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SECTION 3.
CIRCUIT DESCRIPTION
3-1.
GENERAL.
a.
The Keithley Model 610C is basically an extreme-
ly stable and linear de voltmeter with a full-scale
sensitivity of 1 millivolt and an input impedance of
tol4
ohms shunted
by 20
picofarads.
By
using the
front panel controls, shunt resistors and capacitors
are selected to make measurements over a total of 79
voltage, current, resistance, and coulomb ranges.
Current and resistance are measured using precision
resistance standards, from 10 ohm wirewound resistors
to
toll
ohm glass-sealed, deposited carbon resistors.
Coulombs are measured using close tolerance polysty-
rene film capacitor standards.
b.
Well-regulated low and high voltage supplies
furnish necessary amplifier power.
3-2.
VOLTMETER OPERATION.
a.
The Model 610C employs matched insulated-gate
field-effect transistors followed by a transistor dif-
ferential amplifier with a high-voltage complementary-
output stage.
Figure 10 shows the block diagram for
the voltmeter mode of operation.
b.
Voltmeter operation of the Model 610C is as
follows:
UHF INPUT
1.
The amplifier is always in a unity-gain, input-
voltage to output current converter configuration.
The internal circuitry is arranged such that a full-
scale input voltage (ei) results in exactly a
1
mil-
liampere output
current~
through the divider string
composed of Rm, Rl78, and the meter.
Voltage gain
of the circuit is determined by the ratio of Rl78 to
Rm-
Output is taken across Rl78.
2.
The voltage drop across the amplifier is
ea
=
ei
k+l
where k is the amplifier loop gain, greater than 106
on all ranges.
c.
The complementary-output stage, Qlll and Qll2,
drives the amplifier ground at the same potential as
the input signal.
Thus the input impedance is main-
tained for any value input voltage and the need for
input dividers is eliminated.
The amplifier ground is
not chassis ground, but it is connected directly to
Jl05, the unity-gain output.
d.
Power supplies are floating with respect to chas-
sis ground.
The ±120 volt supplies are provided for
the complementary-output, Qlll and Qll2, while the
~egulated
±9 volt supplies provide power for the am-
plifier and are referred to the unity-gain output.
r-------------<
Xl OUTPUT
OUTPUT
MULTIPLIER
FIGURE 10.
Voltmeter Configuration, NORMAL Mode.
9
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