SSM2166 データシートの表示（PDF） - Analog Devices

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SSM2166

Microphone Preamplifier with Variable Compression and Noise Gating

Analog Devices 

SSM2166

The gain of the VCA at the rotation point is set by the value of a

resistor, RGAIN, connected between Pin 2 and GND. The relationship

between the VCA gain and RGAIN is shown in Figure 6. The AGC

range can be as high as 60 dB. The VCAIN pin (Pin 3) is the non-

inverting input terminal to the VCA. The inverting input of the

VCA is available at the VCAR pin (Pin 4) and exhibits an input

impedance of 1 kΩ, as well. As a result, this pin can be used for

differential inputs or for the elimination of grounding problems

by connecting a capacitor whose value equals that used in series

with the VCAIN pin to ground (see Figure 26 for more details).

The output impedance of the SSM2166 is typically less than 75 Ω,

and the external load on Pin 13 should be >5 kΩ. The nominal

output dc voltage of the device is approximately 2.2 V. Use a

blocking capacitor for grounded loads.

The bandwidth of the SSM2166 is quite wide at all gain settings.

The upper 3 dB point is approximately 30 kHz at gains as high as

60 dB (using the input buffer for additional gain, circuit bandwidth

is unaffected). The gain bandwidth (GBW) plots are shown in

Figure 11. 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 optimizing

the usable dynamic range of the part. A plot of wideband peak-

to-peak output noise is shown in Figure 10.

LEVEL DETECTOR

The SSM2166 incorporates a full-wave rectifier and 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 averaging capacitor

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

cause a pumping effect for some signals, while too large a value

may result in slow response times to signal dynamics. Electrolytic

capacitors are recommended for lowest 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 appropriate in voice-band 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

controlled 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 less than 1 ms in most cases.

C6

10µF

C7

10µF (OPTIONAL)

R1

10kΩ

AUDIO

+IN

R2

10kΩ

+

1µF

–IN

6

7

C1

0.1µF

V+

+

BUF OUT VCAIN

14

5

3

BUFFER

1kΩ

+

VCAR

4

1kΩ

VCA

SSM2166

RMS

LEVEL

DETECTOR

CONTROL

CIRCUIT

OUTPUT

13

VOUT

GAIN ADJUST

V+

2

RGAIN

RGATE

NOISE GATE SET

9

RROT PT

ROTATION SET

11

POWER DOWN

12

1

GND

8

AVG CAP

+ CAVG

2.2µF

10

COMP

RATIO SET

RCOMP

Figure 16. Functional Block Diagram and Typical Application

Rev. D | Page 9 of 20

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