December 4, 1997

SIGNAL INTEGRITY: Probing for noise

Howard Johnson, PhD

Many digital engineers worry that weird currents circulate in the ground system waiting for a chance to leap into the measurement equipment, from which they can obfuscate your sensitive measurements.

Mike McKinley and Jeff Shuey of Intel recently asked me about noise problems with their high-speed measurements on SDRAM modules. They were using a 5G-sample/sec digital scope with an FET probe. The FET probe had a 0.75-in. ground wire connected to digital ground. With the probe tip touching the V_CC supply on the SDRAM module, the scope showed what appeared to be inductive spikes of ±300 mV.

By itself, the situation sounds rather usual. All power buses have noise, but what really grabbed McKinley's attention was that when he probed digital ground, he saw the same noise.

What causes this effect? How can a probe pick up noise when looking at its own ground? Does this have anything to do with weird ground currents circulating through the probe? If so, how can you ever tell if the noise is present on ground, V_CC, or both? The key to understanding this situation is Faraday's law. Any conductive loop you place near a high-speed circuit picks up noise voltages. The changing magnetic fields emanating from the board cause these noise voltages. The loop need not even touch the board to pick them up. This effect is mutual inductive coupling, or magnetic pickup. Faraday's law prescribes the exact amount of coupling you see in any situation.

The region your probe's ground wire encircles forms a Faraday pickup loop. The ground-wire loop picks up magnetic noise just as any other conductive loop does. The bigger the area of the ground- wire loop, the more noise you see. Many digital engineers worry that weird currents circulate in the ground system, waiting for a chance to leap into the measurement equipment, from which vantage point they can obfuscate your sensitive measurements. That scenario rarely happens. Faraday's law has a simpler effect and has nothing to do with your probe's ground wire actually touching digital ground.

A simple test lets you determine whether noise has anything to do with weird ground currents or just plain old magnetic pickup:

1. Disconnect the probe (both tip and ground connections) from the board.
2. Touch the probe tip to its own ground wire and nothing else.
3. Move the self-grounded probe configuration near the board, but don't let either the probe tip or its own ground come into electrical contact with the board.

The ground wire on the probe now acts as a magnetic-field-pickup loop. It senses any local magnetic fields on the board. Because it does not electrically connect to the board, it picks up only the magnetically coupled noise. If you do this experiment waving the self-grounded probe near a high-speed design, you will probably see approximately 200 mV of noise. This coupling is magnetic in nature and not the result of any mysterious ground currents.

To reduce magnetic noise, try using a probe with a shorter ground wire. With a smaller pickup loop, less noise couples into the scope. When probing a V_CC node that is, presumably, a very low-impedance source, you can use a straight 50 ohm probe. Forget the fancy FET-input stuff. Directly soldering a 3-ft-long RG-174 coaxial cable to V_CC and ground in the machine under test presents a smaller exposed-pickup-loop area than any FET probe. When you do this, use the built-in 50 ohm termination with ac-coupling mode in the scope.

The primary advantages of the RG-174 coaxial probe are size (only 0.100-in. diameter), mechanical flexibility, and high bandwidth. Furthermore, because V_CC is a low-impedance source, the 50 ohm input impedance of the coax, usually a big disadvantage, doesn't matter.

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