Datasheet

MCP6V01/2/3
DS22058C-page 26 © 2008 Microchip Technology Inc.
4.3.9 PCB DESIGN FOR DC PRECISION
In order to achieve DC precision on the order of ±1 µV,
many physical errors need to be minimized. The design
of the Printed Circuit Board (PCB), the wiring, and the
thermal environment has a strong impact on the
precision achieved. A poor PCB design can easily be
more than 100 times worse than the MCP6V01/2/3 op
amps minimum and maximum specifications.
4.3.9.1 Thermo-junctions
Any time two dissimilar metals are joined together, a
temperature dependent voltage appears across the
junction (the Seebeck or thermo-junction effect). This
effect is used in thermocouples to measure tempera-
ture. The following are examples of thermo-junctions
on a PCB:
• Components (resistors, op amps, …) soldered to
a copper pad
• Wires mechanically attached to the PCB
• Jumpers
•Solder joints
•PCB vias
Typical thermo-junctions have temperature to voltage
conversion coefficients of 10 to 100 µV/°C (sometimes
higher).
There are three basic approaches to minimizing
thermo-junction effects:
• Minimize thermal gradients
• Cancel thermo-junction voltages
• Minimize difference in thermal potential between
metals
4.3.9.2 Non-inverting and Inverting Amplifier
Layout for Thermo-junctions
Figure 4-11 shows the recommended non-inverting
and inverting gain amplifier circuits on one schematic.
Usually, to minimize the input bias current related off-
set, R
1
is chosen to be R
2
||R
3
.
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 4-2:
FIGURE 4-11: PCB Layout and Schematic
for Single Non-inverting and Inverting Amplifiers.
Note: Changing the orientation of the resistors
will usually cause a significant decrease in
the cancellation of the thermal voltages.
V
OUT
≈ V
P
G
P
,V
M
=GND
≈ -V
M
G
M
,V
P
=GND
Where:
G
M
=R
3
/R
2
, inverting gain magnitude
G
P
=1+G
M
, non-inverting gain
magnitude
V
OS
is neglected
V
P
R
3
V
OUT
R
1
R
2
V
M
U
1
MCP6V01
U1
V
M
V
OUT
V
P
R3
R2
R1