Analog to Digital Conversion Parameters

High-Speed Digital Design Online Newsletter: Vol. 9 Issue 03

Students often ask me, "What classes should I take?" or "What subjects should I study?"

My answer comes in several parts. First, if you want to go into engineering, take some math. Take more than you think you will need. Next, take some accounting. You will need that to figure out where all the money goes in your first startup :-)

Then get a mentor.

This last step is the hardest, but the most important. Start by looking at people you know well. Then expand your scope to teachers, friends, and distant acquaintances. You are looking for people who truly LOVE THEIR JOB.

Few people in the world find a career they love. Those lucky few are hardy, adventurous, risk-taking people. They master a craft, and then, (this is the good part) they pass that knowledge to others.

One such person is Bob Pease, the famous analog guru at National Semiconductor®. Bob's work at National, his magazine columns, and outstanding book "Troubleshooting Analog Circuits" inspire me greatly. When I admire someone's work, I try to find out what makes him or her tick.

In 1996, I met Mr. Pease for lunch in the National cafeteria. Bob greeted me and led the way to a quiet table by the window. We discussed transistor physics, electro-optical conversion, and the increasing use of simulators for circuit design. Then he stopped, looked directly in my eyes, and explained why I should write regular columns for Electronic Design magazine. Beyond the general responsibility shared by all engineers to pass along what they know, he said, "Writing a regular column teaches you something new every month, and that is a good thing." With his encouragement, I did just that. My early writing assignments led to my position today as EDN columnist for Signal Integrity, and finally to this newsletter you are reading today.

There you go. One meeting--one word of advice--changes a whole career. Thank you, Bob.

Analog to Digital Conversion Parameters

I recently taped a session of the BOB PEASE REALITY TV SHOW. Part One of this show will appear on the web Feb. 28. Part Two follows about a month later (see the announcement at the end of this letter). The shows will be archived. You can watch them any time.

On the show, Bob and I discuss a simple A/D application running at 1500 MHz (yes, that's 1.5 Giga-samples per second). The A/D posts its sampled data to a Xilinx® Virtex-4™ FPGA for later analysis. Our discussion touches on the architecture of the demonstration platform, the various amplifier stages used, the performance of the A/D converter, the LVDS bus between it and the Xilinx FPGA, and the USB link over to a personal computer for data analysis.

For the show, we taped a solid hour of round-table discussions plus time in the lab looking at the hardware with Ian King, the board designer. Even with all that, I feel we barely scratched the surface. Mixed-signal applications can be quite complex. I hope to return to Bob's show in the future to discuss other topics.

During our session we used the following five terms, but did not give detailed definitions. I would like to provide those definitions here, along with some hints about how "specsmanship" can distort the results.

SNR signal to noise ratio

This is the ratio of signal power to noise power in a signal (analog or digital), expressed in decibels (dB). It is usually measured using a pure sine wave source. SNR is the fuzziest of the five terms defined here. For example, some people count only "white" noise (specifically, noise uncorrelated with the signal), in the "noise" part of the ratio. Others say signal distortion (harmonics) should be counted as well. When reading an SNR specification make sure you know what effects count as "noise".
Make sure you know the bandwidth over which the "noise" power is defined. For example, suppose you have a 1-watt sine wave carrier at 100 MHz embedded in white noise having a power spectral density of 1 micro-watt/Hz. If you integrate the noise over a bandwidth from DC to 100 MHz, you get a noise power of 100 watts, yielding a negative SNR of -40 dB. If, on the other hand, you plan to band pass filter your signal to a limit of +/-50 KHz on either side of the carrier then you care only about noise in a band that is 100 KHz wide. The noise measured over that much tighter band drops considerably, yielding a positive SNR. Make sure you know over what bandwidth the noise is integrated in any SNR specification.

THD total harmonic distortion

For a pure sine wave excitation, this is that ratio of signal power at the fundamental to the signal power at all harmonics (and, in the case of sampled-data systems, to all aliased versions of the harmonics). Before making this measurement on a system, check the harmonic content of the source. Your system cannot improve on a source with poor THD performance. If you need a better source, try placing a linear band pass filter in series with the source to knock down its harmonics. The THD measurement procedure usually only measures harmonic power in the first few harmonics (4 to 7 typically). When comparing THD figures, check how many harmonics the datasheet numbers cover.
When dealing with sampled-data systems, the harmonics will often be aliased to other, weird frequencies. Sometimes they pop up in places that the test routing does not check. That is a cheap way to fake a better THD number on a specification sheet. Ask if aliased harmonics are counted in the THD number.

