LM1972

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SNAS094D – APRIL 1995 – REVISED MARCH 2013

CLICKS AND POPS

So, why is that output buffer needed anyway? There are three answers to this question, all of which are

important from a system point of view.

The first reason to utilize a buffer/amplifier at the output of a

μPot is to ensure that there are no audible clicks or

pops due to attenuation step changes in the device. If an on-board bipolar op amp had been used for the output

stage, its requirement of a finite amount of DC bias current for operation would cause a DC voltage “pop” when

the output impedance of the

μPot changes. Again, this phenomenon is due to the fact that the output impedance

of the

μPot is changing with step changes and a bipolar amplifier requires a finite amount 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”.

Secondly, the

μPot has no drive capability, so any desired gain needs to be accomplished through a buffer/non-

inverting amplifer.

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 AMPUFIER

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 78dB. For most applications 78dB 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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