Datasheet

MCP6V01/2/3
DS22058C-page 30 © 2008 Microchip Technology Inc.
4.4.3 THERMOCOUPLE SENSOR
Figure 4-18 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 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 4-5:
FIGURE 4-18: Thermocouple Sensor;
Simplified Circuit.
Figure 4-19 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 4-19: Thermocouple Sensor.
The MCP9700A senses the temperature at its physical
location. It needs to be at the same temperature as the
cold junction (T
CJ
), and produces V
3
(Figure 4-16).
The MCP1541 produces a 4.10V output, assuming
V
DD
is at 5.0V. This voltage, tied to a resistor ladder of
4.100(R
TH
) and 1.3224(R
TH
), would produce a Theve-
nin equivalent of 1.00V and 250(R
TH
). The
1.3224(R
TH
) resistor is combined in parallel with the
top right R
TH
resistor (in Figure 4-18), producing the
0.5696(R
TH
) resistor.
V
4
should be converted to digital, then corrected for the
thermocouple’s non-linearity. The ADC can use the
MCP1541 as its voltage reference. Alternately, an
absolute reference inside a PICmicro
®
can be used
instead of the MCP1541.
4.4.4 OFFSET VOLTAGE CORRECTION
Figure 4-20 shows a MCP6V01 correcting the input
offset voltage of another op amp. R
2
and C
2
integrate
the offset error seen at the other op amp’s input; the
integration needs to be slow enough to be stable (with
the feedback provided by R
1
and R
3
).
FIGURE 4-20: Offset Correction.
4.4.5 PRECISION COMPARATOR
Use high gain before a comparator to improve the
latter’s performance. Do not use MCP6V01/2/3 as a
comparator by itself; the V
OS
correction circuitry does
not operate properly without a feedback loop.
FIGURE 4-21: Precision Comparator.
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
MCP6V01
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
(R
TH
)/250
0.5696(R
TH
)
(R
TH
)/250
C
(R
TH
)
C
V
4
MCP6V01
Type K
(R
TH
)
4.100(R
TH
)
V
1
MCP9700A
V
DD
MCP1541
V
DD
3kΩ
R
TH
= Thevenin Equivalent Resistance (e.g.: 10 kΩ)
MCP6V01
C
2
R
2
R
1
R
3
MCP6XXX
V
DD
/2
3kΩ
V
IN
V
OUT
R
2
MCP6V01
V
IN
R
3
R
2
V
DD
/2
MCP6541
V
OUT
R
5
R
4
R
1
1kΩ