SSM2017

–7–

REV. C

Figure 23. SSM2017 in Phantom Powered Microphone Circuit

INPUTS

The SSM2017 has protection diodes across the base emitter

junctions of the input transistors. These prevent accidental ava-

lanche breakdown, which could seriously degrade noise perfor-

mance. Additional clamp diodes are also provided to prevent the

inputs from being forced too far beyond the supplies.

a. Single Ended

b. Pseudo Differential

c. True Differential

Figure 22. Three Ways of Interfacing Transducers for High

Noise Immunity

Although the SSM2017’s inputs are fully floating, care must be

exercised to ensure that both inputs have a dc bias connection

capable of maintaining them within the input common-mode

range. The usual method of achieving this is to ground one side

of the transducer as in Figure 22a, but an alternative way is to

float the transducer and use two resistors to set the bias point as

in Figure 22b. The value of these resistors can be up to 10 k

Ω,

but they should be kept as small as possible to limit common-

mode pickup. Noise contribution by resistors themselves is neg-

ligible since it is attenuated by the transducer’s impedance. Bal-

anced transducers give the best noise immunity and interface

directly as in Figure 22c.

REFERENCE TERMINAL

The output signal is specified with respect to the reference ter-

minal, which is normally connected to analog ground. The ref-

erence may also be used for offset correction or level shifting. A

reference source resistance will reduce the common-mode rejec-

tion by the ratio of 5 k

Ω/R

tance is 1

Ω, then the CMR will be reduced to 74 dB (5 kΩ/1 Ω

= 74 dB).

COMMON-MODE REJECTION

Ideally, a microphone preamplifier responds only to the differ-

ence between the two input signals and rejects common-mode

voltages and noise. In practice, there is a small change in output

voltage when both inputs experience the same common-mode

voltage change; the ratio of these voltages is called the common-

mode gain. Common-mode rejection (CMR) is the logarithm of

the ratio of differential-mode gain to common-mode gain,

expressed in dB.

PHANTOM POWERING

A typical phantom microphone powering circuit is shown in

tion for the SSM2017 whenever microphones are plugged in or

unplugged.