MIL-HDBK-419A

b.

Balance the Two Amplifier Inputs - If

are manipulated such that

(6-22)

the common-mode noise voltage at the amplifier output terminals can be made to vanish.

- If Z1 is sufficiently large compared to

is sufficiently large

c.

Increase

compared to

then the common-mode noise voltage at the amplifier output terminals will be

diminished. This approach normally requires a differential amplifier with carefully shielded input

signal lines.

In the case of balanced signal sources or transducers, the basic circuit and equations differ from those given in

Figure 6-14 and by Equation 6-21. Figure 6-15 shows a balanced source with an output voltage

and output

r e s i s t a n c e connected to the two inputs of an amplifier. In this case, the center tap of the source is

connected to the ground reference terminal. As before, if it is assumed that

is small, it can be shown that

is given by

(6-23)

The same conclusions regarding the minimization of the common-mode noise component at the amplifier output

apply in this circuit as for the unbalanced source. However, in this case the amplifier must have a differential

input stage. Otherwise, one-half of the source would be shorted out. In Figure 6-14, the amplifier can have

single-ended or differential inputs.

The common-mode rejection (CMR) ratio of an amplifier is the gain of the amplifier (K) multiplied by the

common-mode noise voltage (Vcm) and divided by the amplifier output due to

The CMR ratio describes a

circuit's ability to avoid converting common-mode noise to normal-mode noise. Expressed as a positive

quantity, the CMR ratio is given by

(6-24)

CMR=

6-21