Monroe Electronics Electrostatic Voltmeter - Isoprobe® - model 244A - Manual (Page 15)

14
SECTION 5
THEORY
A. GENERAL: Principle of Operation (See Figure 5-1)
The electrostatic electrode "looks" at the surface under measurement through a
small hole at the base of the probe assembly. The chopped A.C. signal induced
on this electrode is proportional to the differential voltage between the surface
under measurement and the probe assembly. Its phase is dictated by the D.C.
polarity.
The reference voltage and this mechanically modulated signal, conditioned by
the high input impedance preamplifier and signal amplifier are fed to a phase
sensitive detector whose output D.C. amplitude and polarity are dictated by the
amplitude and phase of the electrostatically induced signal relative to the
reference signal. The output of the phase sensitive detector feeds a D.C.
integrating amplifier. Its output polarity is inverted to that of the unknown. The
output of this amplifier is used to drive a H.V. amplifier (AV = -300) which in turn
drives the probe to the same potential as that of the surface under
measurement.
The probe is driven to a D.C. voltage typically within 0.1 % of the potential of the
unknown for a .040" probe-to-surface spacing. By simply metering the output of
the H.V. amplifier, one has an accurate indication of the unknown potential.
B. NULL BALANCE:
This instrument's basic operating principle, i.e., "field-nulling" provides a high
degree of immunity to errors in measurement caused by variations in probe-to-
surface separation,. (As long as the probe and the surface under measurement
are at the same potential the electrical field between them is zero, neglecting
fringing, regardless of the probe-to-surface distance.)
If, however there exist voltage sources in the vicinity of the sensitive electrode
which are independent of the unknown to be measured, the offsets produced will
detrimentally affect the spacing independence. Such voltage sources include
contact potential differences among the internal probes parts, small specks of
charged dust particles, etc.
The null balance circuitry is provided to neutralize such offsets by applying a
voltage directly to the sensitive electrode.
C. ZERO:
The instrument's zero control is a voltage source connected in the metering
circuitry. It is used to overcome offsets produced by voltage sources external to
the probe.
D. GAIN:
The instrument functions as a closed loop unity gain voltage follower. Its open
loop gain from probe to integrator is determined by the gain of various amplifiers.
The exact gain required for optimum transient responses during a specific
measurement is determined by the probe-to-surface spacing used. In order to
accommodate measurements at various probe-to-surface spacings, a front panel
gain control is provided which controls the gain of the signal amplifier.