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.
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
− 1 +
where a is the open-loop gain of the amplifier and f2 is the frequency
at which the phase of a = Φm – 180°.
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
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
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
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 are used in high-accuracy circuits to improve
the common-mode rejection ratio (CMRR).
R3 = R1
R4 = R1
R4 = R2
Figure 13. Difference Amplifier
In the single amplifier instrumentation amplifier (circuit of
Figure 13), where:
R4 = R2
R 3 R1
( ) VO
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:
CMRR = ADM
where ADM is the differential gain and ACM is the common-mode gain.
( ) VDIFF
= V1 − V2
V1 + V2