Keithley 194 Instruction Manual page 155

PRINCIPLES OF OPERATION
Table 6-1. Attenuator and Gain Values
Xl/Xl0
Range Attenuator Gain Amp
Overall Gain
320 mv Xl
x10 X15.26
3.2 V
Xl
Xl
X 1.526
32 V x0.1
Xl X 0.1526
200 v
x0.01
Xl X 0.01526
6.4 AID CONVERTER
Once the signal is processed and conditioned by the in-
put amplifier, it is applied to the A/D converter circuitry.
The purpose of this circuitry is to convert the analog in-
put voltage into a digital or binary value that can be stored
in memory and used by the microprocessor for data
processing.
The following paragraphs discuss the operation of the A/D
converter in detail. First, a simplified block diagram of the
circuit is presented along with an introductory discussion.
Next, a more detailed analysis of the various A/D circuits
is given. When reading through the discussion of the A/D
converter, it will be helpful to refer to schematic drawing
number 194.126, page 1 located at the end of Section 8.
6.4.1 Converter Block Diagram
A simplified block diagram of the A/D converter is shown
in Figure 6-3. The circuit can be broken down into five ma-
jor sections: the A/D converter itself, the data parallel-to-
serial data conversion circuits, the opto-isolators, the con-
trol serial-to-parallel converters, and the data control
circuits.
The analog input signal is applied to the sample and hold
input of the A/D converter module. The output of the con-
verter is in either 8-bit or lh-bit binary form, depending
on the selected converter mode. The parallel data output
is then converted to serial for transmission across the
isolators to the digital control circuitry discussed later in
this section. Transmission is actually done in dual-serial
form with odd/even bit pairs transmitted simultaneously.
The data control circuit generates a 5MHz clock and other
control signals to synchronize data transmission.
A single trigger line is used to trigger the A/D converter
to make a conversion. Trigger pulses arc generated by the
trigger control circuitry discussed later in this section.
Serial control data is used by the A/D and input amplifier
control section. This data is shifted in with the help of the
clock and strobe lines. A total of five a-bit serial-to-parallel
converters are used to control 8116 bit AID converter
modes, set the offset and gain parameters for the A/D con-
verter, and to control the input amplifier through relays
and FETs.
6-4