Keithley 194 Instruction Manual page 152

SECTION 6
PRINCIPLES OF OPERATION
6.1 INTRODUCTION
This section contains an overall functional description of
the Model 194 in block diagram form, as well as details
of operating principles for various circuits within the in-
strument. Many descriptions include simplified block
diagrams. Detailed schematic diagrams and component
layout drawings for the various circuit boards arc located
in Section 8.
Information in this section is arranged as follows:
6.2
6.3
I
6.4
6.6
6.6
6.7
6.6
6.9
Functional Description:
Presents Model '194 cir-
cuitry in block diagram form and gives an overview
of circuit operation.
Analog Circuitry:
Describes operation of the
analog input circuitry for the instrument
including
input attenuator and signal amplification circuits.
A/D Converter:
Describes the basic operation of
the R/l6 bit A/D converter and associated control
circuitry.
Digital Control Circuitry:
Outlines operation of the
digital circuitry used for trigger control, reading
counter, as well as the 64K dual-port memory.
Microcomputer:
Presents a detailed description of
the 6800%based microcomputer, including ROM
memory, address decoding, front panel interface,
and IEEE-488 bus circuits.
I/O Circuitry:
Details operation of circuits used to
drive the front panel display, read keyboard data,
and control the X, Y, and Z analog outputs.
Power Supplies:
Describes the operation of the
various power supplies used to power instrument
circuitry.
Display Board:
Details operation of the front panel
display and keyboard circuits.
6.2 FUNCTIONAL DESCRIPTION
A simplified block diagram of the Model 194 is shown in
Figure 6-l. Fundamentally, the circuit consists of analog
and AID circuitry, the 68008 MI'U and its support circuits
and power supply circuitry. Channel 2 circuitry for thi,se
instruments outfitted with the optional Modci 1944 A D
module is identical to that shown for channel ~1.
Input signals are applied to the input amplifit,r thr<>ugh
the rear panel VOLTAGE INI'UT jack. The input amplifier
attenuates or amplifies the input voltage, as required,
essentially conditioning input signals into values optimized
for use by the A/D converter. Sclcctablc WkHz and SO(lktl/
analog filters are also part of the input ,unplificr circuitry~
Once the signal is conditioned by the input ampliiic~r, it
is applied to the sample and hold circuitry, \vhich is a--
tually contained within the A/t) module. 'The purpose 111
the sample and hold is to freeze the signal voltage 'lt some
steady-state value during the A/D conversion process.
A/D conversion is performed by the WI6 bit A/D convertct
section of the module. For sampling rates oi 100kHz and
below, the converter operates in the 16.bit mode, resulting
in a usable display resolution of 4% digits. Above ~100kl 17.
the converter operates in the R-bit mode, reducing usable
display resolution to 2% digits, although the 4';1 digit for-
mat is maintained.
Once the signal has been digitized, the resulting datd is
fed over the isolators in serial form to the 64K dual-port
memory, the trigger control circuitry, and the real time out-
put. This isolated serial transmission scheme is used to
maintain electrical isolation between the analog and digital
sections of the instrument.
Because data is dumped directly to the 64K byte memow
data can be sampled at rates as high as 1MHz. Data is
sampled in real time by the trigger control circuitry for use
in determining when to triser the AID converter if trig-
gering from the input signal. Data is also fed to the real
time output of the instrument, which can be used to
transmit data in real time to an external device such as a
computer.
6-1