Datasheet

LM1973
SNAS093B –DECEMBER 1994–REVISED MARCH 2013 www.ti.com
of DC bias current for its operation. As the impedance changes, so does the DC bias current and thus there
is a DC voltage “pop”.
2. Secondly, the μPot has no drive capability, so any desired gain needs to be accomplished through a
buffer/non-inverting amplifer.
3. Third, the output of a μPot needs to see a high impedance to prevent loading and subsequent linearity errors
from ocurring. A JFET input buffer provides a high input impedance to the output of the μPot so that this
does not occur.
Clicks and pops can be avoided by using a JFET input buffer/amplifier such as an LF412ACN. The LF412 has a
high input impedance and exhibits both a low noise floor and low THD+N throughout the audio spectrum which
maintains signal integrity and linearity for the system. The performance of the system solution is entirely
dependent upon the quality and performance of the JFET input buffer/amplifier.
LOGARITHMIC GAIN AMPLIFIER
The μPot is capable of being used in the feedback loop of an amplifier, however, as stated previously, the output
of the μPot needs to see a high impedance in order to maintain its high performance and linearity. Again, loading
the output will change the values of attenuation for the device. As shown in Figure 23, a μPot used in the
feedback loop creates a logarithmic gain amplifier. In this configuration the attenuation levels from Table 1, now
become gain levels with the largest possible gain value being 76dB. For most applications 76dB of gain will
cause signal clipping to occur, however, because of the μPot's versatility the gain can be controlled through
programming such that the clipping level of the system is never obtained. An important point to remember is that
when in mute mode the input is disconnected from the output. In this configuration this will place the amplifier in
its open loop gain state, thus resulting in severe comparator action. Care should be taken with the programming
and design of this type of circuit. To provide the best performance, a JFET input amplifier should be used.
Figure 23. Digitally-Controlled Logarithmic Gain Amplifier Circuit
Figure 24. n-μPot Daisy-Chained Circuit
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