Free Trials

Download a free trial to find out which Altium software best suits your needs

How to Buy

Contact your local sales office to get started on improving your design environment

Downloads

Download the latest in PCB design and EDA software

  • PCB DESIGN SOFTWARE
  • Altium Designer

    Complete Environment for Schematic + Layout

  • CircuitStudio

    Entry Level, Professional PCB Design Tool

  • CircuitMaker

    Community Based PCB Design Tool

  • NEXUS

    Agile PCB Design For Teams

  • CLOUD PLATFORM
  • Altium 365

    Connecting PCB Design to the Manufacturing Floor

  • COMPONENT MANAGEMENT
  • Altium Concord Pro

    Complete Solution for Library Management

  • Octopart

    Extensive, Easy-to-Use Component Database

  • PRODUCT EXTENSIONS
  • PDN Analyzer

    Natural and Effortless Power Distribution Network Analysis

  • See All Extensions
  • EMBEDDED
  • TASKING

    World-Renowned Technology for Embedded Systems Development

  • TRAININGS
  • Live Courses

    Learn best practices with instructional training available worldwide

  • On-Demand Courses

    Gain comprehensive knowledge without leaving your home or office

  • ONLINE VIEWER
  • Altium 365 Viewer

    View & Share electronic designs in your browser

  • Altium Designer 20

    The most powerful, modern and easy-to-use PCB design tool for professional use

    ALTIUMLIVE

    Annual PCB Design Summit

    • Forum

      Where Altium users and enthusiasts can interact with each other

    • Blog

      Our blog about things that interest us and hopefully you too

    • Ideas

      Submit ideas and vote for new features you want in Altium tools

    • Bug Crunch

      Help make the software better by submitting bugs and voting on what's important

    • Wall

      A stream of events on AltiumLive you follow by participating in or subscribing to

    • Beta Program

      Information about participating in our Beta program and getting early access to Altium tools

    All Resources

    Explore the latest content from blog posts to social media and technical white papers gathered together for your convenience

    Downloads

    Take a look at what download options are available to best suit your needs

    How to Buy

    Contact your local sales office to get started improving your design environment

    • Documentation

      The documentation area is where you can find extensive, versioned information about our software online, for free.

    • Training & Events

      View the schedule and register for training events all around the world and online

    • Design Content

      Browse our vast library of free design content including components, templates and reference designs

    • Webinars

      Attend a live webinar online or get instant access to our on demand series of webinars

    • Support

      Get your questions answered with our variety of direct support and self-service options

    • Technical Papers

      Stay up to date with the latest technology and industry trends with our complete collection of technical white papers.

    • Video Library

      Quick and to-the-point video tutorials to get you started with Altium Designer

    How to Design a 4-20mA Current Loop Receiver with Minimal Components

    June 5, 2017

    I’ve always been a fan of Gordon Ramsay. He never fails to amaze me with his colorful cursing as raw scallops are continuously served by hapless Hell’s Kitchen contestants. But what really amused me was when he tried his hand at Asian cuisine. The tables were turned, and he learned the hard way that good Asian cuisine is a completely different beast than his usual beef wellingtons. 

    shutterstock_518001565.jpg

    Process Control: Where 4-20mA reign supreme.    

    I had a similar experience whens I shifted to analog design in an industry dominated by programmable logic controllers (PLCs), after spending 5 years designing pure digital electronics. I got my first taste of “Ramsay’s foray into Asian delicacies” as I worked on my first project involving multiple 4-20mA current loop receivers with an 8-bit microcontroller as a low-cost localized option for PLCs. For an analog process control expert this would be a walk in the park, but I got sidelined trying to convert current flow into the familiar digital readings I knew and loved. 

    4-20mA Current Loop In A Nutshell.

    If you’re as ignorant as I was, 4-20mA is a pretty common standard in the control industry. You’ll find 4-20mA transducers in HVAC, manufacturing, data centers or any industry where PLCs dominate. It's an industry standard where the parameters measured by a sensor are represented by direct current ranging from 4 mA to 20 mA. 

    When you've spent your entire design career messing with pure digital electronics you'll start to wonder, why are we not using voltage to transmit measured signal? And why not 0-20mA? 

    First of all, you need to understand that process control and automation relies heavily on the accuracy of the measured parameters and boards often operate in harsh electrical environments. Analog voltage signals that work well in confined digital electronics can experience electrical interference from relays and solenoids or suffer drops through lengthy transmission lines. 

    shutterstock_509525080 (1).jpg

    Back To Basics With Ohm’s law

    You need to get back to the basics to understand 4-20mA current loop transmission. Remember Ohm’s Law with the famous equation of I = V / R? Voltage can drop as it travels along the sensor’s cables, but the current that flows through them remains constant. This makes current flow a better measurement parameter in terms of stability and consistency. As for why parameters start at 4mA instead of 0mA, it simply because it is easier to detect faults. Anything that goes below 4mA is indicative of a faulty sensor or a broken cable.

    Simple and Effective 4-20mA Current Loop Receiver With Minimal Components.

    Before I started setting up my small contract manufacturing business I was a pure electronics designer. Back then, if I needed to convert current to voltage I’d use any solution based integrated circuit (IC) to get the job done quickly. It would have been a no brainer to use a precision 4-20mA amplifier receiver since I wouldn’t have considered its cost and ease of application. For designers who are unfamiliar with analog design, it’s a brave new world. You’ll be working with signal noises, interference and ground loops. A specialised precision IC can be an appealing solution, but will add a significant cost to the project itself.

    Now that I am a small business owner, I have to also consider what is good for sales and production. I take pleasure in reducing costs while keeping my designs functional. For converting current to voltage, the best solution aligns with my philosophy of keeping things simple; use a single low tolerance resistor. I discovered that the answer to designing a minimal 4-20mA receiver once again comes back to  Ohm’s Law, coupled with some good PCB ground plane design skills and the ability to maintain signal integrity.

    shutterstock_275724074.jpg

    Simple solutions always works best

    To convert current into a measurable voltage, simply pass the 4-30 mA current signal through a high precision resistor before it returns to the ground. A 250 Ohm resistor gives a 5V reading at 20mA. In my case, I used a 165 Ohm resistor for a full 3.3V reading since that’s the voltage my microcontroller was operating at. Once you've selected the right resistor value, it’s only a matter of connecting the resistor to the microcontroller’s analog digital converter (ADC) pin to turn the readings into digital values.

    Is A Single Precision Resistor Good Enough In Real Life Process Control Applications?

    To be frank, I was doubtful that a single precision resistor, which is ~1% of the cost of a specialized high precision IC, would be sufficient in all applications. These ICs are there for a reason, to produce accurate readings from sensors. But by keeping analog grounds and digital grounds separated by a single point, and by isolating analog signals from high-speed data, you can get a consistently reliable reading with just a single resistor. 

    I personally have produced reliable PCBs, that only use high precision resistors for multiple 4-20mA receivers, for industrial air dryer controllers and operating theater environmental monitoring systems. As for additional signal filtering and processing, today’s microcontrollers have powerful ADCs that you should take advantage of. 

    Now that you know how to speed up your 4-20mA current loop receiver design, you need the right software to do it. CircuitStudio can help you design professional PCBs while keeping cost in check.

    Have a question about 4-20mA current loop receivers? Contact an expert at Altium today.

    Check out Altium Designer® in action...

    Powerful PCB Design

    most recent articles

    Back to Home