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SSM2167_07

Low Voltage Microphone Preamplifier with Variable Compression and Noise Gating

Analog Devices 

LEVEL DETECTOR

The SSM2167 incorporates a full-wave rectifier and a true rms

level detector circuit whose averaging time constant is set by an

external capacitor (CAVG) connected to the AVG CAP (Pin 6).

For optimal low frequency operation of the level detector down

to 10 Hz, the value of the capacitor should be 2.2 μF. Some experi-

mentation with larger values for CAVG 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, whereas too large a value can result in slow response

times to signal dynamics. Electrolytic capacitors are recommended

here for lowest cost and should be in the range of 2 μF to 22 μF.

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 increase due to a decrease in input signal.

The attack time, the time it takes for the gain to be reduced

because of a sudden increase in input level, is controlled mainly

by internal circuitry that speeds up the attack for large level

changes. In most cases, this limits overload time to less than 1 ms.

The performance of the rms level detector is illustrated in

Figure 14 for a CAVG of 2.2 μF and Figure 13 for a CAVG of 22 μF.

In Figure 13, Figure 14, and Figure 15, the input signal to the

SSM2167 (not shown) is a series of tone bursts in six 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 these figures, 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.

CONTROL CIRCUITRY

The output of the rms level detector is a signal proportional to

the log of the true rms value of the buffer output with an added

dc offset. The control circuitry subtracts a dc voltage from this

signal, scales it, and sends the result to the VCA to control the

gain. The gain control of the VCA is logarithmic—a linear change

in control signal causes a dB change in gain. It is this control

law that allows linear processing of the log rms signal to provide

the flat compression characteristic on the input/output charac-

teristic shown in Figure 2.

15:1

5:1

2:1

1:1

SSM2167

VCA GAIN

1

1

VDE

INPUT (dB)

VRP

Figure 18. Effect of Varying the Compression Ratio

SETTING THE COMPRESSION RATIO

Changing the scaling of the control signal fed to the VCA causes

a change in the circuit compression ratio, r. This effect is shown

in Figure 18. Connecting a resistor (RCOMP) between Pin 8 and

VDD sets the compression ratio. Lowering RCOMP gives smaller

compression ratios as indicated in Table 4. AGC performance is

achieved with compression ratios between 2:1 and 10:1, and is

dependent on the application. Shorting RCOMP disables the AGC

function, setting the compression equal to 1:1. If using a compres-

sion resistor, using a value greater than 5 kΩ is recommended.

If lower than 5 kΩ is used, the device may interpret this as a

short, 0 Ω.

Table 4. Setting Compression Ratio

Compression Ratio

1:1

2:1

Value of RCOMP

0 Ω (short to V+)

15 kΩ

3:1

35 kΩ

5:1

75 kΩ

10:1

175 kΩ

Rev. C | Page 9 of 12

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