Introduction; Cvmeasurementmethod - Keithley 595 Instruction Manual

SECTI
THEORY OF
7.1 INTRODUCTION
This section contains an overall functional description of
the Model 595 in block diagram form as well as details of
the various sections of the instrument. Information con-
cerning the voltage source, preamplifier, X10 amplifier,
mulitiplewer, -2V reference, A/D converter, digital, analog
output and main power supply circuitry is included.
Information is arranged to provide a description of each
of the fonctional blocks within the instrument. Many of
these descriptions include simplified schematics and block
diagrqs. Detailed schematic diagrams and component
layout drawings for the various circuit boards are located
at the end of Section 9.
7.2 CV MEASUREMENT METHOD
The Model 595 uses the feedback charge method for mak-
ing CV mea sumnents. As shown in Figure 71, one~terminal
of the capacitance to be measured (C,) is connected to the
voltage source and the other end is connected to the input
of the feedback charge amplifier (integrator). Initially, the
feedback capacitor (C,) of the integrator is discharged by
closing switch S in parallel with it. When the measurement
is ready to begin, S is opened. Any charge transferred to
the input of the integrator will now cause a change in the
integrator output as follows:
AV,, = -nQ/C,
The v&age source is then changed by a small amount (dv)
causing a charge to to be transferred to Cx. The charge on
Cx is proportional to the voltage change (dQ 7 CxdV). The
charge is then applied to the integrator and causes a voltage
change at its output. The charge on the feedback capacitor
is determined by measuring the integrator output before
and after the voltage source step and making the follow-
ing calculation:
adQ = -C,AVw
3N 7
DPERATION
All = CXAV; AYou, = d&a
cF
I
Figure 7-I. Feedback Charge Method
The unknown capacitance (C,) is then calculated as follows:
Cx = aQ/aV = -C,aV.JaV.
Figure 72 describes the way in which the charge waveform
is measured. The symbols Qlz Q2 and Q3 represent three
measqrqnents of charge made at three specific times. Ql
is the baseline charge immediately preceding the voltage
step. Q3 is measured after a specified DELAY TIME (tDEUY)
and is an indication of the final charge transferred through
C. Q2 is measured before Q3 (preceding it by tJ and is
used to determines the slope of the charge waveform. This
slope indicates the amount of current (Q/t) flowing in C,
during the final portion of the DELAY TIME;t,. Q/t
represents the leakage in C or the system. Significant
leakage currents can beg compensated for with the cCAP
(corrected capacitance) feature. With cCAF enabled, the in-
strument performs the corrected capacitance calculation
shown in Figure 7-2.