Monsoon season just ended here in southern Arizona. During this time of year, brief but intense storms roll through with lots of lightning — one of nature's most spectacular sources of electromagnetic interference (EMI).
A recent storm got me thinking about the EMI challenges we face with precision systems. I wondered if I could actually use EMI to my advantage — to detect lightning during a thunderstorm.
Making an antenna
I needed an antenna to convert the radiated EMI from the lightning bolt into a signal conducted to the input of my amplifier. I made a loop antenna on a PCB (4.65 inches square, eight turns) with an inductance of 20μH tuned to 10.27MHz using a 12pF capacitor.
A simulation from TINA-TI, TI's SPICE-based analog simulation program, shows the output of my antenna circuit. Here it's excited by a pulsed current source (IG1), representing the current induced in the loop from the magnetic field of a lightning bolt.
current pulse (green, top), representing a lightning bolt.
The antenna circuit produces a sinusoid at 10.27MHz. This will serve as an input EMI signal to an amplifier.
Designing the amplifier circuit
The EMI rejection ratio (EMIRR) measures how well an amplifier resists converting EMI into a DC offset. Designers normally select amplifiers with a high EMIRR to avoid EMI problems. In this case, I wanted the exact opposite. The OPA347 does not have an input EMI filter, and it exhibits low EMI rejection at 10MHz (around 13.5dB). With these stats, I knew it would make a great lightning detector.
The SBOA128 application report provides equations to predict the offset resulting from an input EMI signal. For example, we can calculate the change in the OPA347's input-referred offset for a 10mV-pk, 10MHz input signal.
Now, 211μV may not seem significant, but if the amplifier is configured for high gain, EMI might drastically affect the output voltage.
I configured the OPA347 for a gain of 100 using the topology shown in Figure 2. (The design equations for this topology can be found here.) Coupling capacitor C2 forms a high pass filter with the DC biasing resistors R1 and R2 to eliminate mains interference.
Now for the fun
After receiving a prototype PCB for this circuit (Figure 3), I set up the detector in my backyard and monitored the amplifier output as a thunderstorm approached.
and a one-shot circuit for camera shutter release.
Figure 4 shows the amplifier's output during intense storm activity. The amplifier's output dips as each lightning bolt strikes. The magnitude of this change is proportional to the EMI intensity; lightning discharges that are more intense and closer to the detector produce the biggest dips. A particularly strong bolt caused the output to shift by 728mV (at 0.189s) and was followed by smaller discharges.
in amplifier output indicate lightning discharges.
In the future, I'll use the board to trigger my camera for lightning photos. EMI rectification can be a major headache for engineers, but sometimes we can use it to our benefit.
Need EMIRR information?
In recent years, we started including an EMIRR curve in the datasheets for our new amplifiers. We're also providing reports for many amplifiers developed before this change. When available, these reports appear on the product page below the datasheet and have “EMI Immunity Performance” in the title.
If the amplifier you're considering for your design has not been characterized, visit our Precision Amplifiers forum, and ask us to characterize it for you. Here's a curve I took for a customer who asked for help.
For weekly tips, tricks, and techniques from TI precision analog experts, visit our companion blog, the TI Precision Designs Hub.
About the author:
John Caldwell is an applications engineer in the Precision Linear group at Texas Instruments, where he supports operational amplifiers and industrial linear devices. He specializes in precision circuit design for sensors, low-noise design and measurement, and electromagnetic interference issues. He received his MSEE and BSEE from Virginia Tech with a research focus on biomedical electronics and instrumentation.