Keithley 610C, Instruction Manual

Page: 17 / 44

SECTION 3. CIRCUIT DESCRIPTION
3-1.
GENERAL.
s.
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
1014 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 lOll 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 6l0C 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 fu11-
scale input voltage (ei) results in exactly a 1 mil­
liampere output current� through the divider string
composed of Rm. RI7S. and the meter.
Voltage gain
of the circuit is determined by the ratio of Rl7S to
Rm.
Output is taken across RI7S.
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, QlIl and Ql12,
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, QlIl and Q112, while the
�egulated ±9 volt supplies provide power for the am­
plifier and are referred to the unity-gain output.
Xl OUTPUT
OUTPUT
MULTIPLIER
FIGURE 10. Voltmeter Configuration, NORMAL Mode.
9