Capacitive vs. Inductive Sensing

Touch Trade-Offs

Touch sensing has become an indispensable technology across a wide range of embedded systems. In this article, Nishant discusses capacitive sensing and inductive sensing, each in the context of their use in embedded applications. He then explores the trade-offs between the two technologies, and why inductive sensing is preferred over capacitive sensing in some use cases.

By Nishant Mittal

Touch sensing was first implemented using resistive sensing technology. But resistive sensing had a number of disadvantages, including low sensitivity, false triggering and shorter operating life. All of that discouraged its use and led to its eventual downfall in the market.

Today whenever people talk about touch sensing, they’re usually referring to capacitive sensing. Capacitive sensing has proven to be robust not only in a normal environmental use cases but also underwater, thanks to its water-resistant capabilities. As with any technology, capacitive sensing comes with a new set of disadvantages. These disadvantages tend to more application-specific. That situation opened the door for the advent of inductive sensing technology.

In this article, we’ll discuss capacitive sensing for embedded applications and how it can be used in various applications. We will then explore the use of inductive sensing in embedded products and why inductive sensing is preferred over capacitive sensing in some use cases. Finally, we’ll compare the advantages of inductive sensing over capacitive sensing in these applications.

Capacitive Sensing for Embedded

Capacitive sensing operates on the principle of monitoring the change in parasitic capacitance due to a finger touch (Figure 1). Capacitive sensing has been used primarily in two different forms: self-capacitance and mutual-capacitance. In self-capacitance mode, the net capacitance due to a finger touch and board capacitance is additive. This capacitance includes PCB traces and PCB materials like FR4, which has more capacitance compared to Flex materials and many similar dielectrics. Self-capacitance mode is useful in general touch application like buttons for touch-and-respond applications. In contrast, mutual capacitance is well-suited for applications involving more complex sensing such as gestures, multi-touch and sliders.

Capacitive sensing technique

Mutual capacitance sensing uses two different lines: TX(Transmitter) and RX(Receiver). The Transmitter sends a PWM signal with respect to the system VDD and GND. The Receiver detects the amount of charge received on the RX electrode.

One of the difficult use cases of capacitive sensing is that it cannot operate perfectly underwater. It also requires relatively strict design guidelines to be followed for error-free operation. Capacitive sensing performance is also impacted by nearby LEDs and power lines on PCBs. Implementing auto-tuning with variation in trace capacitance, variation in capacitive sensing buttons and different slider sizes and shapes all require different designs. Implementation challenges in industrial applications include using capacitive sensing with thicker glass material (display glass) and meeting capacitive sensor sensitivity requirements with those types of materials.

Inductive Sensing for Embedded

Inductive sensing enables the next-generation of touch technology in applications involving metal-over-touch use cases such as in automotive, industrial and many embedded and IoT applications. Inductive sensing is based on the principle of electromagnetic coupling, between a coil and the target (Figure 2). When a metal target comes closer to the coil, its magnetic field is obstructed and it passes through the metal target before coupling to its origin. This phenomenon causes some energy to get transferred to the metal target—referred to as eddy current—that causes a circular magnetic field. Eddy current induces a reverse magnetic field, in turn leading to a reduction in inductance.

Inductive sensing technique [1]

To cause the resonant frequency to occur, a capacitor is added in parallel to the coil to cause the LC tank circuit. As the inductance starts reducing, the frequency shifts upward changing the amplitude throughout. In contrast to a capacitive sensor, inductive sensing is able to operate reliably in the presence of water thanks to the removal of a dielectric from the sensor. This advantage brings inductive sensing touch sensing to a wide range of applications that involve liquids such as underwater equipment, flow meters, RPM detection, medical instruments and many others. Inductive sensing also supports biomedical applications. In general applications, inductive sensing enables replacement of mechanical switches and proximity sensing of metal objects. For example, in automotive applications, inductive sensing can be used to replace mechanical handles as well as detect car proximity. Some of these examples will be discussed in detail later..

Read the full article in the May 346 issue of Circuit Cellar
(Full article word count: 1842 words; Figure count: 6 Figures.)

Don’t miss out on upcoming issues of Circuit Cellar. Subscribe today!

Note: We’ve made the October 2017 issue of Circuit Cellar available as a free sample issue. In it, you’ll find a rich variety of the kinds of articles and information that exemplify a typical issue of the current magazine.

