IL34118
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
The output voltage at V
B
(Pin 15) is
≈(V
CC
- 0.7)/2,
and provides the ac ground for the system. The output
impedance at V
B
is
≈400 Ω,
and in conjunction with
the external capacitor at V
B
, forms a low pass filter
for power supply rejection.
Since V
B
biases the microphone and hybrid
amplifiers, the amount of supply rejection at their
outputs is directly related to the rejection at V
B
, as
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
CC
, the IC is powered
down. In 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 kΩ, has a threshold of
≈1.5
volts, and the
voltage at this pin must be kept within the range of
ground and V
CC
. If CD is not used, the pin should be
grounded.
POWER SUPPLY,
DISABLE
V
B
, AND CHIP
The power supply voltage at V
CC
(Pin 4) is to be
between 3.5 and 6.5 volts for normal operation, with
reduced operation possible down to 2.8 volts.
PIN DESCRIPTION
Pin No
1
2
3
4
Designation
FO
FI
CD
V
CC
Description
Filter output. Output impedance is less than 50Ω.
Filter input. Input impedance is greater than 1.0 MΩ.
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 KΩ.
A supply voltage of +2.8 to +6.5 Volts is required, at
≈5.0
mA. As V
CC
falls from
3.5 to 2.8 Volts, an AGC circuit reduces the receive attenuator gain by
≈25
dB
(when in the receive mode).
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.
Output of the first hybrid amplifier. The gain of the amp is set by external resistors.
Input and summing node for the first hybrid amplifier. DC level is
≈V
B.
Output of the transmit attenuator. DC level is approximately V
B
.
Input to the transmit attenuator. Max. signal level is 350 mVrms. Input impedance
is
≈10
KΩ.
Output of the microphone amplifier. The gain of the amplifier is set by external
resistors.
Input and summing node of the microphone amplifier. DC level is
≈V
B
.
(continued)
5
6
7
8
9
10
11
HTO+
HTO-
HTI
TXO
TXI
MCO
MCI
PIN DESCRIPTION
7
INTEGRAL [ INTEGRAL CORP. ]
