Page 195 - The Art and Science of Analog Circuit Design

Jim Williams

10pF-22pR The recommended voltmeters, which will be discussed, have
significantly different input characteristics. Figure ll-A2's table shows
higher input resistances and a range of capacitances. Because of this the
probe must be compensated for the voltmeter's input characteristics.
Normally, the optimum compensation point is easily determined and
adjusted by observing probe output on an oscilloscope. A known-
amplitude square wave is fed in (usually from the oscilloscope calibrator)
and the probe adjusted for correct response. Using the probe with the
voltmeter presents an unknown impedance mismatch and raises the prob-
lem of determining when compensation is correct.
The impedance mismatch occurs at low and high frequency. The low
frequency term is corrected by placing an appropriate value resistor in
shunt with the probe's output. For a 10MO voltmeter input, a 1.1MO
resistor is suitable. This resistor should be built into the smallest possible
BNC equipped enclosure to maintain a coaxial environment. No cable
connections should be employed; the enclosure should be placed directly
between the probe output and the voltmeter input to minimize stray ca-
pacitance. This arrangement compensates the low frequency impedance
mismatch. Figure 11-A4 shows the impedance-matching box attached to
the high voltage probe.
Correcting the high frequency mismatch term is more involved. The
wide range of voltmeter input capacitances combined with the added
shunt resistor's effects presents problems. How is the experimenter to
know where to set the high frequency probe compensation adjustment?
One solution is to feed a known value RMS signal to the probe-voltmeter
combination and adjust compensation for a proper reading. Figure 11-A3
shows a way to generate a known RMS voltage. This scheme is simply a
standard backlight circuit reconfigured for a constant voltage output. The
op amp permits low RC loading of the 5.6K feedback termination without
introducing bias current error. The 5.6kn value may be series or parallel
trimmed for a 300V output. Stray parasitic capacitance in the feedback
network affects output voltage. Because of this, all feedback associated
nodes and components should be rigidly fixed and the entire circuit built
into a small metal box. This prevents any significant change in the para-
sitic terms. The result is a known SODY,^ output.
Now, the probe's compensation is adjusted for a 300V voltmeter indi-
cation, using the shortest possible connection (e.g., BNC-to-probe
adapter) to the calibrator box. This procedure, combined with the added
resistor, completes the probe-to-voltmeter impedance match. If the probe
compensation is altered (e.g., for proper response on an oscilloscope) the
3
voltmeter's reading will be erroneous. It is good practice to verify the

The translation of this statement is to hide the probe when you are not using it. If anyone wants
to borrow it, look straight at them, shrug your shoulders, and say you don't know where it is.
This is decidedly dishonest, but eminently practical. Those finding this morally questionable may
wish to reexamine their attitude after producing a day's worth of worthless data with a probe that
was unknowingly readjusted.

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