4.3.2 Precision Current Sources and Input Switching
Please refer to Figure 6.2 (Current Sources, Input Connections,
Switching Schematic #2) for the following discussion.
An explanation of the constant current sources will be limited to
the Sensor A source of the silicon diode since the four current source
circuits are exactly the same with the exception of resistor values be-
tween the platinum and diode sources to achieve the two different values
of
current.
A precision reference voltage is generated by an internal temperature
stabilized precision voltage reference (U12). Resistors R11 and trimpot
R12 vary this voltage to match the voltage generated by feedback resistor
R13. Resistor R13 has been selected to generate 4.99V (by an adjustment
of the reference voltage to equal this feedback voltage). Op amp U15
drives an FET (U16) to generate a current flow through Pins A and B of the
sensor connector.
The voltages generated by the sensors are fed to front panel push
button switches S7, S8 and
S9.
Selector switch S7 selects the type of
sensor selected, platinum or silicon.
S10
then routes the appropriate
voltage and type information to switches S8 and
S9.
Switch S9 selects
which sensor,
A
or B,
will
be the display sensor. This switch takes the
appropriate sensor voltage and feeds
it
to the A/D converter as well as
informing the microprocessor of its position. Switch S8 selects which
switch is the control sensor. This switch takes the voltage and feeds
it
to an input buffer amplifier to be used as the control voltage. This
switch also identifies its position to the microprocessor.
Switch
S10
identifies whether or not the scale expand is in use.
Switch S6 selects the power setting and will be explained in detail
in the power amplifier section (Section 4.3.5).
Buffer Amplifier U19 gives a buffered output of the display sensor
which is available at the monitor plug. This amplifier can have an offset
voltage as high as 0.5 mV. This is adjusted to zero with trim potentio-
meter, R20.
4.3.3 A/D Converter and Microprocessor Hardware
Please refer to Figure 6.3 (DRC-84C Schematic #3) for the following
discussion.
The Analog-to-Digital converter consists of a precision
4½
digit
IC pair (8052A/7103A, U31 and U32) that produces a multiplexed BCD output
that is accurate to
±1
count over the entire
40,000
count range. The
7103A (U31) runs on a
50K
Hz clock cycle generated by U33. This clock
frequency allows for one reading every
.8
seconds (since one clock pulse
is required to make one count; 50,000 pulses per second would allow for
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