Flex PCB Design Services

Building Boards that Bend

While not exactly a brand-new technology, flexible printed circuit boards are a critical part of many of today’s challenging embedded system applications from wearable devices to mobile healthcare electronics. The PCB production service vendors listed here feature a variety of Flex PCB manufacturing services to help bring your embedded system design from prototype to production.

By Jeff Child, Editor-in-Chief

Location: Santa Clara, CA
Accutrace serves the electronics industry’s needs for quick turn PCB fabrication from prototype to production. They can manufacture only what is needed, when it’s needed—and in the amount needed. No need to worry about long shelf life or tying up your capital in bare board inventory.
Services Summary: PCB capabilities: Capabilities: Up to 50 layers; any layer HDI; blind and buried vias; 2-mil line width and spacing; copper up to 16 oz. Materials supported: FR4, Rogers, ISOLA, Polymide-flex, Metal core, Taconic and Magtron
Flexible PCB Services: Flex and Rigid-Flex


Beta Layout’s Flex-Pool PCBs use polyimide foils as their base material.

Beta Layout
Location: Shannon, Ireland
Beta Layout’s PCB-POOL operation is Europe’s largest manufacturer of PCB prototypes, with over 36,000 customers worldwide. PCB-POOL cost sharing principle enables system designers to benefit from low prices. You can calculate the prices instantly online using the PCB-POOL’s Online price calculation. PCBs using special technologies and materials can also be sourced directly from the company, from prototype to series production quantities.
Services Summary: Prototype PCBs or small series PCBs in 1-6 working days; Laser-cut SMD-Stencils in 1-3 working days; Fully populated SMD and THT prototype boards in 2 working days; Support for PCBs with embedded UHF RFID modules
Flexible PCB Services: All of Beta Layout’s Flex-Pool PCBs are manufactured exclusively in Germany. Only polyimide foils are used as the base material. These provide different benefits compared to other foils, differing substantially from rigid PCB materials.

Location: Shenzhen, China
EzPCB is an online provider of PCB manufacture and PCB assembly products and services. Based in China, the company’s worldwide business has been growing rapidly since its launch in 2004. EzPCB has supplied high quality PCBs—and related products and services—for over 2,000 customers from more than 40 countries. Its customers include amateurs, small businesses, universities and many world-class companies and organizations including Jet Propulsion Laboratory, Micron, Siemens, STMicroelectronics and Rohm.
Services Summary: PBC manufacturing, PCB assembly, cabling, enclosures, keypads, stencils and components, one-stop turnkey services and design consultation
Flexible PCB Services: Flex PCB support: Components mounted on flexible plastic substrates, such as polyimide, PEEK or transparent conductive polyester film. Additionally, flex circuits can be screen printed silver circuits on polyester. Flexible electronic assemblies may be manufactured using identical components used for rigid printed circuit boards, allowing the board to conform to a desired shape, or to flex during its use. Rigid-Flex PCB support: Flexible and rigid board structures are bonded together in the same units as one part of so called rigid-flex boards. Most rigid flex boards consist of multiple layers of flexible circuit substrates attached to one or more rigid boards externally and/or internally, depending upon the design of the application. Rigid-Flex PCBs contain a flexible part with an insulating layer and a conductor layer, and a rigid part with circuit wiring.

Location: New Bedford, MA
Epec specializes in custom, build to print electronics. It has a global team of engineers, designers, R & D innovators, product managers, manufacturing/supply chain professionals, quality assurance personnel and sales/customer service staff, all of whom are experts in their fields. Rather than limit its production capacity to its US and UK manufacturing centers, Epec has developed UL certified world class production facilities which are ISO-9001, QS-9002, TS-16949, with aligned technology roadmaps and quality systems.
Services Summary: Provides complete engineering and design services, from concept through production, in a quick and efficient time frame. Capabilities include: battery pack design and assembly, PCB electronic design, cable assembly design, flex and rigid-flex circuit design, user interface and more.
Flexible PCB Services: Epic’s flex and rigid-flex solutions and capabilities span from single- or double-sided circuitry to higher technology multilayer designs up to 6 layers. Its flexible circuits are built to withstand the rigors of aerospace, medical and military applications. As a high reliability replacement for wire and wire harness assemblies, flex circuits provide a significant cost savings with no reduction in performance.


