Section 6-Theory Of Operation; Introduction; Overall Functional Description; Input Preamplifier - Keithley 617 Instruction Manual

SECTION 6
THEORY OF OPERATION
6.1 INTRODUCTION
This section contains an overall functional description of the
Model 617 in block diagram form as well as details of the
various sections of the instrument. Information concerning
the electrometer section, mother board circuitry, IEEE-488 in-
terface, power supplies, and display circuitry is included.
Information is arranged to provide a description of each of
the functional blocks within the instrument. Many of these
descriptions include simplified schematics and block dia-
grams. Detailed schematic diagrams and component layout
drawings for the various circuit boards are located at the end
of Section 8.
6.2 OVERALL FUNCTIONAL DESCRIPTION
A simplified block diagram of the Model 617 is shown in
Figure 6-1. The instrument may be divided into four discrete
sections: analog, digital, voltage source, and power supplies.
The analog, digital, and voltage source sections are electrical-
ly isolated from one another by using opto-isolators for
con-
trol and communications. Separate power supplies for the
various analog sections, digital section, and the voltage
source ensure proper isolation. Because of these isolation
techniques, the analog low connection may be floated up to
rt5OOV above chassis ground, while voltage source common
may be floated up to *lOOV ground, and digital common
may be floated up to +3OV above ground.
The analog section consists of the input stage, output stage,
ranging amplifier, A/D converter, feedback and switching
elements. The input stage is a propietary FET amplifier
designed for high input impedance (2COM) and low input off-
set current (less than 5fA). The output stage provides further
amplification,
thus
allowing the preamp output to go as high
as *2lOV, depending on the selected range and function.
Further control of the input and output stages are provided
by the feedback and switching elements, which set gain and
transfer function according to the selected range and func-
tion. In addition, zero check and zero correct provide a con-
venient means to zero the instrument, allowing cancellation
of internal offsets.
The ranging amplifier conditions the output stage signal into
a 0-2V signal for the A/D converter. The A/D converter uses
both charge balance and single-slope conversion techniques.
The heart of the dig&l section is the 146805E2 CMOS pro-
cessor that supervises the entire operation of the instrument.
Additional digital circuits include the display made up of a
4% digit mantissa and a 2-digit alpha or numeric exponent,
the IEEE-488 interface, and the front panel switch matrix. The
switch matrix decodes front panel switch closure information
that controls instrument operation from the front panel.
The voltage source is a fully programmable, isolated unit that
is also controlled by the microprocessor. An internal 12 bit
D/A converter transforms the digital control information in-
to analog voltages that set the voltage output to a maximum
of +102.4V, -102.35V with 50mV resolution.
Because of the diversity of circuitry within the Model 617, a
number of power supply voltages are required. The voltage
source requires both +15V and *llOV supplies, while the
analog section requires +5V (input stage) and i-210V and
+24V supplies (output stage). Additional supplies include a
separate +5V and -9.lV supply for A/D circuits, and a
separate +5V supply for digital circuitry.
In order to ensure
proper isolation, two separate power transformers are used:
one for the digital and voltage source power supplies, and the
other for analog power supplies.
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 cur-
rent 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 sec-
tions: an input stage, which provides the necessary input im-
pedance functions, a gain stage, which provides the needed
amplification, and an output stage, which supplies the re-
quired voltage or current drive capability. Additional feed-
back and switching elements configure the amplifier accord-
ing to the selected measuring function.
6-1