-11-
THEORY OF OPERATION
Figure 1
Input
Selector
Switches
AC
Converter
Ohms
Converter
Current
Shunt
V
V
Ω
I
Voltage
Divider
Selector
Switches
V
A/D
Converter
& Display
Driver
Display
Decimal
Point
DC
Analog
Data
For resistance measurements, an internal voltage
source drives the test resistor in series with a known
resistor. The ratio of the test resistor voltage to the
known resistor voltage is used to determine the value
of the test resistor.
The input of the 7106 IC is fed to an A/D converter.
Here the DC voltage is changed to a digital format.
The resulting signals are processed in the decoders
to light the appropriate LCD segments.
Timing for the overall operation of the A/D converter
is derived from an external oscillator whose
frequency is selected to be 25kHz. In the IC, this
frequency is divided by four before it clocks the
decade counters. It is then further divided to form the
three convert-cycles phases. The final readout is
clocked at about two readings per second.
The digitized measurements are presented to the
display as four decoded digits (seven segments) plus
polarity. The decimal point position on the display is
determined by the selector switch setting.
A/D CONVERTER
A simplified circuit diagram of the analog portion of
the A/D converter is shown in Figure 3. Each of the
switches shown represent analog gates which are
operated by the digital section of the A/D converter.
The basic timing for switch operation is keyed by the
external oscillator. The conversion process is
continuously repeated. A complete cycle is shown in
Figure 3.
Any given measurement cycle performed by the A/D
converter can be divided into three consecutive time
periods, autozero (AZ), integrate (INTEG) and read.
A counter determines the length of the time periods.
The integrate period is fixed at 1,000 clock pulses.
The read period is a variable time that is proportional
to the unknown input voltage. It can vary from zero
counts for zero input voltage to 2,000 counts for a full
scale input voltage. The autozero period varies from
1,000 to 3,000 counts. For an input voltage less than
full scale autozero gets the unused portion of the
read period. The value of the voltage is determined
by counting the number of clock pulses that occur
during the read period.
During autozero a ground reference is applied as an
input to the A/D converter. Under ideal conditions,
the output of the comparator would also go to zero.
However, input-offset-voltage errors accumulate in
the amplifier loop and appear at the comparator
output as an error voltage. This error is impressed
across the AZ capacitor where it is stored for the
remainder of the measurement cycle. The stored
level is used to provide offset voltage correction
during the integrate and read periods.
A block diagram of the M-1008K is shown in Figure 1.
Operation centers around a custom LSI chip. This
chip contains a dual slope A/D (analog to digital)
converter, display latches, seven segment decoder
and display drivers. A block diagram of the IC
functions is shown in Figure 1. The input voltage or
current signals are conditioned by the selector
switches to produce an output DC voltage with a
magnitude between 0 and 199mV. If the input signal
is 100VDC, it is reduced to 100mVDC by selecting a
1000:1 divider. Should the input be 100VAC, it is first
rectified and then divided down to 100mVDC. If
current is to be read, it is converted to a DC voltage
by internal shunt resistors.
