Other Parts Discussed in Post: LMP2022, LMP2021

Chemical Plant at NightIn part 1 of this blog series I talked about harsh environments which include mines, mills, chemical plants, et. al.  Designing for those environments means taking into consideration extremes which normal products (e.g. consumer goods) would never experience.  We covered (briefly) some thermal issues found in these environments, but beyond high temperatures, corrosives and dirt there are dangers lurking that even humans cannot sense.  I’m talking about Electromagnetic Interference or EMI susceptibility.

Most engineers think about EMI damage caused by electrical overstress.  A good example of that is lightning damage - but equipment doesn’t need to be hit directly to take it out of commission.  The lighting that does the actual damage may occur many miles above the equipment and may not even be seen!

In large networks where wire is strung out over miles, there can be damage caused by electrical overstress due to a phenomenon called “Cross Striking” (see the figure). This occurs when two electrically charged clouds drift near one another.  An electrical potential appears between these two enormous capacitors and grows stronger as they get closer.  On the ground, an opposite charge appears on the cabling caused by the static charge in the cloud above.  It appears very slowly so no apparent problem is seen – yet. 

Cross strike begins as two oppositely charged clouds drift near each other

Cross strike occurs leaving charge on wires below

Once the electric field strength between the two clouds exceeds the break-down voltage of the air between them, a bolt of lightning forms and rapidly dissipates the charge.  This rapid discharge has now left an extremely large charge on the wires below with nothing holding it.  This charge also rapidly dissipates causing extremely high voltage spikes on the cable which typically will destroy anything unprotected on either end. I know this… I’ve seen it first-hand.  Nothing but ashes and a hole where my transceivers used to be! It just makes you go “hummmm… wow!”

To protect against this type of “invisible” damage, gas discharge tubes, spark gaps and ESD diodes need to be placed on either end of the cables to provide a path for the current to travel as the charge dissipates.  This will keep the voltages seen by drivers, receivers or amplifiers within absolute maximums and prevent damage.

Another type of EMI susceptibility doesn’t damage the circuits, but actually causes them to fall out of specification.  This problem often manifests itself in industrial environments where strong RF fields are used (e.g. microwave heaters in processing plants).  It can also be caused by deliberate transmissions such as radio towers.  This phenomenon can be seen by placing a cell phone next to most speaker phones while a call is active.  A humming or clicking is often heard in the speaker phone due to interference with the transmitter in the cell phone.  This is caused by the RF energy impinging on parasitic (or intentional) diode structures within amplifiers and injecting currents into the circuit. 

The rectified signals can show up as offsets in the output of precision amplifiers as well.  This is the critical problem in industrial systems which make very precise measurements in processes that require extremely accurate temperatures or pressures. To combat this, semiconductor designers add additional circuitry to harden the device against these external fields.  An example of this is the LMP2021 or LMP2022 (dual) which is EMI hardened for precision measurements. For more detail on this topic check out application note SNOA497b – A Specification for EMI Hardened Operational Amplifiers.  This white paper discussed a new parameter called the EMIRR or Electro-Magnetic Interference Rejection Ratio which is used to quantify the susceptibility of op-amps to EMI.

I hope this post sheds some light on some of the unseen perils that can damage or derail your precision circuits. For more information on industrial components, see TI's industrial offering page.  I’ll cover some more ideas on helping build industrial strength designs in my next post… I welcome your comments or ideas as well. Till next time…

Note: You can now read Part III of Rick's Industrial Strength Design blog post HERE.