U4082B Datasheet, PDF (5/30 Pages) ATMEL Corporation – Low-voltage Voice-switched IC for Hands-free Operation

U4082B
3.2 Transmit and Receive Attenuators TI, TO and RI, RECO
The attenuators are functionally complementary; that is, when one is at maximum gain
(+6.0 dB), the other is at maximum attenuation (–46 dB), and vice versa. Neither is ever fully on
or off. The sum of their gains remains constant (within a nominal error band of ±0.1 dB) at a typ-
ical value of –40 dB (see Figure 7-1 on page 17). Their purpose is to control the transmit and
receive paths to provide the half-duplex operation required in a speakerphone.
The non-inverting attenuators have a –3.0 dB (from maximum gain) frequency of approximately
100 kHz. The input impedance of each attenuator (TI and RI) is nominally 10 kΩ (see Figure
3-1). To prevent distortion, the input signal should be limited to 350 mVrms. The maximum
recommended input signal is independent of the volume control setting. The diode clamp on the
inputs limits the input swing, and thus the maximum negative output swing. This results in a
specific V_RECO and V_TOL definition as given in the table “Electrical Characteristics” on page 14.
The output impedance is less than 10Ω until the output current limit (typically 2.5 mA) is reached.
Figure 3-1. Attenuator Input Stage
11 kΩ
V_B
RI 21
TI 9
5 kΩ
95 kΩ
to Attenuator
Input
The attenuators are controlled by the single output of the control block, which is measurable at
pin C_T (pin 14). When pin C_T is at +240 mV with respect to V_B, the circuit is in receive mode
(receive attenuator is at +6.0 dB). When pin C_T is at –240 mV with respect to V_B, the circuit is in
transmit mode (transmit attenuator is at +6.0 dB). The circuit is in an idle mode when the C_T
voltage is equal to V_B causing the attenuators' gain to be halfway between their fully on and fully
off positions (–20 dB each). Monitoring the C_T voltage (with respect to V_B) is the most direct
method of monitoring the circuit's mode.
The attenuator control has seven inputs: two from the comparators operated by the level detec-
tors, two from the background noise monitors, volume control, dial-tone detector, and AGC.
They are described in the sections that follow.
3.3 Level Detectors
There are four level detectors, two on the receive side and two on the transmit side. As shown in
Figure 3-2 on page 6, the terms in parentheses form one system, and the other terms form the
second system. Each level detector is a high-gain amplifier with back-to-back diodes in the feed-
back path, resulting in nonlinear gain which permits operation over a wide dynamic range of
speech levels. Refer to the graphs of Figures 7-2, 7-3 and 7-4 on page 18 for their DC and AC
transfer characteristics. The sensitivity of each level detector is determined by the external resis-
tor and capacitor at each input (TLI1, TLI2, RLI1, and RLI2). Each output charges an external
capacitor through a diode and limiting resistor, thus providing a DC representation of the input
AC signal level. The outputs have a quick rise time (determined by the capacitor and an internal
350Ω resistor) and a slow decay time set by an internal current source and the capacitor. The
capacitors on the four outputs should have the same value (±10%) to prevent timing problems.
As can be seen in Figure 1-2 on page 2, on the receive side, one level detector (RLI1) is located
at the receive input, receiving the same signal as at Tip and Ring, and the other (RLI2) is at the
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