Noise Hardened Linear DC Signal Opto Isolator Schematic

Noise Hardening for the Industrial Environment, Part 2

In my experience, the most difficult problems encountered involved electronic circuit boards that were sensitive to industrial EMP (Electro-Magnetic Pulse) radiated and conducted RF noise. Circuits may work flawlessly on the bench, but be useless in real-world industrial applications. This is because ICs are highly susceptible to both EMI and EMP. This discussion covers three basic rules for hardening circuit boards containing integrated circuits. This, I learned by experience rather than in engineering school. Note that what we do not learn from others, we must learn ourselves the hard way. Politics is astonishingly similar: “those who do not learn from history, repeat history.”

Note that there are other sources of noise, but this discussion centers around EMP induced noise that affects circuit board electronics. I sure wish that I knew all this when I started my career some 45years ago.

Reference schematics

Noise Hardened Linear DC Signal Opto Isolator Schematic

(Noise Hardened Linear DC Opto Isolator)

Rule 1 – eliminate EMP noise at the source

Eliminate or reduce EMP type noise at the source(s). The previous article (Part 1) discusses this issue.

Rule 2 – isolate all I-O lines

The 2nd rule is to isolate all I-O (Input–Output) lines with a resistor or ferrite bead and bypass all such lines to common via a capacitor. Serial I-O that must operate at a much higher speed for such isolation means requires a differential line driver and common mode inductor. Note that common mode ferrite bead inductors are a compromise, having both common mode and differential impedance components. Unbalanced high speed RS232 or RS423 lines may remain troublesome and require optical isolation.

If this simple, inexpensive means is not implemented, expect problems and/or failure in the presence of industrial EMP. To make matters worse, failure may occur at some unspecified future time when there will be no discernable cause –much like ESD (Electro-Static Discharge) failure. Note that none of these components are critical –virtually any type of resistor and capacitor will do, however the garden-variety multi-layer capacitors are superior.

Rule 3 – beef up the ground circuit

Beef up the ground circuit with wider and/or redundant traces. In the past, I always screened circuit board layouts by highlighting the ground (or common) circuit to spot potential weak spots. Note that a redundant ground circuit that includes circles does not constitute a “ground loop.” A ground loop occurs when high currents drop a significant voltage from one critical voltage point to another –this could be self induced as in a switching regulator power supply that generates substantial current, or could be externally induced by conducted noise current. Ground loops will be covered in a future discussion.

Akin to this is making sure that all IC rails are bypassed near each connected IC device. This, however, does not necessarily mean that all ICs require dedicated bypass capacitors as a strategically located capacitor may suffice for more than one IC.

The two circuit board schematics –hardened vs unhardened

It is very easy to spot the differences in the two schematics. Electronically, both function in exactly the same way, but the hardened circuit contains (7) additional inexpensive components. Whether actually needed or not, the addition of these components is a good practice and could potentially prevent much grief … and perhaps even protect your job.

Why are ICs sensitive to EMP?

Note that both analog and digital ICs are sensitive to EMP. This happens when the noise voltage on input(s) or output(s) exceed the power supply rails (>Vcc in the positive polarity and/or

One curious observation that tends to defy logic is that outputs are generally more sensitive than inputs –as a result, output noise decoupling must not be neglected.

Another important observation is that in multiple section ICs, noise often affects adjacent sections rather than the one actually connected to the outside world.

Why capacitors alone do not work

Many mistakenly think that simply adding capacitors between critical points and common should fix the EMP problem. All it does is to alter the resonant frequency of the external circuit wiring. While it tends to make the targeted point a voltage node, the resonance has not been attenuated in any way.

Op amp stability –the series 47W resistor

Adding a capacitor from the output of an op amp to common often causes instability (oscillation). A series 47W adds stability in most applications.

Slew rate limiting in op amps

The presence of EMP type noise can actually cause the output of an op amp to well exceed its slew rate –I have observed sub-microsecond transitions on an oscilloscope. While decoupling an output via an R-C (Resistor–Capacitor) network generally attenuates the noise, the capacitor itself plays an important role in correcting the problem by limiting the slew rate. The op amp current limit feature limits the output slew rate by limiting the charge rate of the capacitor. The result is that the output cannot exceed the slew rate in the presence of noise. By obtaining the output current limit from the spec sheet and using the relationship I*T = C*V, the capacitor value may be selected to slightly exceed the op amp slew rate –this will provide best stability.

Input R-C low-pass filter

The input R-C low-pass filter is a no-brainer. Most already understand its importance. The series resistor also provides ESD protection and may be made high in resistance due to the high input impedance of op amps and CMOS logic.

Noise generator for testing

In the lab where I worked, we had an old sewing machine motor that we used for noise testing. Teasing the power plug generated lots of nasty, wide-spectrum noise and circuit boards were subjected to this simple test. While this noise source was completely without specification, circuit boards that passed were considered good –if a circuit board did not pass, nothing was actually proven and it might just work OK in the industrial environment. While crude, this technique is far cheaper than testing via an independent lab, and actually should precede such testing.

Soft ferrite products from DigiKey

Ferrite bead photos

Datasheets, typical

Undocumented words and idoms (for our ESL friends)

no-brainer –idiom –no brains required for this task

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