AN929 Datasheet, PDF (15/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

resistance RTD and a higher performance comparator.
The trade-off, however, will be that the comparator’s
current consumption will be much higher.
Design Procedure:
Set R₁ = RTD sensor, R₂ = R₃ = R₄ = R and R ≅ 10 x
R_o, where: R_o = RTD resistance at 0°C.
1. Select a desired nominal oscillation frequency.
2. C₁ = 1/(1.386 R_o f_o)
3. Select a comparator with an Output Short
Circuit Current (I_SC), which is at least five times
greater than the maximum output current, to
ensure start-up at cold and a relatively good
accuracy.
I_{OUT_MAX} = V_DD/R_{1_MIN}
I_SC = I_{OUT_MAX} * 5
where: R_{1_MIN} = RTD resistance at coldest
sensing temperature and V_DD is equal to the
supply voltage.
V_DD
R₁ = RTD
C₁
V_IN-
V_IN+
R₂
V_DD
A₁
V_SS
R₃
R₄
V_OUT
If R₂ = R₃ = R₄
then
f_O
=
-----------------¹-------------------
( 1.386 ) ( R₁ C₁ )
Voltage
V_OH
V_OL
V_OH
V_THL
V_TLH
V_OL
V_IN-
V_IN+
t_charge t_discharge t_charge
time
FIGURE 21:
Relaxation RTD Oscillator.
AN929
Thermistor Circuits
VOLTAGE DIVIDER CIRCUIT
Thermistors offer the advantages of a high sensitivity
(∆R vs. temperature) and a linear change in resistance
between approximately 0°C and 70°C. Figure 22
shows the conventional circuit used with thermistors.
The circuit consists of a voltage divider and a voltage-
follower op amp with a gain of one. The voltage divider
network consists of reference voltage V_REF and series
resistor R_S. A low-pass, noise-reduction filter is formed
by R₂ and C₁. The equation listed below can be used
to select R_S.
R_s
=
R_----T---1---R_{----T---2----+-----}R_----T---2--R_{----T---3----–----}2_----R_----T---1--R_----T---3-
R_T1 + R_T3 – 2R_T2
Where:
R_T1 = thermistor resistance at the low
temperature.
R_T2 = thermistor resistance at the mid-point
temperature.
R_T3 = thermistor resistance at the high
temperature.
V_REF = 5V
R_S
4.53 kΩ
100 kΩ
R_T
10 kΩ
1 µF
MCP60X
V_OUT
V_OUT
=


R^----_S^---R-+---T--R^----_T^{--}
V_RE
F
FIGURE 22:
Voltage Divider Circuit.
A plot of the output of the divider circuit is shown in
Figure 23. While a microcontroller can use a software
routine to improve the linearization, a high-bit ADC is
required to resolve the small change in the output volt-
age at temperatures less than 0°C and greater than
70°C. Figure 24 shows the change in voltage or slope
of the output voltage. The ADC must be able to resolve
a voltage of approximately 50 mV at 35°C and a volt-
age of less than 20 mV at temperatures less than -5°C
and greater than 90°C. Table 2 provides the resolution
of an ADC, assuming that the ADC’s Effective Number
of Bits (ENOB) is equal to one bit less than the maxi-
mum available resolution. If this aggressive ENOB
assumption is made, an 8-bit ADC is required to mea-
sure temperatures between 10°C and 60°C, with an
11-bit ADC being required to measure temperatures at
the cold and hot end points.
 2004 Microchip Technology Inc.
DS00929A-page 15