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Chapter 4
4.6.3
Set Point DAC
The microprocessor loads the digitally stored current set point value into the set point
16-bit DAC. The Set Point DAC converts a digital set point signal from the
microprocessor to a voltage which becomes the set temperature input to the PI
control loop. The TEC current set point value is updated at power-up, at a "bin"
recall, and whenever a TEC set point value is changed.
4.6.4
A/D Converter
The 16-bit A/D converter measures the sensor voltage and the current of the bipolar
output stage. The sensor measurement is used by the microprocessor in the
calculation of temperature or thermistor resistance. The current measurement is used
for the ITE value.
4.6.5
Sensor Select
Sensor selection is accomplished in the Sensor Select block of the TEC board.
Precision 100µA and 10µA current sources may be selected for thermistor control.
RTD, LM335 and AD590 IC temperature sensors may also be selected. The AD590
has a +5 VDC bias voltage, the LM335 has a 1 mA bias current, and the RTD has a
precision 1 mA current source.
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.
4.6.6
Difference Amplifier
Differential amp provides a proportional difference signal to the PI control. This
signal is the difference between set temperature and actual temperature voltage.
4.6.7
Proportional Amplifier and Integrator
The proportional amplifier is part of a digitally controlled gain stage consisting of the
analog switches and their associated resistors. The analog switches vary the ratio of
resistance in the feedback circuit to change the gain.
The signal from the difference amplifier is sent to an integrator which reduces the
difference between the set point temperature and the actual temperature to zero,
regardless of the gain setting. An analog switch discharges the integrating capacitor
whenever integration is not required to prevent unnecessary difference signal
integration.
4.6.8
Bipolar Output Stage
The Bipolar Output Stage consists of circuits which limit the TEC output, sense the
TEC output polarity, sense voltage and current limit conditions, as well as supply the
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Temperature Controller Module Operation
65
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