Passing conducted noise immunity tests can be particularly challenging for electronic products equipped with a projected capacitive (PCAP) touchscreen for interface with the user, especially those targeted at industrial or medical markets that must meet 61000-4-6 Level 2 or Level 3 specifications.
Touchscreen controller selection and firmware optimisation each have a major influence on performance and should be applied in conjunction with best-practice filter design and board layout.
What we'll cover in this Whitepaper:
- Touchscreen Control and EMC Regulation
- Conducted Noise Immunity
- Passing the 61000-4-6 Tests
- Looking at an example from working with our customers
- Learning from Experience
Capacitive touchscreen control is a popular upgrade for next-generation products targeted at consumer, industrial, or medical applications. It can not only enhance the user experience but can also eliminate components like switches and lamps thereby simplifying the mechanical design and bill of materials. In addition, reliability can be improved and sealing simplified to exclude moisture or contaminants.
On the other hand, it is important to ensure compliance with electromagnetic compatibility (EMC) regulations prevailing in the intended geographical market, such as EMC Directive 2014/30/EU applicable in the EU. This directive references numerous standards, many of which have been originated by the International Electrotechnical Commission (IEC), and adopted as corresponding EN standards by the European Commission. These include the IEC 61000 series, among many others.
The challenges associated with passing conducted-noise immunity tests, as described in IEC 61000-4-6 and regional derivatives (EN 61000-4-6 in Europe) are frequently underestimated, especially when a PCAP touchscreen is implemented into a product. The reason 61000-4-6 testing is so challenging for capacitive touchscreens is that the test requires equipment to be unaffected by external electromagnetic disturbances entering the equipment, while at the same time the principle of a PCAP touch relies on detecting measurable disturbances caused by the user touching the sensor. This imposes demands on the touchscreen controller, which must ensure enough sensitivity to detect when the user has touched the screen while remaining unaffected by the noise that could be wrongly interpreted as a user input.
The tests specified in IEC 61000-4-6 require a disturbance signal to be applied in common mode to external cables (figure 1). The test is repeated through the frequency range from 150kHz to 80MHz, and the equipment behaviour is monitored to ensure the noise signal does not cause malfunctioning.
Figure 1. The IEC 61000-4-6 conducted-noise immunity tests verify that disturbance signals of a specified amplitude, and from 150kHz to 80MHz, injected into connecting cables are not capable of causing the equipment to malfunction.
The magnitude of the noise signal that must be applied without causing the equipment under test to malfunction is not determined by IEC 61000-4-6, but by the type of product category and target market. Table 1 describes the standard levels that can be applied to products.
Table 1. Industrial products and other mission-critical equipment must be immune to signals of higher amplitude than are applied to ordinary consumer products.
As the table shows, tests applied to Level 1 require a disturbance signal of 1V amplitude. This test can be passed with most consumer-grade ICs in the market. Other products – such as mission-critical industrial equipment or patient-critical medical equipment - may suffer more severe consequences if conducted noise were to interfere with correct operation. These must be able to operate correctly in the presence of stronger noise signals. As table 1 shows, the specified noise-signal amplitude is 3V for Level 2 where a more robust, industrial-grade IC is recommended. When 10V for Level 3 is required, High-end grade ICs would be recommended.
The choice of PCAP Touchscreen Integrated Circuit (or IC), as well as other factors such as board layout, cable lengths, power supply filtering, and the design and placement of any EMI filters all have an influence over immunity to conducted- noise. It is recommended to understand the appropriate level of immunity before product development begins because making significant changes to these aspects of the design are more expensive and time-consuming later in the project. Moreover, extra costs and delays are incurred each time the product has to be resubmitted to an approved test house for evaluation.
In practice, re-optimising controller firmware can increase conducted noise immunity to a limited extent. However, achieving a significant increase may require more fundamental changes to the design.
The Anders engineering team has helped several customers tackle conducted noise immunity challenges, applying an understanding of controller firmware, and hardware-design issues – seeking to demonstrate a suitable level of noise immunity according to IEC 61000-4-6.
One such project, a consumer-grade home-automation console featuring a 5-inch TFT display with a capacitive touch panel and custom cover lens, had already demonstrated immunity to the appropriate 3V (Level 2 Class A) disturbance signal and had qualified for CE compliance permitting the device to be placed on the EU market. The manufacturer then saw an opportunity to retarget the product for the smart-home-security market, which promised the possibility of increased sales and significant extra revenue. However, the mission-critical nature of the home-security application demands conducted-noise immunity to level 3, which required new test validation with a noise source of amplitude 10V.
The engineering team at Anders was asked to help make the transition into the home-security market, by improving the noise performance of the human-machine interface (HMI) design to pass Level 3 testing. Finding that noise filters were generally well designed and well positioned, we focused attention on optimising the touchscreen controller firmware to optimise touch sensitivity and noise immunity.
Leveraging our understanding of firmware optimisation and the opportunities for improving its performance, as well as our established links with chip makers, we produced and tested several iterations as figure 2 expresses.
Figure 2. Anders involvement in re-engineering the touchscreen controller firmware in pursuit of a solution to the challenge of raising conducted noise immunity from 3V to 10V. The Firmware Optimisation Process has taken a total of 3 months, with a 8V test passed only.
Ultimately, we demonstrated immunity with a maximum noise amplitude of 8V, which was insufficient to pass the 61000-4-6 Level 3 specification. At this point, we concluded that no further improvements were possible by adjusting the firmware and that more fundamental hardware changes were needed to achieve the desired 10V immunity. We suggested that further improvement may be possible by changing to a higher-specification (and likely more expensive) controller IC meaning new design of the touch controller driver board.
We have worked with many customers to improve the EMC performance of graphical and touchscreen user interfaces. We have been able to draw several inferences, based on our experience:
- Conducted noise immunity has a significant bearing on the performance of a capacitive touchscreen.
- The test specifications applied in major markets worldwide are based on IEC 61000-4-6. Appropriate levels for noise immunity are 1V for general consumer products, 3V for low-risk industrial equipment, and 10V for high-end mission-critical industrial or patient-critical medical equipment
- There are only limited opportunities to significantly increase the noise performance of an existing design through simple hardware or firmware changes.
- Filtering, firmware, and physical layout should all be optimised to ensure the most cost-effective solution meeting the desired target for noise immunity.
- Repeated product testing adds significantly to overall project time and cost
Designing with touchscreens presents engineers with tough noise-management challenges, to ensure immunity to external disturbances while maintaining adequate sensitivity to user commands.
When creating a new product, design for noise immunity requires knowledge of specific market requirements, and should ultimately be considered at the beginning of the project before the hardware design and component selection become firm. It is also worth considering potential alternative markets for the product as early as possible, to minimise the time and budget that must be committed to re-engineering.
Ensuring best design practice from the outset of the project requires an investment in time and expertise, and may add to the bill of materials (BoM) cost, but can deliver greater dividends by saving expensive redesign and re-testing.