Keithley 610C, Instruction Manual

Page: 18 / 44

3-3. VOLTMETER CIRCUIT.
a. The amplifier input stage is a pair of insulated­
gate, field-effect transistors, QIOI and QI02, in a
differential configuration. The gate of QI02 is re­
turned to amplifier common, the unity-gain output.
b. Depressing the ZERO CHECK Button, 5103, places
the gate of the active insulated-gate devices at zero
potential.
c. The input stage is followed by a transistor dif­
ferential amplifier composed of QI03-QIIO. QI08 and
QI09 make up the output gain stage, which is utilized
in a gain-multiplier configuration. This stage pro­
vides the remainder of the high gain required by the
amplifier. Also, this stage prevents fold-over and
lock-up with positive input overloads. Diode DlOI,
between base and emitter of QIIO, prevents fold-over
and lock-up under negative input overloads.
d. Frequency compensation is provided by capacitors
Cl14, CllS, resistors R14S, R148, and capacitor Cl16.
The compensation networks provide a controlled fre­
quency characteristic to insure stability under all
conditions of capacitive loading on input and output
while on any range.
e. The recorder output is derived from the current
flow from Qlll and Ql12 through the divider, Rm, R178,
and the meter. With the Output Switch, 5108, on 3V,
±3 volts for full-scale deflection is obtained at out­
put connector, JI03, by %1.1 milliamperes flOWing
through the divider.
With 5108 at lMA, Rl76 and R177
are connected across Jl03, allowing %1 milliampere,
±5%, to pass through an external load.
3-4. AMMETER OPERATION.
a. Normal Method. In the normal method of current
measurements (FEEDBACK Switch in NORMAL position), one
of the Range Switch resistors, RI02 through Rl12,
shunts the input.
(See Figure 11). The MOdel 610C
then measures the voltage drop across the resistor.
The meter is calibrated to read the current in amperes
for the appropriate range.
b. Fast Method. In the fast method of current mea­
surements (FEEDBACK Switch in FAST position), the Mod­
el 610e functions as an ammeter with negative feedback.
The differential amplifier output is divided by the .
Multiplier Switch resistors, R16S to R17S, and fed
back to the amplifier input through a feedback resis­
tor selected with the Range Switch.
(See Figure 11).
Floating ground is connected to the low impedance side
of the input, and the output ground is floating. This
method increases the response speed by minimizing the
effects of input capacity; it also reduces the input
drop to less than 100 microvolts.
10
3-5. OIlMl!ETER OPERATION.
a. Normal Method. In the normal method of resis­
tance measurements (FEEDBACK Switch in NORMAL posi­
tion), the Model 6l0C uses a constant-current, voltage­
drop circuit. Refer to Figure 12. Rx is the unknown
resistor.
A voltage source, E, applies a potential
across Rx. The source is obtained from the +9 volt
supply through the resistor divider network, R136
through R13 8 . E varies from 0.1 to 1.3 volts. depend­
ing upon the OHMS range used. The voltage source is
connected between floating ground and the input gate
of QlOl through Rs. the range resistor.
Rs is one of
the resistors, RI02 through Rl12. I is equal to E/Rs.
regardless of the value of Rx. as long as the voltage
drop across Rx does not exceed the Multiplier Switch
setting. This circuit provides a true source regard­
less of the input. The Model 6lOC can then measure
the voltage drop across
Rx
and indicate the resistance
value on its calibrated meter.
b. Guarded Method. In the guarded method of resis­
tance measurements (FEEDBACK Switch in FAST position
and the sample resistance connected between the input
terminal, JIOl, and the OHMS Guard terminal, JI04) ,
feedback is applied through the sample. The circuit
is similar to the normal method, except for the feed­
back. This reduces the slowing effect of the instru­
ment's input capacity. Leakage error is also reduced
since the potential across �he input terminal is small.
In this mode, floating ground is connected to the low
impedance side of the input and the output ground is
floating. The guard terminal is at output ground po­
tential.
3-6. COULOMBMETER OPERATION. The Model 610C circuit
for measuring charge is similar to that used for an
ammeter with the fast method.
A negative feedback is
applied around a shunt capacitor, el08 to Clll, se­
lected with the Range Switch. The shunt capacitor re­
places Rs in Figure 11. The stored charge is propor­
tional to the voltage across the capacitor, which is
measured by the Model 6l0e voltmeter circuits.
3-7. POWER SUPPLY. The power supply operates from
the line voltage and furnishes Tl20 and T9 volts to
the differential amplifier and output stages.
a. A 24-volts rms output from the transformer,
TIOl, is full-wave rectified by diodes DllO through
0113. The resultant 30 volts is filtered by capaci­
tors Cll7 and CllS and regulated to i-9 volts by diodes
DI02 through DIOS.
b.
The ±l20 volts is obtained similarly through
filtering and regulating by capacitors Cl19 and C120,
and diodes DlOS and DI09.