Monroe-electronics Electrostatic Voltmeter - Isoprobe® - model 279L Instrukcja Użytkownika Pobierz pdf (Strona 11)

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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 AC 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 DC 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 DC magnitude 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 DC integrating amplifier. Its output polarity is inverted to that of the unknown. The

output of this amplifier is used to drive a high voltage amplifier, which in turn drives the probe to the same potential

as that of the surface under measurement.

The probe is driven to a DC voltage typically within 0.1% of the potential of the unknown for a 2-3mm probe-to-

surface spacing. By simply metering the output of the H.V. amplifier, one has an accurate indication of the unknown

potential.

Figure 5-1

B. Null balance: (ZERO 1)

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.

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Monroe-electronics Electrostatic Voltmeter - Isoprobe® - model 279L Instrukcja Użytkownika Strona 11