Microchip Technology MCP6V01DM-VOS Data Sheet

© 2008 Microchip Technology Inc.

DS22058C-page 27

4.3.9.3

Difference Amplifier Layout for 
Thermo-junctions

 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:

4.3.9.4

Dual Non-inverting Amplifier Layout 
for Thermo-junctions

The dual op amp amplifiers shown in 

 and

 produce a non-inverting difference gain

greater than 1, and a common mode gain of 1 .They
can use the layout shown in 

. 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:

 

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

V

P

R

1

R

3

V

REF

U1

V

M

V

OUT

V

P

R4
R2

R1

R3

V

REF

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

V

IA

R

3

V

OA

R

1

R

2

U

1

R

3

R

2