Cypress CY8CKIT-025, Руководство пользователя

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CY8CKIT-025 PSoC® Precision Analog Temperature Sensor Expansion Board Kit Guide, Doc. # 001-65791 Rev. *J 26
Hardware
Figure 4-2. Thermistor Circuit Diagram
In Figure 4-2 ,
V1
is the voltage across the thermistor resistance,
R
t
and
V2
are the voltage across the
reference resistance
R
r
.
Equation 3 provides the thermistor resistance.
Equation 3
The voltage ratio ensures that any gain error and drift associated with the ADC are eliminated. The
accuracy of the thermistor resistance,
R
t
, is determined mainly by the accuracy of the reference
resistance,
R
r
. From
R
t
, the temperature can be calculated using the Steinhart-Hart equation.
PSoC Creator provides a thermistor component for PSoC Creator, which makes it easier to interface
a thermistor to PSoC 3 or PSoC 5LP. The component provides the flexibility of using either a lookup
table (LUT) or the Steinhart-Hart equation and provides simple C functions to compute temperature
from resistance.
4.4 RTD
Application note AN70698 - PSoC 3 and PSoC 5LP Temperature Measurement with RTDs explains
the RTD fundamentals and RTD temperature measurement. This section assumes that you are
aware of the RTD terminology.
In an RTD, the resistance variation with temperature is given by Callender Van Dusen equations
(equations 4 and 5).
Equation 4
Equation 5
where,
-
R
RTD
is the resistance at temperature
T
expressed in °C
-
R
0
is the resistance at 0 °C (equal to 100

for PT100 RTD, one of the commonly used RTDs)
-
A
,
B
, and
C
are Callender Van Dusen coefficients
By determining the resistance of the RTD, you can find the temperature using equations 4 or 5.
Figure 4-3 shows the circuit used to measure RTD resistance.
V
DC
ADC
R
t
R
r
10 K
0.1%
M
U
X
V2
V1
0
1
2
r t
R *
V2
V1
R

0 T , ) BT AT (1 R R
2
0 RTD
   
0 T , ) 100)T C(T BT AT (1 R R
2 2
0 RTD
     