Theory Of Operation; Introduction; Overall Functional Description - Keithley 6512 Instruction Manual

6.1

Introduction

This section contains an overall functional description of the
Model 6512 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
interface, 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 6512 is shown in
Figure 6-1. The instrument may be divided into three discrete
sections: analog, digital, and power supplies. The analog and
digital sections are electrically isolated from one another by
using opto-isolators for control and communications. Sepa-
rate power supplies for the various analog and digital sec-
tions ensure proper isolation. Because of these isolation
techniques, the analog low connection may be floated up to
±500V above chassis ground, while digital common may be
floated up to ±30V above ground.
The analog section consists of the input stage, output stage,
ranging amplifier, A/D converter, and feedback and switch-

Theory of Operation

ing elements. The input stage is a proprietary FET amplifier
designed for high input impedance (200T Ω ) and low input
offset current (less than 5fA). The output stage provides fur-
ther amplification, thus allowing the preamp output to go as
high as ±210V, depending on the selected range and func-
tion. Further control of the input and output stages is provid-
ed by the feedback and switching elements, which set gain
and transfer function according to the selected range and
measuring function. In addition, zero check and zero correct
provide a convenient 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 digital 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
1/2 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.
Because of the diversity of circuitry within the Model 6512,
a number of power supply voltages are required. The analog
section requires ±5V (input stage) and ±210V and ±24V sup-
plies (output stage). Additional supplies include a separate
+5V and -9.1V supply for A/D circuits, and a separate +5V
supply for digital circuitry.
6
6-1