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SSM2165-2S Datasheet(PDF) 5 Page - Analog Devices |
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SSM2165-2S Datasheet(HTML) 5 Page - Analog Devices |
5 / 10 page REV. A SSM2165 –5– THEORY OF OPERATION Figure 10 illustrates the general transfer characteristic for the SSM2165 where the output level in dBu is plotted as a function of the input level in dBu (0 dBu = 0.775 V rms). 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 over 15:1 via a single resistor, RCOMP. Input signals above VRP are compressed with a fixed compression ratio of approximately 10:1. This region of opera- tion 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 “rotation point,” and is different for the SSM2165-1 and SSM2165-2, see Table I. Table I. Characteristics vs. Dash Number SSM2165 Rotation Point Gain Output* –1 40 mV (–25.7 dBu) 18 dB 320 mV (–6 dBu) –2 100 mV (–17.7 dBu) 8 dB 250 mV (–8 dBu) *At limiting. 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. When the compression is set to 2:1, a –2 dB change of the input signal level in the compression region causes –1 dB change of the output level. Likewise, at 10:1 compression, a –10 dB change of the input signal level in the compression region causes a –1 dB change in the output level. The gain of the system with an input signal level of VRP is fixed regardless of the compression ratio, and is different for the SSM2165-1 and SSM2165-2 (see Figures 1a and 1b). The “nominal gain” of the system is 18 dB for the SSM2165-1, and 8 dB for the SSM2165-2. System gain is measured at VRP and is (VOUT – VIN) in dB. Input signals below VDE are downward expanded at a ratio of approximately 1:3. As a result, the gain of the system is small for very small input signal levels below VDE, even though it may be quite large for input signals above VDE. The downward expansion threshold, VDE, is fixed at 500 µV (–64 dBu) for both dash versions. APPLICATIONS INFORMATION The SSM2165 is a complete microphone signal conditioning system in a single integrated circuit. Designed primarily for voiceband applications, this integrated circuit provides amplifi- cation, rms detection, limiting, variable compression, and down- ward expansion. The internal rms detector has a time constant set by an external capacitor. An integral voltage-controlled amplifier (VCA) provides up to 40 dB of gain in the signal path with approximately 30 kHz bandwidth. The device operates on a single +5 V supply, accepts input signals up to 1 V 1, and pro- duces output signal levels at limiting of 320 mV and 250 mV for the SSM2165-1 and SSM2165-2 respectively, into loads > 5 k Ω. The SSM2165 contains an input buffer and automatic gain control (AGC) circuit for audio and voice band signals. Circuit operation is optimized by providing user-adjustable compression ratio and time constant. A downward expansion (noise gating) feature reduces background and circuit noise below 500 µV. The rotation point determines the output signal levels before limiting (referred to the input), and is 40 mV for the SSM2165-1 and 100 mV for the SSM2165-2. INPUT – dB LIMITING REGION LIMITING THRESHOLD (ROTATION POINT) COMPRESSION REGION 1 r 1 1 DOWNWARD EXPANSION THRESHOLD (NOISE GATE) DOWNWARD EXPANSION REGION Vrp VCA GAIN VDE Figure 10. General Input/Output Characteristics of the SSM2165 10 0% 100 90 20mV 10 s TA = +25 C CAVG = 2.2 F RL = 10k COMP RATIO = 1:1 VIN = 12.5mV (–1) VIN = 40mV (–2) Figure 8. Small Signal Transient Response 10 0% 100 90 10 s 200mV TA = +25 C CAVG = 2.2 F RL = 10k COMP RATIO = 1:1 VIN = 125mV (–1) VIN = 400mV (–2) Figure 9. Large Signal Transient Response 1All signals are in rms volts or dBu (0 dBu = 0.775 V rms). |
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