5.2.2
Limit D/A
,
The Limit
D/A
converts a digital limit signal from the microprocessor to a voltage which becomes the limit voltage
for the Bipolar'Output Stage.
The
ITE
current limit
D/A
converter is made up of U215 and U201:B (see Figure 7.18). The microprocessor loads
the digitally stored current limit value into the 12-bit
DIA
(U215) which converts it to a voltage at the output of
U20
1
:B.
The current limit Galue is updated at power-up, at a "bin" recall, and whenever the
LIM
ITE
value is changed.
5.2.3
Set Point D/A
The Set Point
D/A
converts a digital set point signal from the microprocessor to a voltage which becomes the set
temperature input to the PI.control loop.
The Set Point
D/A
co~iverter is made up of U214 and U201:C (see Figure 7.18). The microprocessor loads the
digitally stored current set point value into the 12-bit
D/A
(LJ214) which converts it to a voltage at the output of
u20
1:C.
The
TEC
current set point value is updated at power-up, at a "bin" recall, and whenever a TEC set point value is
changed.
5.2.4
AID
Converter
The 16-bit
A/D
converter,
U205
(see Figure 7.18), measures the actual temperature voltage and the current output
of the bipolar output stage. The value of actual temperature measurement
is
used by the microprocessor in the
calculation of temperature or thermistor
resistance
for display on the front panel.
The ITE
measurement
is
used to
drive the
LED
bar graph and the
ITE
measurement display mode on the front panel.
5.2.5
Sensor Select
Sensor selection is accomplished 'in the Sensor Select block of the TEC board. Precision 100
pA
and
10
pA
current sources
(U22j
and
Q200)
may
be
selected for thermistor control (see Figure 7.20).
LM335
and
AD590
IC
temperature sensors may also be selected. Sensor selection is accomplished via the back panel SENSOR SELECT
switch,
S20
1.
The
AD590
has a
+8
VDC
bias voltage and the LM335
has
a
1
mA
bias current. These biases are
provided by
U226.
The output of the Sensor Select block of the
TEC
board is a voltage which is proportional to the actual
temperature. This voltage is fed to the
A/D
converter which provides a digital measurement to the microprocessor
and to the
PI
control loop to close the feedback loop when temperature is being controlled.
3.2.6
Difference
Amplifier
.
Differential amp
U233
(see Figure
7.16)
provides
a
proportional dflerence signal to the
PI
control. Thrs signal is
the difference between set temperature and actual temperature voltages.
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