A standards-based approach to capacitive-sensor EMC problems

J.D. van Wyk, M. Visser, and D.B. Rademeyer, Azoteq

April 04, 2012

J.D. van Wyk, M. Visser, and D.B. Rademeyer, AzoteqApril 04, 2012

By following basic principles based on industry standards, it is possible to implement capacitive sensing designs with high signal-to-noise ratios that will withstand a barrage of noisy abuse. This article looks at the most relevant EMC standards and noise threats, and discusses what can be done to ensure compliance for a given capacitive sensing application.

Capacitive sensing is a highly accurate analog measurement process that detects changes in capacitance on the order of 10-15 to 10-12 Farads and is supported by extensive digital signal processing. If it was feasible to position sensing ICs extremely close to sensing electrodes, noise would not be a problem. Alas, that's not the case. In most real-life capacitive sensing applications, a fair distance (a few mm's – cm's) exist between a sensing electrode and the IC.

Given the extremely small capacitance changes measured, it is no surprise that noise can wreak havoc if allowed to couple in between electrode and IC. Noise can cause digital data corruption via illegal bit detections or timing anomalies.

But all is not lost. By following some basic principles, making use of sensing solutions with high signal-to-noise ratios (1000:1 SNR is available in industry) and noise mitigation technology, it is possible to realize applications that will withstand all kinds of noisy abuse. This article will look at relevant EMC standards, noise threats, and what to do to achieve compliance for capacitive sensing applications.

Relevant Standards
Deciding which of the plethora of EMC standards apply to a given product or design can be confusing for the uninitiated. Even for people active in EMC, clarity is not always possible without a bit of effort.

EMC standards can be split into two groups, generic and product-specific standards. Generic standards give general guidance on what is applicable and how to test for specific compliance. However, for a given family of products such as. household appliances, committees for specific products determine what is required for compliance. Product-specific standards generally call upon or refer to the generic standard. Obviously, the number of product-specific standards is quite large. Therefore, we will review only generic standards.

For EMC, the most important considerations are: Does the Device Under Test (DUT) cause other devices or systems to malfunction, and does the DUT itself malfunction due to a lack of immunity to unintentionally received electromagnetic energy?

To determine whether the Device Under Test (DUT) cause other devices or systems to malfunction, we do emissions testing to determine the amount of energy radiated into the space around the DUT or conducted into the cables connecting the DUT to other devices and systems.

These emissions needs to be less than the limit of the relevant EMC standard. Capacitive sensing ICs consume extremely little power, typically in the low µW range. Therefore, emissions by capacitive sensing applications are typically in the nW - mW range, and compliance is seldom a problem. (Our emissions testing have shown that typical levels are at least -15dB below the CISPR22 limit). So for brevity’s sake, emissions will be not be discussed further.

But determining whether the DUT itself malfunctions due to a lack of immunity to unintentionally received electromagnetic energy is another beast altogether, because capacitive sensing circuits are so sensitive. Given the large number of potential interfering sources and the high sensitivity of the circuits, thorough immunity testing is advisable. Specifically, attention should be paid to those noise sources which result in capacitive currents flowing to earth using common mode EMC tests. (This does not mean differential mode immunity can be ignored.) Typical noise sources include lightning, supply voltage fluctuations, 50Hz magnetic fields, arcing due to breaks in inductive circuits, radio transmitters, electrostatic discharges, and switch mode power supplies.

In terms of energy, lightning strikes and related surges on power lines are the most destructive. The standard IEC 61000-4-5 (Surge immunity test) is most commonly referred to. Normally, surge immunity is addressed during design of the PSU front-end of an application and immunity of the capacitive sensing circuit per se is not tested. http://en.wikipedia.org/wiki/List_of_EMC_directives
Mains supply fluctuations should not affect a capacitive sensing application if a quality power supply is used. However, on occasion, cost may force use of a power supply with little fluctuation immunity. This may cause voltage rails variation for the sensing IC. In such cases, immunity testing according to IEC 61000-4-11 (Voltage dips, short interruptions and voltage variations immunity tests) should be done.

A capacitive sensing application using a mains-derived power supply or that is in the vicinity of mains-powered equipment is likely to see some sort of 50Hz interference. Often, this is due to coupling of 50Hz magnetic fields into the product or the cables feeding it. Use of intelligent DSP-based 50Hz filters, as are available in some capacitive sensing ICs, may help ensure immunity. For formal 50Hz magnetic fields immunity compliance, refer to IEC 61000-4-8 (Power frequency magnetic field immunity test).

Another noise source which should always be considered in capacitive sensing applications is Electrical Fast Transient Bursts (EFT/B). These typically occur due to fast breaks in inductive circuits and high voltage breakdown of air gaps, with the arc forming and decay happening in the ns range, implying frequencies in the tens to hundreds of MHz.. Testing according to IEC 61000-4-4 (Electrical fast transient/burst immunity test) is advisable, as EFT/B can be a headache for capacitive sensing.

And then we have wireless, with radio waves from cellular telephones, Wi-Fi, and other transmitters arguably posing the biggest threat to capacitive sensing applications. Coupling into the sensing system may be via a number of paths, always requiring some sort of inadvertent antenna structure. Radiated Immunity testing is described by IEC 61000-4-3 (Radiated, radio-frequency, electromagnetic field immunity test), covering the frequency range from 80MHz upwards.

However, additional testing beyond IEC 61000-4-3 is well advised. It may also happen that the radiation is too strong to filter or compensate for. In such cases, sensing ICs with integrated RF-detection become invaluable. Often, radiated interference with frequencies below 80MHz are coupled into long cables feeding a particular system, which has led to the establishment of IEC 61000-4-6 and which describes testing for immunity against 150kHz to 80MHz conducted interference. It should be noted that the interference here is a continuous wave, with AM, unlike that for immunity against surge of electrical EFT/B.

