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[Design View / Design Solution]
How To Ground Mixed-Signal Circuits Properly

Nicholas Gray  |   ED Online ID #5944  |   October 27, 2003


DESIGN VIEW is the summary of the complete DESIGN SOLUTION contributed article, which begins on Page 2.

There are many ways, and just as many opinions on the best method, to configure grounds for ADCs and other mixed-signal products. An earlier article written by the author ("Attack The Noise Gremlins That Plague High-Speed ADCs," Electronic Design, Dec. 17, 1999, p. 107) discussed the limitations of popular ADC grounding techniques and suggested a split ground plane. Though the method works for obtaining good noise performance for high-speed ADCs, a split ground plane isn't good for RFI/EMI performance.

Despite that grounding problem, there is a way to enhance noise performance while minimizing layout-related radiation. Three concepts dictate how to handle grounding for a combination of good dynamic performance and minimal RFI/EMI: the skin effect, the proximity effect, and a layout that considers these effects. The skin effect indicates that at high frequencies, the conductor's effective cross-sectional area is reduced below the actual full cross-sectional area. The proximity effect is when a flow of return current in the reference plane is restricted to a narrow area below or above the trace carrying the corresponding outgoing current.

This article discusses the difficulties surrounding skin and proximity effects and offers methods to deal with them. Among the topics are use of power traces instead of power planes, careful component placement, and the grounding of open areas. In particular, attention to component placement and power-supply routing makes it possible to achieve low noise performance while providing acceptable RFI/EMI levels.

HIGHLIGHTS:
The Skin Effect When ac flows primarily near the outside surface of the conductor, or on its skin, that is the skin effect. It becomes more prominent as frequency increases. Therefore, the effective cross-sectional area of the conductor is smaller than the actual dimension.
The Proximity Effect The magnetic field surrounding outgoing and return currents causes those currents to want to flow in close proximity to each other, ultimately restricting the return current to a narrow area below or above the trace.
Performance Needs Three fundamental principles help determine what's necessary for best overall performance: all currents follow the path of least impedance; transmission-line impedance is proportional to the distance between conductors; and the amount of signal-path radiation increases with the loop area defined by the outgoing and return current paths.
Grounding Open Areas It's common practice to cover all unused areas on the top and bottom of the pc board with grounded copper. Copper areas should be grounded at more than one point to avoid an antenna effect that would result in radiated energy at the frequencies contained in the ground plane.
   Obtaining the best performance from analog-to-digital converters (ADCs) has long eluded many circuit designers. One of the main obstacles involves proper grounding. Inadequate grounding techniques in high-speed ADC and mixed-signal circuits and systems can lead to excessive noise when digital return currents find their way into analog circuit areas.

Full article begins on Page 2


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    Reader Comments

    it is better to provide some small electronic circuit with grounding techenics

    GCGOUD -September 16, 2005   (Article Rating: )

    I have understood a little bit more about this theme. I would like to know if the analog ground is the same than the digital ground. I have a current transformer which I want to measure the analog signal, first it gets into a differential amplifier, then it is rectified and then it is filtered to have a DC signal. This signal goes into the ADC of a microcontroller , PIC16F870. the design: http://my.netomat.net/sardineta/Current_Transformer/ What you recommend me to do? am I right? Happy New Year.

    Gerardo -December 31, 2004

    It is very good on EMI and RFI. It's help me to design and in many ways.

    DIVAKAR NATH SINGH -December 13, 2003

    It will be nice if all the color illustrations are carried out in the printed edition of the magazine. One can then print out the stuff on the web in a B/W laser printer.

    Hareesh Janakiraman -November 21, 2003

    Please make your articles available in PDF as other online magazines do.

    Mike Allman -November 19, 2003

    I do all my reading while commuting while riding public transport to and from work every day. Having to access the Internet to read your articles forces me to print them out. What I find disconcerting is that you have figures on different pages when you can easily place several on one page, as in the case of the figures in artical 5944. That way less paper trees are used. Of course, someone there may have stock in the paper companies ...

    Thanks for listening!

    Troy Watson -November 13, 2003

    How about split plan with power routing? Will it give better noise and low EMI?

    bp khor -November 11, 2003

    Provide a pdf version of published articles for personal and local archiving. It also makes it easier to reference figures, tables, schematics, etc..

    Barry Smith -November 11, 2003

    Good article, but please embed the figures. A pdf would be even better.

    Gary Crowell -November 11, 2003

    Provide a PDF link for the DRILL DEEPER articles.

    john maclellan -November 08, 2003

    A good article but perhaps a bit empirical on the antenna theory. For example, the author states: "Radiation is, therefore, a function of the loop area outlined by the current path. It doesn’t matter whether one or both of these conductors is a piece of wire, a pc-board trace, or a plane". This is not necessarily the case. A pc-board trace placed above a ground plane creates an antenna having twice the "current-area" (length X current), as compared to the same trace driven against another trace. The radiation from the "ground plane" antenna is 6 dB higher, given the same antenna current. The radiation resistance is 4X for the ground plane case. This is due to the "image" antenna within the ground plane. For an electrically short trace, the vias are the real antenna.

    Another generalization implies that the radiation is proportional to the loop area. It is not stated whether the resulting electric field or the resulting radiated energy is proportional to the loop area. Given a square, or a circular small loop antenna (<1/10 wavelength circumference), the radiated power is proportional to the area. However, given a very rectangular loop antenna, such as a microstrip trace above a ground plane, the electric field will be proportional to the via height and not the horizontal trace length. This is due to radiation from the horizontal trace being largely canceled by the magnetic-field cancellation from the ground plane. The vias, being far apart, do not exhibit such high magnetic-field cancellation, and radiate rather well as two very short dipoles.

    It is also stated that proper termination can reduce EM radiation. EM radiation is due to the RF current flowing in the "antenna" portions of the circuit. As stated before, this can be mainly the vias. So, if termination reduces the RF current in the antenna portions, then it will reduce EM radiation. If it doesn't, then there is no reduction in EM radiation.

    The article also mentions EM radiation from slot. EM radiation from long, narrow slots is largely from the short ends and can be reduced by making the slot very narrow.

    Dave Cuthbert -November 05, 2003

    Gees... Please just insert all of the illustrations into the text and put all of the text on one big page... This jumping back and forth from page to page is a foolish waste of time.

    Richard Shelquist -November 05, 2003

    Very good article. Clearly written with good detail.

    dirk christensen -November 03, 2003

    As an electronics engineer who has designed over 100 printed-circuit boards, this was valuable information. I welcome these articles, especially the additional information that the website articles provide.

    Thank you.

    H Shanko/h.a.s. designn -October 30, 2003

    This may be nit-picking, but the story on skin effect isn't quite correct. The formula given applies to a round conductor. Replacing Pi X diameter with 2 X (w + h) isn't the same thing. It isn't that high frequencies like to flow on the surface, they just hate the center of the conductor. In a flat conductor, the currents crowd to the edges and leave the center of the trace with very little current flow. To optimize current flow at high frequencies, round conductors are best and big conductors are better. This presumes litz wire is not suitable.

    bill siebert -October 28, 2003

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