mobile | classic
Dataweek Electronics & Communications Technology Magazine

Follow us on:
Follow us on Facebook Share via Twitter Share via LinkedIn


Electronics Buyers' Guide

Electronics Manufacturing & Production Handbook 2019


Using guard rings to minimise noise
28 January 2004, Design Automation

Although the effects of noise problems in digital and analog portions of circuitry can be very different, they do not necessarily need different solutions. Often the root cause is the same - the board layout has placed sensitive tracks or pins next to noisy tracks or pins. Clearly, the first thing to try is to physically separate the noisy traces from the high-impedance lines. If this is not possible, there is another way.

Let us consider a digital example - a PWM pulse interfering with a low-power oscillator circuit. Since the oscillator circuit is designed to draw very little power during operation, it has a very high internal impedance. As a result, the switching noise of the PWM pulse is capacitively-coupled from trace to trace into the oscillator circuit. This will cause clock jitter and, at worst, clock irregularity. If it is not possible to separate the traces sufficiently, the answer is to use a guard ring, as shown in Figure 1A.

Figure 1. Digital circuits may require a guard ring (A) around the oscillator pins of the controller or processor. Analog circuits may require a guard ring (B) around high impedance pins of the amplifier device
Figure 1. Digital circuits may require a guard ring (A) around the oscillator pins of the controller or processor. Analog circuits may require a guard ring (B) around high impedance pins of the amplifier device

The results of noise in an analog circuit can be equally destructive, even though the way in which the problem arises may be very different. A typical example would be a high-impedance pin of an operational amplifier located close to a track with a different voltage potential or, even worse, switching noise. It is very easy to overlook the first possibility.

For example, if the input pins of an operational amplifier have a very high impedance, the input bias current will be as low as a few picoamps at room temperature. The leakage current between the higher-potential track and the high-impedance one will make the input bias current of the amplifier appear to be higher than it really is. This leakage current could be a result of close traces and aggravated by humidity, board contamination or dust. The leakage current can be as much as five to 10 times higher than the input bias current, and can create severe problems. This is another example where the pins may need a guard ring as shown in Figure 1B.

A low-impedance guard ring, which separates sensitive operations from higher voltage or noisy events, is a very useful technique. Low impedance implies that it has low resistance as well as low inductance. In Figure 1A, the guard ring surrounds the processor or controller pins, oscillator clock and capacitors. Ground is connected to the guard ring to establish a low impedance potential. The guard ring in Figure 1B is connected to a low-impedance voltage that has a similar level to that on the input pins of the amplifier.

There are three common places to find these low impedance voltages. Ground is the most likely connection and this is the best solution if one of the amplifier inputs is connected to ground. Alternatively, the guard ring can be connected to the inverting input of the amplifier if the feedback impedance and source impedance to the amplifier are relatively low. A third option is to connect the guard ring to the non-inverting input of the amplifier, provided this pin is connected to a low impedance voltage source.

In both circuits described here, the underlying cause of the problem was an impedance mismatch. Once this is understood, the solutions, including guard rings, can be used to good effect.

For more information contact Avnet Kopp, 011 809 6100, Memec SA, 011 897 8600 or Tempe Technologies, 011 452 0530.

Supplied By: Avnet South Africa
Tel: +27 11 319 8600
Fax: +27 11 319 8650
Supplied By: Tempe Technologies
Tel: +27 11 455 5587
Fax: 086 517 8411
  Share on Facebook Share via Twitter Share via LinkedIn    

Further reading:

  • eSPI-to-LPC bridge
    26 June 2019, Avnet South Africa, Computer/Embedded Technology
    Microchip Technology’s new ECE1200 bridge allows developers to implement the eSPI standard in boards with legacy LPC connectors and peripherals. Product longevity is critical in industrial computing ...
  • Designing certified power solutions for LED lighting
    26 June 2019, Avnet South Africa, Power Electronics / Power Management
    LED lighting has become the product of choice in all types of environments, from domestic to industrial through to specialist areas such as stage lighting. The combination of efficiency in terms of lumens ...
  • Automotive smart hub ICs
    26 June 2019, Avnet South Africa, Telecoms, Datacoms, Wireless, IoT
    Microchip Technology has added new single-port USB smart hub ICs to its family of USB automotive products. The USB4912 and the USB4712 can be used to add a single port under the radio, in the centre console ...
  • Flash memories with embedded MAC addresses
    26 June 2019, Avnet South Africa, DSP, Micros & Memory
    Microchip Technology’s SST26VF Serial Quad I/O (SQI) 3 V Flash family is the industry’s first NOR Flash devices to offer integrated MAC address options. Pre-programmed with EUI-48 and EUI-64 addresses, ...
  • Passive RFID using UHF delivers long-range benefits in the IoT
    29 May 2019, Avnet South Africa, This Week's Editor's Pick, Telecoms, Datacoms, Wireless, IoT
    In order to support the use of UHF RFID as an IoT solution, a global alliance was formed in 2014 by Google, Intel, Impinj, Smartrac and AIM.
  • Wi-Fi and Bluetooth module
    29 May 2019, Avnet South Africa, Telecoms, Datacoms, Wireless, IoT
    Murata Manufacturing has collaborated with Cypress Semiconductor to develop the Type 1LV (CYW43012) low-power, small form factor Wi-Fi and Bluetooth module. This product is designed to improve battery ...
  • Dual- and single-core DSCs
    29 May 2019, Avnet South Africa, DSP, Micros & Memory
    New dual- and single-core dsPIC33C digital signal controllers (DSCs) have been released by Microchip Technology, with more options to meet changing application requirements across memory, temperature ...
  • AC-DC linear regulator
    29 May 2019, Avnet South Africa, Power Electronics / Power Management
    Texas Instruments expanded its portfolio of linear regulators by introducing the TPS7A78, which uses a unique switched-capacitor architecture to eliminate discrete components, including external inductors ...
  • Aluminium profile enclosures
    29 May 2019, Avnet South Africa, Enclosures, Racks, Cabinets & Panel Products
    In addition to its wide range of plastic enclosures, OKW also offers elegant aluminium enclosures. The two product groups, Synergy and Smart-Terminal, feature good stability, heat dissipation and a high-quality ...
  • 5G – the slicing and dicing of IoT?
    29 May 2019, Avnet South Africa, Telecoms, Datacoms, Wireless, IoT
    The new buzzword in the wireless industry is 5G, and it is often used in relation to the Internet of Things (IoT). This has contributed to some confusion as IoT has been broadly gaining traction in the ...
  • Connectors 101 for lighting designers
    30 April 2019, Avnet South Africa, Interconnection
    Selecting the best connector for a lighting application is a matter of matching application requirements to available connectors, with proper consideration given to performance and costs.
  • Altium Designer gets 2019 update
    30 April 2019, EDA Technologies, Design Automation
    Altium Designer 19, the latest version of the company’s flagship PCB design software, introduces new features aimed at making the design of complex, high-quality projects easier, faster and more accurate, ...

Technews Publishing (Pty) Ltd
1st Floor, Stabilitas House
265 Kent Ave, Randburg, 2194
South Africa
Publications by Technews
Dataweek Electronics & Communications Technology
Electronics Buyers’ Guide (EBG)

Hi-Tech Security Solutions
Hi-Tech Security Business Directory

Motion Control in Southern Africa
Motion Control Buyers’ Guide (MCBG)

South African Instrumentation & Control
South African Instrumentation & Control Buyers’ Guide (IBG)
Terms & conditions of use, including privacy policy
PAIA Manual


    Classic | Mobile

Copyright © Technews Publishing (Pty) Ltd. All rights reserved.