OP1177/OP2177/OP4177

Rev. G | Page 18 of 24

PROPER BOARD LAYOUT

The OPx177 is a high precision device. To ensure optimum

performance at the PCB level, care must be taken in the design

of the board layout.

To avoid leakage currents, the surface of the board should be

kept clean and free of moisture. Coating the surface creates a

barrier to moisture accumulation and helps reduce parasitic

resistance on the board.

Keeping supply traces short and properly bypassing the power

supplies minimizes power supply disturbances due to output

current variation, such as when driving an ac signal into a heavy

load. Bypass capacitors should be connected as closely as possible

to the device supply pins. Stray capacitances are a concern at the

outputs and the inputs of the amplifier. It is recommended that

signal traces be kept at least 5 mm from supply lines to

minimize coupling.

A variation in temperature across the PCB can cause a mismatch in

the Seebeck voltages at solder joints and other points where dissi-

milar metals are in contact, resulting in thermal voltage errors. To

minimize these thermocouple effects, orient resistors so heat

sources warm both ends equally. Input signal paths should contain

matching numbers and types of components, where possible to

match the number and type of thermocouple junctions. For

example, dummy components such as zero value resistors can

be used to match real resistors in the opposite input path.

Matching components should be located in close proximity and

should be oriented in the same manner. Ensure leads are of equal

length so that thermal conduction is in equilibrium. Keep heat

sources on the PCB as far away from amplifier input circuitry as

is practical.

The use of a ground plane is highly recommended. A ground

plane reduces EMI noise and also helps to maintain a constant

temperature across the circuit board.

DIFFERENCE AMPLIFIERS

Difference amplifiers are used in high accuracy circuits to improve

the common-mode rejection ratio (CMRR).

R1

V1

V2

R3 = R1

R4 = R1

OP1177

2

3

=

R4

R3

R2

R1

R2

100kΩ

6

7

4

V+

V–

Figure 63. Difference Amplifier

In the single instrumentation amplifier (see Figure 63), where

R1

R2

R3

R4 =

()

1

2

O

V

V

R1

R2

V

−

=

a mismatch between the ratio R2/R1 and R4/R3 causes the

common-mode rejection ratio to be reduced.

To better understand this effect, consider that, by definition,

CM

DM

A

A

CMRR =

where ADM is the differential gain and ACM is the common-

mode gain.

CM

O

CM

DIFF

O

DM

V

V

A

V

V

A

and

=

=

()

2

1

CM

2

1

DIFF

V

V

V

V

V

V

+

=

−

=

2

1

and

For this circuit to act as a difference amplifier, its output must

be proportional to the differential input signal.

From Figure 63,

2

1

O

V

R4

R3

R1

R2

V

R1

R2

V

1

1

⎥

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛ +

⎟

⎠

⎞

⎜

⎝

⎛ +

+

⎟

⎠

⎞

⎜

⎝

⎛

−

=

Arranging terms and combining the previous equations yields

R2R3

R4R1

R4R2

R3R2

R4R1

CMRR

2

2

2

−

+

+

=

(1)

The sensitivity of CMRR with respect to the R1 is obtained by

taking the derivative of CMRR, in Equation 1, with respect to R1.

⎟

⎠

⎞

⎜

⎝

⎛

−

+

+

−

δ

δ

=

δ

δ

R2R3

R1R4

R2R3

R2R4

R2R3

R1R4

R1R4

R1

R1

CMRR

2

2

2

2

2

()

R1R4

R2R3

R1

CMRR

2

2

1

−

=

δ

δ

Assuming that

R1 ≈ R2 ≈ R3 ≈ R4 ≈ R

and

R(1 − δ) < R1, R2, R3, R4 < R(1 + δ)

the worst-case CMRR error arises when

R1 = R4 = R(1 + δ) and R2 = R3 = R(1 − δ)