OP4177 datasheet(13/20 Pages) AD

13 / 20 page

REV. B

OP1177/OP2177/OP4177

–13–

Reducing Electromagnetic Interference

A number of methods can be utilized to reduce the effects of

EMI on amplifier circuits.

In one method, stray signals on either input are coupled to the

opposite input of the amplifier. The result is that the signal is

rejected according to the amplifier’s CMRR.

This is usually achieved by inserting a capacitor between the inputs

of the amplifier as shown in Figure 11. However, this method may

also cause instability depending on the value of capacitance.

VOUT

V+

OP1177

–

Figure 11. EMI Reduction

Placing a resistor in series with the capacitor (Figure 12) increases

the dc loop gain and reduces the output error. Positioning the

breakpoint (introduced by R-C) below the secondary pole of the

op amp improves the phase margin and hence stability.

R can be chosen independently of C for a specific phase margin

according to the formula

ajf

=− +









where a is the open-loop gain of the amplifier and f2 is the frequency

at which the phase of a =

Φm – 180°.

VOUT

OP1177

V+

–

Figure 12. Compensation Using Input RC Network

Proper Board Layout

The OP1177 is a high-precision device. In order to ensure optimum

performance at the PC board 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 will minimize 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 pos-

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

at the output 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 PC board can cause a

mismatch in the Seebeck voltages at solder joints and other

points where dissimilar metals are in contact, resulting in thermal

voltage errors. To minimize these thermocouple effects, resistors

should be oriented so heat sources warm both ends equally.

Input signal paths should contain matching numbers and types

of components where possible in order to match the number

and type of thermocouple junctions. For example, dummy com-

ponents 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. Leads should be of equal length so that ther-

mal conduction is in equilibrium. Heat sources on the PC board

should be kept as far away from amplifier input circuitry as

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).

100k

V+

OP1177

VOUT

R3 = R1

R4 = R1

Figure 13. Difference Amplifier

In the single amplifier instrumentation amplifier (circuit of

Figure 13), where:

O =−

()

a mismatch between the ratio R2/R1 and R4/R3 will cause the

common-mode rejection ratio to be reduced. To better under-

stand this effect, consider the following:

By definition:

CMRR

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

DIFF

and

DIFF

=−

()

and

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