CY8CKIT-025 PSoC® Precision Analog Temperature Sensor Expansion Board Kit Guide, Doc. # 001-65791 Rev. *J
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Example Projects
5.2.2.4.4
Simultaneous Temperature Display of all Six Sensors
The sensor-to-sensor variation in temperature is 0.5 °C to 1.1 °C without calibration. The signal
chain can add additional error. Remember the following thermal considerations to understand the
temperature difference between the sensors in the simultaneous display of six temperatures.
■
The thermocouple measures the air temperature; the RTD and thermistor measure the board
temperature; the IC measures the temperature of the exposed pad below it; and diodes are
screwed on to the board although they hang in the air.
■
Because the thermocouple measures the air temperature, it is about a degree Celsius lower than
the board at room temperature. Place the tip of the thermocouple on the board and you will see
the thermocouple temperature rise by a degree.
■
Place the board in the sun; you will note that the temperature display shown by the onboard sen-
sors slowly rise. In tropical regions, you can see the board temperature rise to greater than 50 °C,
while the thermocouple temperature will be around 35 °C to 37 °C.
■
The IC sensor has a thermal pad underneath it. Ideally, it is expected to be isothermal with the
cold junction. Ensuring an isothermal connection requires providing good thermal contact
between the thermocouple cold junction and the exposed IC pad. If the thermal contact is not pro-
vided, having the cold junction IC very close to the cold junction terminals is sufficient. The tem-
perature difference between the cold junction terminals and the IC pad will not be more than
0.5 °C. Most thermocouple applications do not demand high accuracy and it is sufficient to have
the cold junction IC very close to the cold junction. The IC temperature is about 0.3 °C to 0.5 °C
lower than the RTD and thermistor temperature because the exposed pad underneath the IC is
connected to ground. This is an offset error. This offset error can be eliminated by performing an
offset correction, as shown in
.
Apart from the above thermal considerations, the sensor and bias resistor tolerances should deter-
mine the sensor temperature. Typically, RTD and thermistor temperatures are within about 0.3 °C of
each other; IC temperature and diode temperature is within about 0.5°C lower than the thermistor
temperature; and thermocouple temperature is about 1 °C to 2 °C lower than the thermistor temper-
ature.
5.2.2.4.5
CY8CKIT-001 PSoC DVK Thermistor Temperature
The CY8CKIT-025 connects to the Port A connector on the CY8CKIT-001. Through this connection,
the resistor/thermistor network connects to pin 5_0. On PSoC port pins 0_0, 0_1, 3_6, and 3_7 con-
nect directly to the opamp output. Thus, there is no direct connection from an opamp to the thermis-
tor/resistor network, which is connected to pin 5_0. Therefore, an unbuffered VDAC is used to
supply the required voltage for the thermistor/resistor network. Due to internal routing this connec-
tion has some parasitic resistance, which causes a voltage drop. The ADC measures this voltage
drop and compensates. However, thermistor measurement on CY8CKIT-001 may result in higher
error up to 0.5 °C. For details on internal analog routing, see
Internal Analog Routing Considerations
5.2.3
Project Details
5.2.3.1
Project Schematic
shows the PSoC Creator schematic of the project. An eight-channel ADC is used to
sense the six sensors. The IDAC has four channels to provide excitation to the RTD and the diodes.
Whenever IDAC channel 2 is connected, the IDAC output flows through the diode connected transis-
tor Q1, Pin 3_7 and to ground. The ground connection is provided on the pin in firmware. PSoC's pin
structure allows easy connection to the ground.
