LMC1983 Datasheet, PDF (7/14 Pages) National Semiconductor (TI) – Digitally-Controlled Stereo Tone and Volume Circuit with Three Selectable Stereo Inputs

General Information (Continued)
FIGURE 1. Typical Application
DS011279-3
Application Information
INPUT SELECTOR
The LMC1983’s input selector and mode control are shown
in Figure 2. The input selector selects one of three stereo
signal sources or a mute function with typical attenuation of
100 dB. The selected signals are then sent to a mode control
matrix. As shown in Table 1, the matrix provides normal ste-
reo or can direct any given channel to both LEFT or RIGHT
SELECT OUTPUTs. The third matrix mode is normal stereo.
The control matrix output is buffered and appears on each
channel’s respective SELECT OUT pin (7, 22). Switching
noise is kept to a minimum when mute is selected by using
a 50 kΩ bias resistor.
Noise performance is optimized through the use of emitter
followers in the mode control matrix’s output. Internal 50 kΩ
resistors are connected to each input selector pin to provide
the proper bias point for the emitter follower buffers. Each in-
ternal 50 kΩ bias resistor is connected to a common
half-supply (V⁺/2) source. This produces a voltage at pins 7
and 22 (SELECT OUT) that is 1.4V below V⁺/2 (typically
3.1V with V⁺ = 9V). Since a DC voltage is present at the in-
put pins (4, 5, 6, 23, 24, and 25), input signals should be AC
coupled through a 1 µF capacitor.
The output signal at pins 7 and 22 can be used to drive ex-
teral audio processing circuits such as noise reduction
(LM1894–DNR or Dolby) or graphic equalizers (LMC835). It
is important that if any noise reduction is used it be placed
ahead of any tone controls or equalizers in the external cir-
cuit path to preserve the frequency spectrum of the selected
input signal. Otherwise, any frequency equalization could
prevent the proper operation of the noise reduction circuit. If
no external processor is used, a capacitor should be used to
couple the SELECT OUT signals directly to pins 8 and 21,
respectively.
MINIMUM LOAD IMPEDANCE
The LMC1983 employs emitter-followers to buffer the se-
lected stereo channels. The buffered signals are available at
pins 7 and 22 (SELECT OUT). The SELECT OUT buffers op-
erate with a typical bias current of 1 mA.
The Electrical Specifications table lists a maximum input sig-
nal of 2.0 V_rms (2.8 V_peak) for 1% THD at the SELECT OUT
pins. This distortion level is achieved when the minimum AC
load impedance seen by the SELECT OUT pins is 2.5 kΩ
(2.5V/1 mA). Using lower load impedances results in clipping
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