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SSM2019BRW Datasheet(PDF) 7 Page - Analog Devices |
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SSM2019BRW Datasheet(HTML) 7 Page - Analog Devices |
7 / 8 page REV. 0 SSM2019 –7– RG 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 VOUT +18V –18V C1, C2: 22 F TO 47 F, 63V, TANTALUM OR ELECTROLYTIC Z1–Z4: 12V, 1/2W RG1 RG2 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 W/RREF. 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. |
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