LM1971MX/NOPB National Semiconductor, LM1971MX/NOPB Datasheet - Page 9

• Manufacturer Part Number: LM1971MX/NOPB
• Description: IC AUDIO ATTENUATOR DIG 8-SOIC
• Manufacturer: National Semiconductor
• Series: Overture™r
• Type: Audio Attenuatorr

Specifications of LM1971MX/NOPB

• Applications: Cell Phones, PC, MIDI
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM1971MX; *LM1971MX/NOPB; LM1971MX

Application Information
quency power supply fluctuations, a smaller capacitor
(0.01 µF–0.1 µF) could be added in parallel to the 10 µF
capacitor.
LOGARITHMIC GAIN AMPLIFIER
The µPot is capable of being used in the feedback loop of an
op amp to create a gain controlled amplifier as shown in
Figure 8. In this configuration the attenuation levels from
Table 1 become gain levels with the largest possible gain
value being 62 dB. For most applications, 62 dB of gain will
cause signal clipping to occur. However, this can be con-
trolled through programming. It is important to note that
when in mute mode the input is disconnected from the
output, thus placing the amplifier in open-loop gain state. In
FIGURE 7. Active Reference with Active Gain Buffering
FIGURE 6. Higher Performance
Active Half-Supply Buffering
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this mode, the amplifier will behave as a comparator. Care
should be taken with the programming and design of this
type of circuit. To provide the best overall performance, a
high input impedance, low input bias current op amp should
be used.
MUTE FUNCTION
A major feature of the LM1971 is its ability to mute the input
signal to an attenuation level of 102 dB. This is accom-
plished internally by physically disconnecting the output from
the input while also grounding the output pin through ap-
proximately 2 kΩ.
The mute function is obtained during power-up of the device
or by sending any binary data of 0011 1111 and above
serially to the device. The device may be placed into mute at
any time during operation, allowing the designer to make the
mute command accessible to the end-user.
DC INPUTS
Although the µPot was designed to be used as an attenuator
for signals within the audio spectrum, it is also capable of
tracking and attenuating an input DC voltage. The device will
track voltages to either supply rail.
One point to remember about DC tracking is that with a
buffer at the output of the µPot, the resolution of DC tracking
will depend upon the gain configuration of that output buffer
and its supply voltage. Also, the output buffer’s supply volt-
age does not have to be the same as the µPot’s supply
voltage. Giving the buffer some gain can provide more reso-
lution when tracking small DC voltages.
FIGURE 8. Logarithmic Gain Amplifier Circuit
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