SL34118

System Logic

Semiconductor

SLS

As a low pass filter (Figure 7), it can be used to

roll off the high and frequencies in the receive circuit,

which aids protecting against acoustic feedback

problems. With an appropriate choice of an input

coupling capacitor to the low pass filter, a band pass

filter is formed.

Figure 7. Low Pass Filter

POWER

SUPPLY,

AND

CHIP

DISABLE

between 3.5 and 6.5 volts for normal operation, with

reduced operation possible down to 2.8 volts.

≈(V

provides the ac ground for the system. The output

≈400 Ω, and in conjunction with

power supply rejection.

the microphone and hybrid

amplifiers, the amount of supply rejection at their

well as their respective gains.

The Chip Disable (Pin 3) permits powering down

the IC to conserve power and/or for muting purposes.

With CD

≤0.8 volts, normal operation is in effect. With

CD

≥2.0 volts and ≤V

the powered down mode, the microphone and the

hybrid amplifiers are disabled, and their outputs go to

a high impedance state. Additionally, the bias is

removed from the filter (Pins 1, 2), the attenuators

(Pins 8, 9, 21, 22), or from Pins 13, 14, and 15 (the

attenuators are disabled, however, and will not pass a

signal). The input impedance at CD is typically 90 k

Ω,

has a threshold of

≈1.5 volts, and the voltage at this

If CD is not used, the pin should be grounded.

PIN DESCRIPTION

Pin No

Designation

Description

1

FO

Filter output. Output impedance is less than 50

Ω.

2

FI

Filter input. Input impedance is greater than 1.0 M

Ω.

3

CD

Chip Disable. A logic low (<0.8 V) sets normal operation. A logic high (>2.0 V)

disables the IC to conserve power. Input impedance is nominally 90 K

Ω.

4

A supply voltage of +2.8 to +6.5 Volts is required, at

≈5.0 mA. As V

to 2.8 Volts, an AGC circuit reduces the receive attenuator gain by

≈25 dB (when in

the receive mode).

5

HTO+

Output of the second hybrid amplifier. The gain is internally set at -1.0 to provide a

differential output, in conjunction with HTO-, to the hybrid transformer.

6

HTO-

Output of the first hybrid amplifier. The gain of the amp is set by external resistors.

7

HTI

Input and summing node for the first hybrid amplifier. DC level is

≈V

B.

8

TXO

9

TXI

Input to the transmit attenuator. Max. signal level is 350 mVrms. Input impedance is

≈10 KΩ.

10

MCO

Output of the microphone amplifier. The gain of the amplifier is set by external

resistors.

11

MCI

Input and summing node of the microphone amplifier. DC level is

≈V

(continued)

PIN DESCRIPTION