LMC1982 National Semiconductor, LMC1982 Datasheet - Page 9

• Manufacturer Part Number: LMC1982
• Description: Digitally-Controlled Stereo Tone and Volume Circuit with Two Selectable Stereo Inputs
• Manufacturer: National Semiconductor

Application Information
if the total emitter current exceeds 5 mA. Thus, maximum
output voltage can be increased and much lower distortion
levels can be achieved using load impedances of at least
25 k .
INPUT IMPEDANCE
The input impedance of pins 4, 5, 24 and 25 is defined by in-
ternal bias resistors and is typically 50 k .
EXTERNAL SIGNAL PROCESSING
The SELECT OUT pins (6 and 23) enable greater system
design flexibility by providing a means to implement an ex-
ternal processing loop. This loop can be used for noise re-
duction circuits such as DNR (LM1894) or multi-band
graphic equalizers (LMC835). If both are used, it is important
to ensure that the noise reduction circuitry precede the
equalization circuits. Failure to do so results in improper op-
eration of the noise reduction circuits. The system shown in
Figure 3 utilizes the external loop to include DNR and a
multi-band equalizer.
TONE CONTROL RESPONSE
Bass and treble tone controls are included in the LMC1982.
The tone controls used just two external capacitors for each
stereo channel. Each has a corner frequency determined by
the value of C2 and C3 (see Figure 4 ) and internal resistors
in the feedback loop of the internal tone amplifier. The
maximum-boost or cut is determined by the data sent to the
LMC1982 (see Table 1 ).
The typical tone control response shown in Typical Perfor-
mance Curves were generated with C2 = C3 = 0.0082 µF
and show the response for each step. When modifying the
FIGURE 2. Input and Mode Select Circuitry
(Continued)
9
The SELECT IN pins have an input impedance that varies
with the BASE and TREBLE control settings. The input im-
pedance is 100 k at DC and 19 k at 1 kHz when the con-
trols are set at 0 dB. Minimum input impedance of 30.4 k at
DC and 16 k
lected. At 10 kHz the minimum input impedance, with the
tone controls flat, is 6.8 k
maximum boost, is 2.5 k .
tone control response it is important to note that the ratio of
C3 and C2 sets the mid-frequency gain. Symmetrical tone
response is achieved when C2 = C3. However, with
C2 = 2(C3) and the tone controls set to “flat”, the frequency
response will be flat at 20 Hz and 20 kHz, and +6 dB at
1 kHz.
The frequency where a tone control begins to deviate from a
flat response is referred to as the turn-over frequency. With
C = C2 = C3, the LMC1982’s treble turn-over frequency is
nominally
at 1 kHz occurs when maximum boost is se-
and, with the tone controls at
DS011028-6
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