CY8CKIT-025 PSoC® Precision Analog Temperature Sensor Expansion Board Kit Guide, Doc. # 001-65791 Rev. *J
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Example Projects
The main sources of IDAC error are its offset, gain error, and INL. The IDAC errors are completely
removed by passing the IDAC current through a precise reference resistor and measuring the volt-
age across it.
The reference resistor tolerance determines the error caused by it. A 0.1% reference resistance is
used in both RTD and thermistor temperature measurements.
The error component caused by the signal chain for each sensor is listed in
.
Cypress application notes on each of the four sensors explain the signal chain errors in detail.
5.2.2.4.2
Resolution (peak to peak)
The temperature measurement resolution is the minimum temperature increment possible. A 0.1 °C
resolution is achieved on all sensors. A software IIR filter (see
Sensor Output Filter on page 51
applied on some sensors to achieve 0.1 °C resolution. The application notes associated with each
sensor gives more details on how 0.1 °C resolution is achieved on each sensor. Generally, a small
flicker is seen on the LCD temperature display. A small flicker of 0.2 °C on thermocouple readings,
0 °C on IC and thermistor, 0.1 °C on RTD, and 0.3 °C on diode at a very small rate has been
observed in the example projects. It can be reduced by additional filtering at the cost of temperature
settling time. The IIR filter can be changed in firmware, as shown in
Sensor Output Filter on page 51
.
5.2.2.4.3
Test Results
This section shows the test results on the sample board. Wherever possible, measurement tech-
niques are used to identify the signal chain performance of PSoC.
Thermocouple
Thermocouple input can be simulated by a millivolt source. This eliminates any error due to thermo-
couple tolerance and allows you to test the signal chain in the whole temperature range without a
heat source. A millivolt source is fed to the thermocouple inputs and the cold junction temperature is
forced to 0 °C in the example project. The temperature shown by PSoC is noted and the voltage
input is measured using an accurate voltmeter; the temperature corresponding to that voltage is cal-
culated using NIST tables. The expected results and actual results are listed in
Table 5-2. Signal Chain Errors for PSoC 3
Sensor
Signal Chain Error Components
Typ/Max Error at 25 °C
Max Error at 100 °C
Thermocouple
ADC gain error, INL, cold junction IC
sensor error (± 0.5 °C)
0.1 / 2.1
2.2
Thermistor
ADC INL, reference resistor tolerance
0 / 0.05
0.6
RTD
ADC INL, reference resistor tolerance
0.1 / 0.5
0.9
Diode
ADC gain error, INL
0.3 / 2.6
3.8
Table 5-3. Thermocouple Test Results
Simulated thermo-emf
(mV)
Voltage read by
multimeter (mV)
Expected
temperature (°C)
Obtained
Temperature (°C)
Error (°C)
–5
–4.695
–141
–141.1
0.1
–4
–3.666
–103.7
–103.7
0
–3
–2.575
–69.6
–69.5
–0.1
–2
–1.741
–45.9
–45.7
–0.2
–1
–0.654
–16.77
–16.7
–0.1
0
0
0
0
0
