Page 70 - Troubleshooting Analog Circuits

Kelvin Connections Improve Measurement Accuracy 57

when the device’s 6 ground pins cany the heat out of the package into the foil. But
some people point out that leaving too much foil on a PC board can cause warping
after wave-soldering.
Engineers often assume that a printed-circuit trace has virtually no resistance and
no IR drop. But, when you run large currents through a foil run, you will be unpleas-
antly surprised by the IR drop you’ll see. The classic example is a layout where the
signal ground for a preamplifier is mixed and shared with the ground-return path for
the power supply’s bridge and filter capacitor. This return path will see ampere-size
surges 120 times per second. Needless to say, that preamplifier won’t have ”low
noise” until the path for the current surges is essentially divorced from the preamp’s
ground. For precision work, your PC-board layout must include well-thought-out
ground paths for your sensitive circuitry.
When you think about it, separating power-supply grounds that carry lots of cur-
rent from voltage-sensing circuitry is similar to Kelvin connections, which are com-
monly used in test instruments. A Kelvin connection uses four wires: one pair of
leads is meant to carry current and the other pair senses voltage across a device.
Keeping the idea of Kelvin connections in mind when designing your PC board will
help you optimize your grounding scheme. In fact, when I draw up my schematic. I
label each set of grounds separately. If it has a lot of dirty currents flowing, I keep
that separate as Power Ground; if it has to be especially clean, that’s a High-Quality
ground, indicated with a triangular ground symbol. Then the grounds will be con-
nected together at only one point.
I don’t have very much to say about printed-circuit boards for surface-mount de-
vices, because I have not worked on them, and I am not an expert about them. I hear
that they are, of course, more challenging, and require more meticulous work in
every way. In other words, if you are an expert at engineering ordinary PC boards.
you can probably study really hard and do it well. If you are not an expert, well, it’s
not a good place to start. After you get your board built, one of the worst problems is
that of thermal cycling and stresses. Packages such as LCCs (Leadless Chip Carriers)
have problems because of their zero lead length-they have no mechanical compli-
ance. If the PC board material does not have the same thermal coefficient of expan-
sion as the IC package, the soldered joints can be fatigued by the stresses of temp
cycling, and may fail early. This is especially likely if you have to cycle it over a
wide (military) temperature range-xactly where people wish they had perfect
reliability. The commercial surface-mount ICs with gull-wing leads or J-leads are
more flexible and more forgiving, and may cause less problems, but you have to do
your homework. Too much solder and the leads get stiffened excessively; too little.
and there’s not enough to hold on.

Kelvin Connections Improve Measurement Accuracy
The usefulness of Kelvin contacts and connections is not widely appreciated. In fact
when Julie Schofield, an Associate Editor at EDN, asked me some questions about
them recently, I was surprised to find very little printed reference material on the
subject. I looked in a few dozen reference books and text books and didn’t find a
decent definition or explanation anywhere. I did find some “Kelvin clips,” which
facilitate Kelvin-connected measurements, in a Keithley catalog. I also found some
Textool socket data sheets, which mentioned, in a matter-of-fact way, the advantages
of Kelvin contacts. I’ll try to explain their usefulness in more detail here, since nei-
ther Julie nor I could find a good description in any technical encyclopedia.
Perhaps the most common use of Kelvin connections is the remote-sense tech-
nique. Kelvin connections and sockets let you bring a precise voltage right to the