Shown here is an example of Flexible Circuit Technology’s multi-layer flex PCB. It combines several single-sided or double-sided circuits with complex interconnections, shielding and/or surface mounted technologies in a multi-layer design. The layers may or may not be continuously laminated together throughout the production process.

Flexible Circuit Technologies
Location: Minneapolis, MN
Flexible Circuit Technologies (FCT) has an internationally recognized flex circuit Application Engineering and Design Team offering over 250 years of combined industry expertise, allowing FCT to provide customers with superior design services and attentive overall support. By co-chairing the IPC (Association Connecting Electronics Industries) Flexible Circuit Design committee, FCT stays abreast of industry standards and helps set standards the industry.
Services Summary: Supports multiple types of flexible PCBs; value added Assembly, Testing Services to complete box builds; Design consultation, optimization, DFM (Design for Manufacturing), to final flex design; Supply chain management for components and materials; Prototypes through production; Low mix-high volume and high mix-low volume program support; Assembly services; Testing Services.
Flexible PCB Services: Supports single, double, multi-layer flex, rigid flex, flexible heaters, flat flex and membrane switches. Heat sinks can be laminated to flex circuits to dissipate heat away from sensitive components such as LEDs. Supports through hole and surface mount capabilities on Flex PCBs, as well as in circuit testing, conformal coating, and electrostatic protective packaging. Graphic overlays can be placed over flexible circuits, rigid boards or membrane switches.

Location: Elk Grove Village, IL
Imagineering acts as a reliable source for high quality and on time PCBs. Its quick turn prototypes not just intended for testing and verification of designs. Every one of its boards meet IPC-A-600 F (Class2) standard, be it prototype or production. Specializes in quick turn prototypes, as well as rapid turn production.
Services Summary: PCB assembly capabilities summary: SMT, Flip chip, thru hole, Flex circuit assembly, cable assemblies, lead-free assembly and wire harness assembly. Full Turnkey service for all customers if needed as well as partial turn-key and consignment orders. PCB capabilities include: 22-layer fabrication, hole sizes down to 8 mil plated and 5 mil laser drilled, 3 mil line width and spacing, 6 oz. copper and a maximum PCB thickness up to .300″.
Flexible PCB Services: Supports Multi-Layer, Rigid and Flex PCBs.

Location: Shijiazhuang Hebei, China
OurPCB is a multi-national PCB manufacturing and PCB assembly company that provides global service and support while using its own Chinese manufacturing capabilities. Company has provided professional PCB production and assembly services for more than 2,500 customers around the globe.
Services Summary: Assembly capabilities include BGA, LGA, QFN, QFP, DIP and SIP. The smallest SMT footprint it can mount is 0201. Factory can also provide programming and wiring as well as injecting and conformal coating services.
Flexible PCB Services: Can fulfill flex PCB orders of 20 layers maximum; high-quality, quick turn flexible PCB manufacturing services.
Processes and services include:
• Line Width/Space in the Inner Layer: 3/3 mil
• Min. Drilling Hole Diameter: 6 mil
• Tolerance for Impedance Control (+/-): 10% rts

Location: Hangzhou, China
PCBCart is a professional PCB production service provider with more than 10 years of experience in the electronics manufacturing industry. It has manufactured printed circuit boards for more than 10,000 companies and over 80 countries around the world. Fast, affordable prototype assembly they take your unique PCB designs, prepare them for the assembly process and perform comprehensive testing to ensure they meet your precise performance requirements. Can provide a complete turnkey PCB prototype assembly featuring a one-stop shop approach.
Services Summary: Development, manufacturing, assembly and testing of custom PCBs; Rapid PCB prototyping; Circuit boards manufacturing: PCB assembly; Components sourcing services
Flexible PCB Services: Specializes in the fabrication and assembly of flexible PCB products for use in the manufacturing of medical devices and various types of telecommunication and industrial equipment. Can also work with customers to produce a custom flex PCB prototype.
• Quality Grade Standard IPC 2
• Number of Layers 1 – 8layers
• Order Quantity 1 pc – 10,000+pcs
• Build Time 2 days – 5 weeks
• Material DuPont PI, Domestic Shengyi PI
• Board Size Min. 6 mm x 6 mm, Max. 406 mm x 610 mm
• Board Thickness 0.1 mm – 0.8 mm

