Datasheet
MCP6V26/7/8
DS25007B-page 28 © 2011 Microchip Technology Inc.
4.4.2 RTD SENSOR
The ratiometric circuit in Figure 4-14 conditions a three
wire RTD. It corrects for the sensor’s wiring resistance
by subtracting the voltage across the middle R
W
. The
top R
1
does not change the output voltage; it balances
the op amp inputs. Failure (open) of the RTD is
detected by an out-of-range voltage.
FIGURE 4-14: RTD Sensor.
The voltages at the input of the ADC can be calculated
with the following:
4.4.3 THERMOCOUPLE SENSOR
Figure 4-15 shows a simplified diagram of an amplifier
and temperature sensor used in a thermocouple
application. The type K thermocouple senses the
temperature at the hot junction (T
HJ
), and produces a
voltage at V
1
proportional to T
HJ
(in °C). The amplifier’s
gain is set so that V
4
/T
HJ
is 10 mV/°C. V
3
represents
the output of a temperature sensor, which produces a
voltage proportional to the temperature (in °C) at the
cold junction (T
CJ
), and with a 0.50V offset. V
2
is set so
that V
4
is 0.50V when T
HJ
–T
CJ
is 0°C.
EQUATION 4-3:
FIGURE 4-15: Thermocouple Sensor;
Simplified Circuit.
Figure 4-16 shows a more complete implementation of
this circuit. The dashed red arrow indicates a thermally
conductive connection between the thermocouple and
the MCP9700A; it needs to be very short and have low
thermal resistance.
FIGURE 4-16: Thermocouple Sensor.
R
3
100 nF
10 nF
R
2
R
3
100 nF
ADC
V
DD
2.49 kΩ
2.49 kΩ
10 nF
V
DD
R
W
R
W
R
W
R
T
R
B
R
RTD
R
1
R
1
1µF
100Ω
3kΩ
3kΩ
20 kΩ
20 kΩ
100 kΩ
100 kΩ
2.49 kΩ
2.49 kΩ
R
2
2.55 kΩ
2.55 kΩ
U
1A
½ MCP6V27
U
1B
½ MCP6V27
V
DM
G
RTD
V
T
V
B
–()G
W
V
W
+=
V
CM
V
T
V
B
G
RTD
1G
W
–+()V
W
++
2
------------------------------------------------------------------------------=
G
RTD
12R
3
R
2
⁄⋅
+=
G
W
G
RTD
R
3
R
1
⁄
–=
Where:
V
T
= Voltage at the top of R
RTD
V
B
= Voltage at the bottom of R
RTD
V
W
= Voltage across top and middle R
W
’s
V
CM
= ADC’s common mode input
V
DM
= ADC’s differential mode input
V
1
≈ T
HJ
(40 µV/°C)
V
2
= (1.00V)
V
3
=T
CJ
(10 mV/°C) + (0.50V)
V
4
=250V
1
+(V
2
–V
3
)
≈ (10 mV/°C) (T
HJ
–T
CJ
)+(0.50V)
(R
TH
)/250
(R
TH
)
(R
TH
)/250
C
(R
TH
)
C
V
4
Type K
40 µV/°C
(R
TH
)
(R
TH
)
V
1
V
3
(hot junction
(cold junction
V
2
Thermocouple
at T
HJ
)
at T
CJ
)
R
TH
= Thevenin Equivalent Resistance
U
1
MCP6V26
(R
TH
)/250
0.5696(R
TH
)
(R
TH
)/250
C
(R
TH
)
C
V
4
Type K
(R
TH
)
4.100(R
TH
)
V
1
Temp. S e nsor
V
DD
VREF
V
DD
3kΩ
R
TH
= Thevenin Equivalent Resistance (e.g., 10 kΩ)
U
3
MCP6V26
U
1
MCP1541
U
2
MCP9700A