REV. 0

SSM2019

–7–

C4

200pF

Z1

Z2

Z3

Z4

R1

10k

R2

10k

R3

6.8k

1%

R4

6.8k

1%

+IN

–IN

R5

100

C3

47 F

C1

C2

+18V

–18V

C1, C2: 22 F TO 47 F, 63V, TANTALUM OR ELECTROLYTIC

Z1–Z4: 12V, 1/2W

SSM2019

+48V

Figure 4. SSM2019 in Phantom Powered Microphone Circuit

INPUTS

The SSM2019 has protection diodes across the base emitter

junctions of the input transistors. These prevent accidental

avalanche breakdown, which could seriously degrade noise

performance. Additional clamp diodes are also provided to prevent

the inputs from being forced too far beyond the supplies.

(INVERTING)

(NONINVERTING)

TRANSDUCER

SSM2019

a. Single-Ended

R

TRANSDUCER

SSM2019

R

b. Pseudo-Differential

TRANSDUCER

SSM2019

c. True Differential

Figure 3. Three Ways of Interfacing Transducers for

High Noise Immunity

Although the SSM2019 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 3a. An alternative way is to float

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

Figure 3b. The value of these resistors can be up to 10 k

W, but

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

pickup. Noise contribution by resistors is negligible since it is

attenuated by the transducer’s impedance. Balanced transducers

give the best noise immunity and interface directly as in Figure 3c.

For stability, it is required to put an RF bypass capacitor directly

across the inputs, as shown in Figures 3 and 4. This capacitor

should be placed as close as possible to the input terminals. Good

RF practice should also be followed in layout and power supply

bypassing, since the SSM2019 uses very high bandwidth devices.

REFERENCE TERMINAL

The output signal is specified with respect to the reference terminal,

which is normally connected to analog ground. The reference

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

ence source resistance will reduce the common-mode rejection

by the ratio of 5 k

1

W, then the CMR will be reduced to 74 dB (5 kW/1 W = 74 dB).

COMMON-MODE REJECTION

Ideally, a microphone preamplifier responds to only the difference

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

Figure 4. Z1 to Z4 provide transient overvoltage protection for

the SSM2019 whenever microphones are plugged in or unplugged.