Page 128 - The Art and Science of Analog Circuit Design

James M. Bryant

It is often easy to deduce where currents flow in a ground plane, but in
complex systems it may be difficult. Breadboards are rarely that com-
plex, but if necessary it is possible to measure differential voltages of as
little as 5M-V on a ground plane. At DC and LF this is done by using an
instrumentation amplifier with a gain of 1,000 to drive an oscilloscope
working at 5 mV/cm. The sensitivity at the input terminals of the inamp
is S^tV/cm; there will be some noise present on the oscilloscope trace,
but it is quite possible to measure ground voltages of the order of l(iV
with such simple equipment. It is important to allow a path for the bias
current of the inamp, but its common-mode rejection is so good that this
bias path is not critical.
The upper frequency of most inamps is 25-50kHz (the AD830 is an
exception—it works up to 50 MHz at low gains, but not at xl,000).
Above LF a better technique is to use a broadband transmission line
transformer to remove common-mode signals. Such a transformer has
little or no voltage gain, so the signal is best displayed on a spectrum
analyzer, with jiV sensitivity, rather than on an oscilloscope, which only
has sensitivity of 5mV or so.

Decoupling

The final issue we must consider before discussing the actual techniques
of breadboarding is decoupling. The power supplies of HF circuits must
be short-circuited together and to ground at all frequencies above DC.
(DC short-circuits are undesirable for reasons which I shall not bother to
discuss.) At low frequencies the impedance of supply lines is (or should
be) low and so decoupling can be accomplished by relatively few elec-
trolytic capacitors, which will not generally need to be very close to the
parts of the circuit they are decoupling, and so may be shared among
several parts of a system. (The exception to this is where a component
draws a large LF current, when a local, dedicated, electrolytic capacitor
should be used.)
At HF we cannot ignore the impedance of supply leads (as we have
already seen in Figure 9-6) and ICs must be individually decoupled
with low inductance capacitors having short leads and PC tracks. Even
2-3mm of extra lead/track length may make the difference between the
success and failure of a circuit layout.

DECOUPLING
Figure 9-14.

Supplies must be short-circuited to each other
and to ground at all frequencies.
(But not at DC.)

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