Electrostatic Discharge (ESD) will cause your capacitive sensing system to fail if no protection is provided. ESD immunity is split into two categories: the immunity of devices or ICs is described by the JEDEC Human Body Model (HBM); the immunity of complete systems that incorporate devices is described by the IEC 61000-4-2 (Electrostatic discharge immunity test) standard.

The below sections will take a more detailed look at four of the above standards that we have found to be particularly relevant to capacitive sensing applications. http://en.wikipedia.org/wiki/Human-body_model

Radiated Immunity
A large number of intentional (cell-phones, Wi-Fi, gaming consoles) and unintentional (lightning, arcing of contactors, spark plugs, products not conforming to EMC standards) transmitters will likely operate in proximity to your capacitive sensing circuit. Ensuring immunity to these transmitters can be challenging. The reader is encouraged to peruse [3] for a proper overview. The present IEC 61000-4-3 was preceded by IEC 801-3, and is a generic standard with a large number of product specific standards calling upon it, two examples being EN 55024: Information Technology Equipment (ITE) and Telecom Products, and EN 55104-2: Household Appliances.

IEC 61000-4-3 requires commercial products to be immune to radiated fields with strengths between 3V/m to 10V/m, which are 80% amplitude modulated with a 1kHz sine wave, and with frequency between 26MHz and 1GHz. Recent editions of the standard have called for further testing in the band between 1.4GHz and 6GHz, albeit not as a continuous sweep. For an excellent review of IEC 61000-4-3’s past and future, refer to [4]. Just testing to the minimum requirements of IEC 61000-4-3 is not wise. Most EMC authorities advise a good margin. Given the possible future requirements [4], we normally advise testing up to 30V/m and 6GHz where possible.

Radiated immunity testing is normally done on an Open Area Test Site (OATS) in a fully Anechoic Chamber, in a Semi-Anechoic Chamber, or in a GTEM cell that has been certified for conformance with an OATS. The capital outlay for all of these test facilities is high, reflected in their booking cost. In-house alternatives to gauge immunity are:

  • Cellular telephones, which typically emit up to 2W at 900MHz / 1.8GHz (EU) or 1.9 / 2.45GHz (US);
  • Wi-Fi routers – 2.45GHz;
  • Zigbee or Bluetooth Transceivers, 2.45GHz band. (Higher power Zigbee modules emit up to 100mW);
  • All Industrial, Scientific, Medical (ISM) band transceivers – typically in the lower mW range;
  • Two way radios, also known as walkie-talkies, which emit up to a few Watt of RF;
  • E-Field and H-Fields probes to inject radiated fields into localized sections of the application.

When using alternative radiated field sources such as these, they should be placed in as many locations and orientations relative to your circuit under test as possible. Always ensure that you are not exceeding local legal radiation limits, which could have severe consequences, including loss of life.

Following the below guidelines should help to ensure maximum immunity to radiated interference:

Decoupling capacitors: Proper decoupling of ICs is the first defense against radiated interference. However, for a specific capacitor value a resonant point exists above which impedance increases again (Figure 1a). Therefore, use a range of capacitors placed as close as possible to the IC. Feed and return path for the capacitors should have minimum inductance (Figure 1b). Use 0402/ 0603 size 10pF, 100pF, 1nF, 100nF, and 1µF if cost/space is not constrained, and 100pF and 1µF if money and board space is tight.

Figure 1a: Typical impedance for surface mount 100nF, 0402 ceramic capacitor

Figure 1b: Minimum inductance path between decouple C and IC, with smallest value C closest to IC

RC- and LC-filters: Classical low-pass RC- and LC- filters can be used on supply and communication lines to provide -20dB/decade and -40dB/decade attenuation respectively. Cascade them to increase filtering.

Unused / Do-Not-Place Components: Tracks running to and from component sites which are not populated may form excellent RF antennas. Use 0 W (zero ohm) resistors to decouple tracks to unused component sites. Watch out for unused connectors, since their open-circuit status also enables realization of antennas.

Grounding and stitching: RF-currents avoid inductances and seek capacitances. Without a low inductance ground, RF-currents may not be shunted to ground correctly, negating all effort with decoupling capacitors, filters and other tricks you felt were quite clever. The following summarizes grounding:
  • Thin ground track = BAD;
  • Wide ground track = BETTER;
  • Ground place with number of slots to accommodate other tracks = GOOD;
  • Solid ground plane only interrupted by via’s = BEST;
  • Solid ground plane only interrupted by via’s, and stitched to ground plane opposite = BEST of the BEST
Grounding is an extensive topic that can fill volumes, so the above needs to be critically applied. Ensure that you do not create islands of ground in your design. Stich sections on different layers together with vias.

Loops & Following the current path: The maxim “Follow the current” is well worth applying. Ensure you do not create loops by tracing paths as far as possible. Consider virtual interference sources or coupling points.

Capacitive Sense Pads: Tracks leading to capacitive sense pads are inherently unconnected on the pad side. Therefore, radiated interference easily couples into your design via these tracks, so keep them as short and thin as possible. This should also improve capacitive sensing sensitivity. Try increasing the series resistance between pad and capacitive sensing ICs to impede the noise. With some industry offerings, series resistance of up to 10kW (Kilo-ohms) will still give good performance.

Sometimes, the strength of the radiated fields is too much and it swamps whatever diligence has been applied. In such cases, the capability to sense excessively high radiated fields and halt any logic outputs is invaluable for immunity. Industry offerings exist where such capability has been integrated into the capacitive sensing IC.

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