SINAD signal to noise and distortion

SINAD is very much like SNR, except that here we are clear that the definition of "noise" includes both white noise and distortion (harmonics). Always ask over what bandwidth the noise is integrated.
What if the signal includes spurious sinusoidal peaks from other, non-correlated sources (like crosstalk from your clock)? Does that count as noise, or distortion, or neither? I do not know. Check your specification sheet.

SFDR spurious free dynamic range

Inject a pure sine wave into your system. The output displays a number of peaks. SFDR measures the ratio of the power in your fundamental to power in the largest (non-fundamental) peak. When calculating SFDR, some datasheets count all the peaks, while some count only peaks not related as harmonics of the data signal. Some count only peaks over certain specific bandwidths (that might be the case when looking at a mixer). Make sure you know which peaks are reported in this number.

ENOB effective number of bits

This is the measurement digital people like best. It condenses everything down into one, easy-to-grasp, simple, wrong number. I say it is wrong because it is so non-specific about what factors are counted. In concept, you first measure one of the other definitions (typically SINAD) and then figure out how quantization levels you would need in an "ideal" A/D converter to attain that same level of performance. The ENOB is the log-base-two of the number of quantization levels required. A good concept, but useful only if two competing manufacturers base their ENOB calculations on the same type of measurement.

In all cases, the measurements may be taken once, or several times and the results averaged.

The evaluation software we used on the show is called "WaveVision". It is discussed in a National technical note here: http://www.national.com/profile/os.cgi?EventID=090805

I hope you enjoy watching the session as much as I enjoyed making it.

Best Regards,
Dr. Howard Johnson

About National Semiconductor’s Analog by Design Show

Media Contact
Megan Carter
National Semiconductor
(408) 721-6929
megan.carter@nsc.com

Analog Guru Bob Pease to Talk Shop with Digital Wizard Dr. Howard Johnson on National Semiconductor’s Analog by Design Show

Reality TV Show for Analog Engineers to Focus on High-speed Data Transfer

February 21, 2006 - The next episode of National Semiconductor Corporation’s (NYSE:NSM) Analog by Design Show, National’s online alternative to reality TV for analog engineers, will be hosted by Bob Pease, National’s analog design guru, and Dr. Howard Johnson, the author of High-speed Digital Design: A Handbook of Black Magic and a leading authority on signal-integrity techniques.

The featured topic of this two-part show is high-speed data transfer, but Pease and Johnson will also cover the latest developments in analog, digital, and everything in between. Engineers who tune in will learn how to minimize the high-speed design challenges they face through real-world illustrations based on National’s gigahertz-speed family of analog-to-digital converters (ADCs), Xilinx’ VirtexTM-4 field-programmable gate arrays (FPGAs), RF signal processing and National’s WaveVision 4 ADC evaluation software. Part one of this episode will premiere on February 28 and is available for viewing at http://www.national.com/nationaltv/. Part two will debut on March 28.

Joining Pease and Johnson are guests Paul Rako, a system design engineer with National’s Webench® online software development team; Cary Eskow, technical director at Avnet Electronics Marketing; Ian King, a systems engineer in National’s Data Conversion Group responsible for applications for National’s gigahertz-speed ADCs; and Panch Chandrasekaran, connectivity marketing manager at Xilinx.

In November 2005, National and Xilinx introduced a new development platform that the companies created to ease the design of high-speed data acquisition systems. The platform includes a Xilinx Virtex-4 FPGA and one of National’s gigahertz-speed ADCs, such as the ADC08D1500. More information about National’s and Xilinx’ products is available at http://xilinx.national.com.

About the Analog by Design Show

Launched in 2003, National’s Analog by Design Show is the semiconductor industry’s first webcast for analog engineers and provides solutions to real-world design challenges in an informative, entertaining talk show format. Each program features in-depth discussion of technical topics and lab demonstrations. Viewers also may link to other useful National sites such as the Analog University® online educational forum, Knowledge Base, Featured Communities and the Webench toolset.

This episode of the Analog by Design Show is sponsored by Avnet Electronics Marketing.

About National Semiconductor

National Semiconductor, the industry's premier analog company, creates high-value analog devices and subsystems. National's leading-edge products include power management circuits, display drivers, audio and operational amplifiers, communication interface products and data conversion solutions. National's key analog markets include wireless handsets, displays and a variety of broad electronics markets, including medical, automotive, industrial, and test and measurement applications. Headquartered in Santa Clara, California, National reported sales of $1.91 billion for fiscal 2005, which ended May 29, 2005. Additional company and product information is available at www.national.com.

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