Keithley 6512, Instruction Manual, Page: 100 / 156

Theory of Operation
6-2
Figure 6-1
Overall block diagram
6.3 Input preamplifier
The input preamplifier provides the high input impedance and
high output voltage capability necessary for the volts and ohms
functions, and the low input impedance and high current output
capability needed for the amps and coulombs functions.
A simplified block diagram of the input preamplifier is
shown in Figure 6-2. The circuit is essentially made up of
three sections: an input stage, which provides the necessary
input impedance functions, a gain stage, which provides the
needed amplification, and an output stage, which supplies
the required voltage or current drive capability. Additional
feedback and switching elements configure the amplifier ac-
cording to the selected range and measuring function.
FEEDBACK
ELEMENTS,
SWITCHES
µ
P DISPLAY
FRONT
PANEL
BUTTONS
IEEE-488
INTERFACE
DIGITAL
INPUT
STAGE OUTPUT
STAGE
RANGING
AMPLIFIER
A/D
CONVERTER
IN
HI
IN
LO
GND
ANALOG
POWER SUPPLY
±
5
±
210
±
24 +5 +5
-9.1
ANALOG DIGITAL
Figure 6-2
Basic configuration of electrometer preamplifier
FEEDBACK
AND
SWITCHING
INPUT
STAGE
(Q308)
GAIN
STAGE
(U309)
OUTPUT
STAGE
(Q301,
Q302,-Q307)
OUTPUT TO
RANGING
AMPLIFIER
SIGNAL
INPUT