Datasheet

© 2011 Microchip Technology Inc. DS25007B-page 29
MCP6V26/7/8
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 4-15).
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
Thevenin 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 4-15), 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
®
device can be
used instead of the MCP1541.
4.4.4 OFFSET VOLTAGE CORRECTION
Figure 4-17 shows an MCP6V27 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 4-17: Offset Correction.
4.4.5 PRECISION COMPARATOR
Use high gain before a comparator to improve the
latter’s performance. Do not use MCP6V26/7/8 as a
comparator by itself; the V
OS
correction circuitry does
not operate properly without a feedback loop.
FIGURE 4-18: Precision Comparator.
C
2
R
2
R
1
R
3
U
2
V
DD
/2
V
IN
V
OUT
R
2
U
1
MCP6V26
R
4
R
5
V
DD
/2
MCP661
V
IN
R
3
R
2
V
DD
/2
U
2
V
OUT
R
5
R
4
R
1
1kΩ
U
1
MCP6V26
MCP6541