Table of Contents
Advertisement
Theory of Operation
I N
HI
I N
LO
GND
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.
6-2
FEEDBACK
ELEMENTS,
SWITCHES
INPUT
STAGE
ANALOG
±5
±210
±24
+5
-9.1
POWER SUPPLY
OUTPUT
RANGING
STAGE
AMPLIFIER
µP
DIGITAL
+5
FRONT
PANEL
BUTTONS
SIGNAL
INPUT
Figure 6-2
Basic configuration of electrometer preamplifier
ANALOG
A/D
CONVERTER
DISPLAY
IEEE-488
INTERFACE
DIGITAL
FEEDBACK
AND
SWITCHING
INPUT
GAIN
STAGE
STAGE
(Q308)
(U309)
OUTPUT
OUTPUT TO
STAGE
RANGING
(Q301,
AMPLIFIER
Q302,-Q307)
Advertisement
Table of Contents
