018).

Conclusion

From this project we can conclude that it growth in the technology, the demands of the users increasing. To fulfil their demands the conventional technology should become faster, the processing speeds should be increased with no compromise in the accuracy of the results. This can be easily achieved using modern flash ADCs. These circuits work in high frequency and with electronic components that interface to it. Different application requires different configurations such as in communication systems, oscilloscopes, LIDAR, real-time monitoring and many more. These ADCs find wide applications in today’s world and are used in almost every sector of technology because they offer greater flexibility and lesser circuits because they replace many conventional circuits. Flash ADCs can do multiple jobs making it a better option than many previously used circuits. Defense organizations are trying to free some of their spectrums for commercial usability. Moreover, more and more devices are being added to this network. So, the management becomes key role along with the protection. This is easily offered by the high speed and frequency ADCs that can operate a greater multiple than the existing ADCs speed.

References

Browne, J. (2008, October 14). High-Speed ADCs and DACs Arm Broadband Communications. Retrieved from https://www.mwrf.com: https://www.mwrf.com/components/high-speed-adcs-and-dacs-arm-broadband-communications

Electronics-tutorial. (2018, September 11). Flash Type ADC. Retrieved from https://www.electronics-tutorial.ne: https://www.electronics-tutorial.net/analog-integrated-circuits/data-converters/flash-type-adc/index.html

Frenzel, L. (2016, May 4). What’s the Difference Between SAR and Delta-Sigma ADCs? Retrieved from https://www.electronicdesign.com: https://www.electronicdesign.com/adc/what-s-difference-between-sar-and-delta-sigma-adcs

Frenzel, L. (2017, September 5). Fast ADC Facilitates Direct RF Sampling at Higher Frequencies. Retrieved from https://www.electronicdesign.com: https://www.electronicdesign.com/analog/fast-adc-facilitates-direct-rf-sampling-higher-frequencies

Jones, A. G., Wingender, M., Chantier, N., Thepaut, G., Amblard, J. P., & Marcelot, E. (2012, March 4). Selecting high-speed ADCs for high-frequency applications. Retrieved from https://www.eetimes.com: https://www.eetimes.com/document.asp?doc_id=1279526&page_number=2

Keim, R. (2017, May 17). Amplify, Then Convert: A New High-Speed ADC Driver from Linear Tech. Retrieved from https://www.allaboutcircuits.com: https://www.allaboutcircuits.com/technical-articles/amplify-then-convert-a-new-high-speed-adc-driver-from-linear-tech/

Keysight Technologies. (2018, September 11). High-speed ADC chipsets set the pace in real-time monitoring and control. Retrieved from https://www.keysight.com: https://www.keysight.com/main/editorial.jspx?ckey=1976527&id=1976527&nid=-35500.0.00&lc=eng&cc=IN

Kincaid, L. (2018, May 21). Analog Devices’ 12-bit 10.25-GSPS Radio Frequency ADC Sets New Performance Benchmarks for Instrumentation and Defense. Retrieved from https://www.analog.com: https://www.analog.com/en/about-adi/news-room/press-releases/2018/5-21-2018-12-bit-10-point-25-gsps-radio-frequency-adc-sets-new-performance.html

Lightwave Staff. (2014, March 4). Semtech announces ultra-high speed ADC/DAC cores. Retrieved from https://www.lightwaveonline.com: https://www.lightwaveonline.com/articles/2014/03/semtech-announces-ultra-high-speed-adc-dac-cores.html

Marjorie, P. (2010, February 1). Driving High-Speed ADCs in Wideband Applications. Retrieved from https://www.highfrequencyelectronics.com: https://www.highfrequencyelectronics.com/Feb10/HFE0210_Plisch.pdf

Mccaney, K. (2016, January 20). Ultra-high-speed chip could make the spectrum feel less crowded. Retrieved from https://defensesystems.com: https://defensesystems.com/articles/2016/01/20/darpa-act-em-spectrum-analog-to-digital.aspx

Reeder, R. (2019, June 19). An Inside Look at High-Speed ADC Accuracy. Retrieved from https://www.electronicdesign.com: https://www.electronicdesign.com/adc/inside-look-high-speed-adc-accuracy