SSM2019BRNZRL Datasheet, PDF (7/10 Pages) Analog Devices – Self-Contained Audio Preamplifier

SSM2019
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)
TRANSDUCER
(NONINVERTING)
SSM2019
a. Single-Ended
R
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 kW, 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.
R
TRANSDUCER
SSM2019
b. Pseudo-Differential
TRANSDUCER
SSM2019
c. True Differential
Figure 3. Three Ways of Interfacing Transducers for
High Noise Immunity
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 kW/R_REF. If the reference source resistance is
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.
+48V
C1
+IN
+18V
R5
100⍀
C3
47␮F
R3
6.8k⍀
1%
R1
10k⍀
R4
6.8k⍀
1%
R2
10k⍀
Z1
Z2
C4
200pF
Z3
Z4
R_G1
R_{G SSM2019}
R_G2
–IN
C2
–18V
C1, C2: 22␮F TO 47␮F, 63V, TANTALUM OR ELECTROLYTIC
Z1–Z4: 12V, 1/2W
V_OUT
Figure 4. SSM2019 in Phantom Powered Microphone Circuit
REV. ^A
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