Page 2 284.CBSM-12/98-RCA
Section 2
GENERAL INFORMATION
The Monroe Electronics Model 284 Nanocoulomb Meter is a convenient battery
powered portable instrument for direct measurement of charge in the range
between 1 picocoulomb (limit of resolution) and 2 microcoulombs (maximum
displayable value). The standard instrument configuration provides two ranges—
±19.99nC and ±199.9nC—with one-decade extension in either direction available
as a factory option at time of order or as an aftermarket upgrade.
The NanoCoulomb Meter charge amplifier consists of an I.C. electrometer
amplifier with a high quality capacitor connected in the negative feedback
position. The input terminal (center pin of a BNC connector) feeds the inverting
input of the amplifier through a current limiting resistor. This inverting input is at
zero volts or virtual ground so all charge that is induced or directly deposited at
the input terminal is transferred to the integrating capacitor. Since Q=CV, the
output voltage of the charge amplifier is directly proportional to the charge
injected in the input, that is, for a 0.1µF integrating capacitor, a +100
nanocoulomb input equals −1.0 volt at the output. The input being at virtual
ground reduces leakages and polarization of insulators, thus minimizing these
effects on the readings.
The NanoCoulomb meter is zeroed by a low leakage momentary contact switch,
which discharges the integrator. A built-in reference quality voltage splitter
corrects for all offsets.
The electrometer is followed by a scaled-gain inverter, which is used to correct
the gain error caused by variations in capacitors and the inversion of the input
amplifier and to drive the 3½ digit Liquid Crystal Display (LCD) and the analog
output circuit.
The readout is thus always expressed directly in nanocoulombs with the decimal
point properly placed and the output is always 2 volts for full scale, regardless of
range.
In order to be fully functional, the Model 284 requires some sort of input source
device. Two types of input devices are readily available from Monroe Electronics
as accessories to the Model 284—Faraday cups and contact probes.
The Faraday cup consists of two concentric cans with expanded polyethylene as
an insulator between them. The outer can is at ground potential and the inner
can is the sensing electrode for the electrometer. These are available in two
standard sizes or may be custom designed to order.
The point contact probe is held in a manner similar to which one would hold a
pen or pencil and is used to make charge measurements by direct contact with
small objects or small areas of objects such as individual pins on IC’s.
Any input device used must have low leakage (>10
11
Ω) and provide an electrical
reference (usually earth ground).
(24 pages)
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