Looking for Common Ground

The term "ground" should be used more carefully. In our equipment, we distinguish between ground and DC common for good reason. Ground refers to chassis or safety ground to take errant AC voltage to ground instead of through an operator's body.

The mixing of the two can have unintended effects, as evidenced in a few equipment failure instances we encountered. In addition to the outright failures, signal integrity (noise) issues become significant, especially in the analog sensor world, when a path is provided between the messy AC world and the internal processing circuitry.

Before the advent of nonvolitile solid-state (Flash) memory, many systems used battery-backed SRAM to hold system data. Remember computer CMOS settings and batteries? Our system held calibration data in BB SRAM. The CPU (single-board computer) had its DC common rail tied to the mounting pads on the PCB. We mounted the board (solidly) to the equipment metal baseplate and chassis with metal standoffs. Guess where safety ground was tied to? After several equipment returns with "calibration lost" as the failure mode, we determined that induced bounce (AC spike?) in the ground plane was exceeding the 1.5V SRAM voltage and erasing the data. Nylon standoffs, anyone?

Recently, I discovered that more than one of the major industrial LCD manufactures tie the metal frame (chassis) of the LCD to DC common. When asked about this, they replied, "To reduce EMI." This may be fine to make the display pass emission standards, but it is just plain wrong from an electronic design point of view. Industrial equipment (the intended market) is often housed in metal enclosures tied to AC ground. This is not throwaway consumer equipment in a plastic (nongrounded) case. This one disturbed the nonvolitile Flash memory, causing the CPU configuration data to be corrupted. The operator was rather dismayed to encounter a "keyboard error" message on an embedded system that had no keyboard. The most amusing part of the message was the line that said to "Press F1 to continue."

The same ground path was demonstrated with dramatic results when the power switch manufacturer changed its design. The switch failure allowed the AC voltage in the switch to come in contact with the 12V DC conductor for the LED indicator light in the switch. The AC chose the path to ground through the 12V rail to the display, through the electronics to the DC common, and directly to AC chassis ground, because they were connected. Several thousands of dollars of electronics were reduced to paperweight status in the blink of an eye, along with the sealed switch literally exploding due to the internal pressure from the vaporized switch internals.

Lesson for the day: Ground and common are not the same, and never the twain shall meet, unless made by monkeys with EMI in their heads.

This entry was submitted by John Lombard and edited by Rob Spiegel.

John Lombard is the engineering manager for an AS9100 company that supports the composite repair industry (aerospace, wind energy, marine) with materials and equipment. He is responsible for product development and lifecycle maintenance, testing and regulatory compliance, and technical support. A former Navy electronics technician with a BSBM degree from the University of Phoenix, he is involved with all aspects of product realization and support.

Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.

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