PCB Unlimited
Location: Tualatin, OR
In 2003, PCB Unlimited’s sister company Stencils Unlimited pioneered the internet SMT stencil quote and order process. In 2008, PCB Unlimited took it one step further by providing a one-stop-shop where engineers can quote and order online US and offshore PCB services and everything else they need for their PCB projects.
Services Summary: US PCB fabrication including Rigid, Flex and Rigid-Flex PCBs; Offshore PCB Fabrication; US Quick-Turn Prototype PCB Assembly and Low Volume Production followed by our offshore operation to service your high-volume production needs
Flexible PCB Services: Flex and Rigid-Flex PCBs from prototype to production.
• Trace and space capabilities down to .003″/.003″ (0.5 oz.), .004″/.004″ (1 oz.)
• Microvia diameter from .003”
• Lead times as short as 5 days
• Multiple offshore partners quoting system to offer excellent pricing (ISO:9001 certified)
• Selective plating of gold on the same flexible circuit
• Minimum tolerance of ZIF connector to edge of flex +/- .003″

San Francisco Circuits
Location: San Mateo, CA
Specializes in the complex, advanced technology of PCB fabrication and assembly, producing high quality multi-layered PCBs from elaborate layouts. Expertise in circuit boards is not limited to PCB manufacturing, but extends to PCB layout & PCB assembly.
Services Summary: PCB fabrication, PBC assembly and PCB layout services. Supports Mil-SPEC PCBs, Flex PCBs, Metal Core PCBs and many others.
Flexible PCB Services: Supports Flex PCBs and Rigid-flex PCBs. Can provide heavy copper flex circuit boards that can also be coupled with flexible thin copper layers designed to handle heavier power currents. Your heavy copper boards can be built with flex portions for lower voltage additions and applications.
Flex PCB specifications:
• Drill to copper – Outer layers 8 mil
• Drill to copper – Inner layers 10 mil
• Copper Thickness 0.5 oz. – 4 oz.
• Minimum Hole Size 0.004″
• Minimum Trace/Space 0.003″/0.003″
• Minimum Pitch 0.5 mm

Sierra Circuits can quickturn Flex PCBs in two days or less.

Sierra Circuits
Location: Sunnyvale, CA
Sierra Circuits is an ISO 9001:2015, ISO 13485:2016 and MIL-spec MIL-PRF-55110 certified, Silicon Valley-based, high-technology PCB manufacturer and assembler. It specializes in quick turn PCBs and medium production. By owning its manufacturing and assembly facilities, the company controls every aspect of the production schedule and quality.
Services Summary: Micro/fine line PCBs, PCB assembly, flexible PCBs, lead-free PCBs, high-reliability PCBs, no-touch PCBs, burn-in PCBs, R&D PCBs, MIL-spec PCBs, PCB design
Flexible PCB Services: Latest offerings include Flex PCBs, Rigid-flex PCBs, and microelectronics PCBs down to 1 mil trace/space (some restrictions apply). Offers high-quality, high-reliability quickturn flexible printed circuit boards in two days or less.
Flex PCB specifications:
• Max. Layer Count 12
• Min. Core Thickness .001″
• Min. Starting
Cu Foil Thickness O.L. 9 micron
• Max. Finished Cu Weight I.L. 2 oz.
• Smallest Mech. Drill Diameter .0079″
• Smallest Laser Drill Diameter .003″

This article appeared in the March 344 issue of Circuit Cellar

Don’t miss out on upcoming issues of Circuit Cellar. Subscribe today!

Note: We’ve made the October 2017 issue of Circuit Cellar available as a free sample issue. In it, you’ll find a rich variety of the kinds of articles and information that exemplify a typical issue of the current magazine.

Closed-Loop Module

CogiscanThe LCR Control Module is a fully integrated and closed-loop system designed to eliminate the risk of placing incorrect passive components on PCBs. Combined with the Offline Job Setup and/or Line Setup Control module, the LCR Control Module’s software is integrated with an electrical LCR meter. The system verifies that the measured values (e.g., inductance, capacitance, resistance) are within the tolerances specified for the component. This electrical verification can be done at any time prior to mounting the reel on the placement machine.

Contact Cogiscan for pricing.

Cogiscan, Inc.