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SSM2167-1RMZ-R7 Datasheet(PDF) 8 Page - Analog Devices |
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SSM2167-1RMZ-R7 Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page SSM2167 Data Sheet Rev. G | Page 8 of 12 APPLICATIONS INFORMATION The SSM2167 is a complete microphone signal conditioning system on a single integrated circuit. Designed primarily for voice-band applications, this integrated circuit provides ampli- fication, limiting, variable compression, and noise gate. User adjustable compression ratio, noise gate threshold, and two different fixed gains optimize circuit operation for a variety of applications. The SSM2167 also features a low power shutdown mode for battery-powered applications. 10 1 2 4 3 9 7 8 5 6 + SSM2167 10µF GND VDD VDD VDD 500kΩ SHUTDOWN INPUT 0.1µF 10µF + 10µF GND RCOMP RGATE 100kΩ OUTPUT 10µF Figure 15. Typical Application Circuit VCA +1 CONTROL C1 0.1µF C2 10µF 1kΩ 1kΩ OUTPUT NOISE GATE AND COMPRESSION SETTINGS LEVEL DETECTOR BUFFER INPUT GND + CAVG C3 10µF SHUTDOWN RC RG VDD VDD BUFOUT VCAIN Figure 16. Functional Block Diagram THEORY OF OPERATION The typical transfer characteristic for the SSM2167 is shown in Figure 2 where the output level in dB is plotted as a function of the input level in dB. The dotted line indicates the transfer char- acteristic for a unity-gain amplifier. For input signals in the range of VDE (downward expansion) to VRP (rotation point), an “r” dB change in the input level causes a 1 dB change in the output level. Here, r is defined as the compression ratio. The compression ratio may be varied from 1:1 (no compression) to 10:1 via a single resistor, RCOMP. Input signals above VRP are compressed with a fixed compression ratio of approximately 10:1. This region of operation is the limiting region. Varying the compression ratio has no effect on the limiting region. The breakpoint between the compression region and the limiting region is referred to as the limiting threshold or the rotation point. The term, rotation point, derives from the observation that the straight line in the compression region rotates about this point on the input/output characteristic as the compression ratio is changed. The gain of the system with an input signal level of VRP is the fixed gain, 18 dBV for the SSM2167, regardless of the compression ratio. Input signals below VDE are downward expanded; that is, a −1 dB change in the input signal level causes approximately a −3 dB change in the output level. As a result, the gain of the system is small for very small input signal levels, even though it may be quite large for small input signals above VDE. The external resistor at Pin 7, RGATE, is used to set the downward expansion threshold (VDE). Finally, the SSM2167 provides an active low, CMOS-compatible digital power-down feature that reduces device supply current to typically less than 2 µA. SIGNAL PATH Figure 16 illustrates the block diagram of the SSM2167. The audio input signal is processed by the input buffer and then by the VCA. The input buffer presents an input impedance of approximately 100 kΩ to the source. A dc voltage of approximately 400 mV is present at INPUT (Pin 5) of the SSM2167, requiring the use of a blocking capacitor (C1) for ground-referenced sources. A 0.1 µF capacitor is a good choice for most audio applications. The input buffer is a unity-gain stable amplifier that can drive the low impedance input of the VCA and an internal rms detector. The VCA is a low distortion, variable gain amplifier whose gain is set by the side-chain control circuitry. An external blocking capacitor (C2) must be used between the buffer output and the VCA input. The 1 kΩ impedance between amplifiers determines the value of this capacitor, which is typically between 4.7 µF and 10 µF. An aluminum electrolytic capacitor is an economical choice. The VCA amplifies the input signal current flowing through C2 and converts this current to a voltage at the output pin (Pin 9) of the SSM2167. The net gain from input to output can be as high as 40 dB, depending on the gain set by the control circuitry. The output impedance of the SSM2167 is typically less than 145 Ω, and the external load on Pin 9 should be >5 kΩ. The nominal output dc voltage of the device is approximately 1.4 V; therefore, a blocking capacitor for grounded loads must be used. The bandwidth of the SSM2167 is quite wide at all gain settings. The upper 3 dB point is over 1 MHz at gains as high as 30 dB. The GBW plots are shown in Figure 5. The lower 3 dB cutoff frequency of the SSM2167 is set by the input impedance of the VCA (1 kΩ) and C2. Whereas the noise of the input buffer is fixed, the input-referred noise of the VCA is a function of gain. The VCA input noise is designed to be at a minimum when the gain is at a maximum, thereby maximizing the usable dynamic range of the part. |
Similar Part No. - SSM2167-1RMZ-R7 |
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Similar Description - SSM2167-1RMZ-R7 |
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