MAX33074E
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The MAX33070E/MAX33071E/MAX33074E are a family of fault-protected RS-485/RS-422 transceivers with high ±65V protection for overvoltage conditions on the...
Datasheet
MAX33074E on Analog.com
In RS-485 systems, long cable runs can introduce significant ground potential differences between devices. Transceivers at the far ends of the bus reference the differential data to their supply and shifts in the ground potential between end nodes can push the common-mode voltage beyond the standard -7V to +12V range.
Extended common-mode range transceivers mitigate the impact of these ground shifts, ensuring stable and error-free communication over vast distances. Now that we’ve familiarized ourselves with RS-485, let’s review why its wide common-mode range tolerance makes make it so robust.
Differential data transmitted on an RS-485 transceiver’s output will have a DC component that is common to both signals in the differential pair. This DC voltage is referred to as the common-mode voltage and is typically set by the transmitting node. The RS-485 communication standard requires that receivers accept a common-mode voltage of -7V to +12V.
In some cases, it can be useful to extend this range, for example to overcome challenges due to equipment setup or operation in a noisy environment. This is particularly beneficial in applications where external electromagnetic interference (EMI) is prevalent, such as industrial automation systems, renewable energy installations, and automotive and transportation networks.
RS-485 best practices recommend driving the differential signaling over a shielded twisted pair of wires. In some cases, shielding is not feasible due to cable costs, connectors, or wire harness size limitations. Thus, noise that is coupled to the RS-485 cable will affect both pairs of wires and present itself as common-mode transients. Extended common-mode range enhances noise immunity by allowing transceivers to tolerate higher levels of coupled noise without impacting signal integrity.
Here’s an interesting real-world case for extended common-mode range. A user was experiencing EMI in their factory automation installation, which coupled sinusoidal noise from 500kHz to 3MHz with a peak-to-peak swing of up to 30V to their RS-485 . Motors and high voltage power supplies on a factory floor can radiate large amounts of EMI due to their operating principles where large amounts of current are switched. These switching currents induce strong magnetic fields that can couple to cable runs.
The MAX33074E with an extended common-mode range of +/-40V was used to successfully overcome this challenge and maintain communications signal integrity in their application. a differential signal of 10Mbps can be recovered when coupled with a sinusoidal common-mode signal. The waveforms on channels 1 and 2 are the A and B input signals to the half-duplex receiver of the MAX33074E and have an amplitude swing greater than –30V to +30V. These signals are comprised of a sinusoidal waveform common to both the A and B inputs and represent a transient common-mode voltage coupled with a 10Mbps differential data pattern which represents the data a user is transmitting on the bus. Channel 3 of the scope shots show the receiver output of the MAX33074E which has successfully recovered the 10Mbps data pattern while rejecting the common-mode transient voltage. Figure 1 below shows successful data recovery with a 500kHz sinusoidal common-mode voltage and Figure 2 shows successful data recovery with a 3MHz sinusoidal common-mode voltage.
Figure 1: 500kHz, 40Vp-p common-mode signal coupled with 10Mbps data. CH1 = RS-485 receiver input A, CH2 = RS-485 receiver input B, CH3 = RS-485 Receiver output RO
Figure 2: 3MHz, 30Vp-p common-mode signal coupled with 10Mbps data. CH1 = RS-485 receiver input A, CH2 = RS-485 receiver input B, CH3 = RS-485 Receiver output RO
Analog Devices offers a wide variety of transceivers with extended common-mode range for applications in factory automation, process automation, heavy machinery, and power grid applications.
MAX33070E/MAX33071E/MAX33074E: +3.3 to +5V half-duplex transceivers with +/-40V common-mode range
LTC2863-1/LTC2863-2/LTC2864-1/LTC2864-2: +3.3 to +5V full-duplex transceivers with +/-25V common-mode range
MAX14775E/MAX14776E: +3.3 to +5V half-duplex transceivers with +/-25V common-mode range
MAX33076E/MAX33078E: +3.3V to +5V quad receivers with +/-25V common-mode range
Next month in the RS-485 blog series, we’ll look at how fault protection can protect your installation and save you time and money.