How to Design Analog Circuits with Reliable Connectivity in High-Noise Environments

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Introduction

Systems designers encounter multiple challenges when it comes to the assurance of robust operation of connectivity solutions in industrial or hostile environments. A demanding environment may cripple hardware or degrade data accuracy. In this situation, a robust connectivity solution requires immunity from noise, voltage faults, and electrostatic discharge, resulting in both reliable and safe performance. This spotlight article discusses reliable connectivity in high-noise or severe environments and the problems it solves.

Defining Reliable Connectivity

In the context of the essentials of analog design, reliable connectivity provides accurate and reliable data communication between devices operating in harsh environments or remote locations. Linked system components must work flawlessly in harsh conditions, such as inside heavy machinery, on automated factory floors, and at exposed outdoor locations.

Device Characteristics

Reliable connectivity devices are built using analog components specially designed to function in extreme environments. These reliable connectivity devices feature wide temperature ranges, over-voltage and over-current protection, fail-safe operation, ESD protection, noise immunity, and vibration immunity. We will discuss a few of them in this article.

• Electrostatic Discharge: Electrostatic Discharge (ESD) events can cause individual component failures. An overvoltage event may occur when two materials having different electrical potentials come into contact, and consequently transfer their respective stored static charges, generating a spark. These accidental sparks may degrade a semiconductor device. ESD may threaten an electronic system if someone touches an I/O port or replaces a cable. Discharges that accompany such routine events may disable the port through the destruction of one or multiple interface ICs. Discrete components like external ESD diodes preserve data lines. Multiple IC devices integrate varying degrees of ESD protection. The IC itself needs no external protection.
• Fault Protection: Faults, like over-voltages, result from wiring errors, pinched or faulty cables, loose connections, or even solder debris that pushes the power line into contact with the data connection in the connector or on the PCB. The track is vulnerable to high voltages for a few seconds, which may even extend to minutes, before it fails. External protection helps the driver outputs and receiver to tolerate voltage faults of up to ±80V. Specifically designed devices can survive such overvoltage faults and reduce intricate designs and reduce PCB size.
• True Fail-Safe Receivers: True fail-safe circuitry assures a logic-high receiver output if receiver inputs are shorted or opened, or if all transmitters on that terminated bus are disabled (high impedance). True fail-safe receivers solve the collapsing bus problem by changing the threshold of receiver input to a minutely negative differential voltage (i.e., - 200mV to 50mV). A terminated bus with all its transmitters disabled has the differential input voltage of the receiver pulled to ground. This results in a logic-high output with a minimum noise margin of 50mV and a logic-low when the input goes below -200mV.
• Hot-Swap Capability: Hot-swap circuitry eliminates false transitions on the data cable during circuit initialization or live backplane connection. Thermal shutdown circuitry shields the driver against excessive power dissipation, thereby limiting the short circuit current. Inserting circuit boards into a powered or hot backplane may lead to voltage transients on receiver inputs and, consequently, data errors. Devices incorporate hot-swap input circuitry to prevent such unwelcome events.

Protocols

Multiple data interface protocols are presently used, and customized for different applications. Each application comes with a specific structure set and protocol specifications. The interfaces can include CAN, RS232, and RS-485/RS-422, among others. Usage of specialized data transceivers and analog ICs extract better performance compared to general products.

Applications

The following application examples show how Maxim Integrated has developed transceivers to overcome the myriad challenges in hostile environment conditions.

RS232 Transceivers for Hostile Environments

RS232 serial communication finds use in low-cost and low-speed serial communication applications such as GPS and automotive telematics. To achieve a robust data communications link in harsh industrial environments, the RS232 port must provide an isolated interface between the RS - 232 cable network and the connected systems to protect against voltage spikes and ground loops. An isolated RS232 communications interface is possible when the devices at both ends are isolated from the RS232 cable connecting them. The cable must be isolated to detach both data signal lines and the drive power. An example of this design is the MAX33251E and MAX33250E. They are isolated 1Tx/1Rx RS232 and 2Tx/2Rx respective transceivers, with galvanic isolation of 600VRMS between the logic field side and the power side. They also break the ground loops. The isolated RS232 transceivers have integrated charge pumps and an inverter to eliminate the need for a high positive and negative voltage supply. Built-in inverter capacitors further reduce PCB space, as well as enabling level translation between the two voltages.

RS-485/RS-422 Data Safeguards in Industrial Environments

The RS-422 and RS-485 serial communication standards must meet specific Electromagnetic compatibility (EMC) regulations to be used in industrial applications. They include three principal transient immunity standards: electrostatic discharge, surge, and fast electrical transients. Most EMC problems are subtle, and thus must be factored in during the product design stage. The MAX14780E high-speed transceiver for RS-485/ RS-422 communication has one driver and a receiver. This device incorporates fail-safe circuitry, along with a hot-swap capability, allowing line insertion without erroneous data transfer. It comprises a half-duplex transceiver operating from a single +5.0V supply. The drivers are limited by output short-circuit current and are shielded from excessive power dissipation by thermal-shutdown circuitry. When activated, this circuitry puts driver outputs into the high-impedance state.

CAN Bus Transceivers for Automotive Applications

The Controller Area Network (CAN) bus is popular with industrial embedded engineers, but was first developed for automotive applications. It can be configured to meet different objectives, such as better fuel economy, or additional auxiliary power for electronic devices and power tools. The MAX33053E and MAX33054E are +3.3V CAN transceivers with integrated protection for industrial applications, supporting up to 2Mbps data. They can support 1.6V to 3.6V logic supply range and have highly integrated protection features. These devices have extended ±65V fault protection for equipment where overvoltage protection is required. They also incorporate high ±25kV ESD HBM and an input common mode range (CMR) of ±25V, exceeding the ISO11898 specification of -2V to +7V. This makes them well-suited for applications that are in electrically noisy environments where the ground planes are shifting relative to each other. The thermal shutdown protects the device from high temperatures and resumes operation once the temperature is lowered to its normal operating range. Standby modes switch the transmitter off or set the receiver to low current and low data rate modes. It is possible to adjust the slew rate to very low rates and minimize EMI effects. The transmitter dominant timeout feature pulls the bus out of a dominant low state to a released bus recessive state to prevent bus lock up.

Short Range Device (SRD) Wireless Transmitter

Many applications, such as Building Automation, Remote Keyless entry, and IoT need to deploy short-range wireless communication. The license-free UHF sub-GHz is a low-cost short range device solution, but it faces challenges of communications range, multipath transmission phenomena, transmitter power, and receiver sensitivity. Enhancements to the wireless range offered are possible with a higher power transmitter (TX) output, better antennas (TX_Ant, RX_Ant), and better receiver (RX) sensitivity. Higher power transmitters can effectively translate into more decibels at the receiver, but will compromise on battery life and size. The MAX41460 family of transmitters can provide an additional +6dB of output power over conventional transmitters without compromising the device’s power-supply requirements. The integrated PLL is fast, enabling the implementation of frequency hopping and spread spectrum protocols. It uses a single low-cost crystal providing fast frequency settling time and reduced temperature dependence. Very low supply current of less than 8mA and auto shutdown features extend battery life.

Examples of Analog ICs designed with Reliable Connectivity

Conclusion

Reliable connectivity encompasses a diverse group of design issues ranging from fault and line protection, operating temperature range, hot-swap capability, ESD, and short circuits. Compromising them at the design stage due to complexity is a severe mistake. Robustness and reliability should be factored in the design phase to ensure system reliability.