Keithley 6512, Instruction Manual, Page: 101 / 156

Theory of Operation
6-3
The exact configuration of the input preamplifier will depend
on the measuring function. Figure 6-3 shows circuit config-
uration for the four measuring functions. In the volts func-
tion, the circuit is set up as a high-input impedance (2
×
10
14
Ω
), unity gain, non-inverting buffer amplifier. In the
ohms function, a bootstrapped reference is placed in series
with a range resistor (R
F
) that drives a constant current
through the measured resistance (R
X
). The reference has a
value of 10V, 1V or 0.1V, depending on the selected range.
The voltage developed across the unknown resistance is pro-
portional to its value.
In the amps and coulombs modes, the circuit is configured as
a feedback type current-to-voltage converter. In the amps
mode, the feedback element is a resistor, with the value de-
pendent on the selected range. In the coulombs mode, the
feedback element is a capacitor.
6.3.1 Input stage
A simplified schematic of the input stage is shown in Figure
6-4. The primary purpose of this stage is to provide low leak-
age characteristics of the input preamplifier.
Stage operation centers around a dual JFET, Q308. Resistors
R314, R342, R351 and R352 provide a means to balance the
circuit with help of jumper W303. Depending on circuit off-
set, jumper W303 should be placed in one of three positions:
A, B, or C.
Signal input is applied to the gate of the left JFET section
through R334. The characteristics of the right JFET section
remain constant since its V
GS
voltage stays constant. Be-
cause of the variation in the characteristics of Q308A, the
current through R335 varies, developing a proportional out-
put signal that is applied to the next stage.
VOUT
VOUT = VIN
A. VOLTS
VOUT
C. AMPS
RF
VOUT = -IIN RF
CF = 1000pF
IIN QIN
D. COULOMBS
VOUT
VOUT =
-QIN
CF
IF
RX
RF
B. OHMS
VR = 10V, 1V
or 0.1V
VIN
VOUT
IF =
VR
RF
VOUT =
RX
RF
x VR
Figure 6-3
Electrometer preamplifier configuration