DATA PRECISION 938 User Manual

42-5124
MODEL 938
If there are no charge-storing units in series with the capacitor,
the capacitor will develop a charge equal to the potential applied
to it, as shown in Figure 4-1. The illustration includes a resistive
component in series with the capacitor that acts to impede the
flow of current, or the rate of storing charge. However, as long
as there is a relatively long charging time, the charge on the
capacitor will develop a potential across the capacitor equal to
the applied voltage. This charging phenomenon is also il 
lustrated graphically in Figure 4-1. In an ideal capacitor (with no
resistive components) the charge will remain without change un 
til the capacitor is connected in a circuit that allows the flow of
current, and the reduction in charge. Figure 4-2 illustrates the
capacitor discharge when connected to a resistance component.
The measurement of capacitance (the ability to store charge
under the “ pressure ” of a voltage “ force ” ) is often accomplished
by observing the effects on the capacitor voltage when the
capacitor stored charge is connected to a precisely known charg 
ing or discharging circuit for a precise time interval. (The
measurement of the amount of stored charge is also possible,
but with more elaborate and expensive equipment.) As shown in
Figure 4-3, the reduction in charge is directly proportional to the
time of the current flow and to the amount of capacitance. Or,
to obtain the capacitance from this relationship, the capacitance
is the change in charge per change in voltage.
Figure 4-2. Charge and Discharge Cycles of a Capacitor
4-2
MODEL 938
PRINCIPLES OF OPERATION
4.1.2 Capacitance Measurement
The ratio definition as stated in the previous paragraph, is the
basis for the measurement as performed in the Model 938
Capacitance-Meter. That is, the instrument uses a dual-slope
analog-to-digital converter that digitizes the ratio of the change
in charge to the change in voltage. The charge reduction is ac 
complished by connection to a circuit that allows current to flow,
and this current is averaged over an integral number of
cycles as a measure the value of the change in charge. The
voltage difference between the values across the capacitor at
the start and at the end of the discharge cycle is the
denominator in the ratio meaurement. The A/D converter
develops the digitized value of this ratio for display in the liquid
crystal display.
AQ1
CURRENT THROUGH R IN *3
Figure 4-3. Relationship of Capacitance to the Change of Charge
and Change of Voltage.
4.2 SYSTEM BLOCK DIAGRAM
4.2.1 General
Figure 4-4 is a simplified block diagram of the instrumentation of
the Model 938 Capacitance-Meter. The illustration is prepared in
some detail with respect to those elements that instrument the
basic concepts of capacitance measurement described briefly in
the previous paragraphs. Other functioning elements are includ 
ed in block form. Their circuit operation will be described in
greater detail in later paragraphs of this chapter.
4-3

DATA PRECISION 938 Specifications

General

Brand	DATA PRECISION
Model	938
Category	Measuring Instruments
Language	English