Datasheet

© 2008 Microchip Technology Inc. DS22058C-page 27
MCP6V01/2/3
4.3.9.3 Difference Amplifier Layout for
Thermo-junctions
Figure 4-12 shows the recommended difference ampli-
fier circuit. Usually, we choose R
1
=R
2
and R
3
=R
4
.
The guard traces (with ground vias at the ends) help
minimize the thermal gradients. The resistor layout
cancels the resistor thermal voltages, assuming the
temperature gradient is constant near the resistors:
EQUATION 4-3:
FIGURE 4-12: PCB Layout and Schematic
for Single Difference Amplifier.
4.3.9.4 Dual Non-inverting Amplifier Layout
for Thermo-junctions
The dual op amp amplifiers shown in Figure 4-16 and
Figure 4-17 produce a non-inverting difference gain
greater than 1, and a common mode gain of 1 .They
can use the layout shown in Figure 4-13. The gain set-
ting resistors (R
2
) between the two sides are not com-
bined so that the thermal voltages can be canceled.
The guard traces (with ground vias at the ends) help
minimize the thermal gradients. The resistor layout
cancels the resistor thermal voltages, assuming the
temperature gradient is constant near the resistors:
EQUATION 4-4:
FIGURE 4-13: PCB Layout and Schematic
for Dual Non-inverting Amplifier.
Note: Changing the orientation of the resistors
will usually cause a significant decrease in
the cancellation of the thermal voltages.
V
OUT
≈ V
REF
+(V
P
–V
M
)G
DM
Where:
Thermal voltages are approximately equal
G
DM
=R
3
/R
1
=R
4
/R
2
, difference gain
V
OS
is neglected
V
OUT
≈ V
REF
+(V
P
–V
M
)G
DM
R
4
V
OUT
R
2
V
M
U
1
MCP6V01
V
P
R
1
R
3
V
REF
U1
V
M
V
OUT
V
P
R4
R2
R1
R3
V
REF
Note: Changing the orientation of the resistors
will usually cause a significant decrease in
the cancellation of the thermal voltages.
(V
OA
–V
OB
) ≈ (V
IA
–V
IB
)G
DM
(V
OA
+V
OB
)/2 ≈ (V
IA
+V
IB
)/2
Where:
Thermal voltages are approximately equal
G
DM
=1+R
3
/R
2
, differential mode gain
G
CM
= 1, common mode gain
V
OS
is neglected
U1
V
IB
V
OA
V
OB
V
IA
R1
R2
R3
R1
R2
R3
V
IB
V
OB
R
1
U
1
½MCP6V02
V
IA
R
3
V
OA
R
1
R
2
U
1
½MCP6V02
R
3
R
2