SSM2166S 데이터 시트보기 (PDF) - Analog Devices

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SSM2166S

Microphone Preamplifier with Variable Compression and Noise Gating

Analog Devices 

SSM2166

C6

10␮F

C7*

10␮F

R1 = 10k⍀

V+

14

5

BUFOUT

3

VCAIN

4

VCAR

AUDIO

7

+IN

0.1␮F

R2 = 10k⍀

+

1␮F

INPUT

BUFFER

RMS

LEVEL

DETECTOR

1k⍀

1k⍀

VCA

SSM2166

CONTROL

CIRCUITRY

OUTPUT 13

GAIN

ADJUST 2

VOUT

V+

NOISE

GATE 9

RGAIN

RGATE

ROTATION

POINT

ADJUST 11

RROT PT

GND

1

GND

AVG

CAP

8 CAVG

2.2␮F

POWER

DOWN 12

COMPRESSION

RATIO SET

POWER

DOWN

10

RCOMP

*OPTIONAL

Figure 14. Functional Block Diagram and Typical Application

bandwidth is unaffected). The GBW plots are shown in Figure

10b. The lower 3 dB cutoff frequency of the SSM2166 is set by

the input impedance of the VCA (1 kΩ) and C6. While 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 a minimum when the gain is at a maximum, thereby optimiz-

ing the usable dynamic range of the part. A photograph of the

SSM2166’s wideband peak-to-peak output noise is illustrated in

Figure 10b.

The Level Detector

The SSM2166 incorporates a full-wave rectifier and a patent-

pending, true rms level detector circuit whose averaging time

constant is set by an external capacitor connected to the AVG

CAP pin (Pin 8). For optimal low frequency operation of the

level detector down to 10 Hz, the value of the capacitor should

be 2.2 µF. Some experimentation with larger values for the

AVG CAP may be necessary to reduce the effects of excessive

low frequency ambient background noise. The value of the aver-

aging capacitor affects sound quality: too small a value for this

capacitor may cause a “pumping effect” for some signals, while

too large a value can result in slow response times to signal dy-

namics. Electrolytic capacitors are recommended here for low-

est cost and should be in the range of 2 µF to 47 µF. Capacitor

values from 18 µF to 22 µF have been found to be more appro-

priate in voiceband applications, where capacitors on the low

end of the range seem more appropriate for music program

material.

The rms detector filter time constant is approximately given by

10•CAVG milliseconds where CAVG is in µF. This time constant

controls both the steady-state averaging in the rms detector as

well as the release time for compression; that is, the time it takes

for the system gain to react when a large input is followed by a

small signal. The attack time, the time it takes for the gain to be

reduced when a small signal is followed by a large signal, is con-

trolled partly by the AVG CAP value, but is mainly controlled

by internal circuitry that speeds up the attack for large level

changes. This limits overload time to under 1 ms in most cases.

The performance of the rms level detector is illustrated in Fig-

ure 15 for a CAVG of 2.2 µF (Figure 15a) and 22 µF (Figure

15b). In each of these photographs, the input signal to the

SSM2166 (not shown) is a series of tone bursts in 6 successive

10 dB steps. The tone bursts range from –66 dBV (0.5 mV rms)

to –6 dBV (0.5 V rms). As illustrated in the photographs, the

attack time of the rms level detector is dependent only on CAVG,

but the release times are linear ramps whose decay times are

dependent on both CAVG and the input signal step size. The

rate of release is approximately 240 dB/s for a CAVG of 2.2 µF,

and 12 dB/s for a CAVG of 22 µF.

100mV

100

90

6dBV

66dBV

10

0%

85dBV

100ms

Figure 15a. RMS Level Detector Performance with

CAVG = 2.2 µF

REV. A

–7–

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