current op amp to keep DC shifts inaudible. Additionally, the

output of µPot needs to see a high impedance to keep linear-

ity errors low.

Attenuation level changes cause changes in the output im-

pedance of a µPot. Output impedance changes in the pres-

ence of a large input bias current for a buffer/amplifier will

cause a DC shift to occur. Neglecting amplifier gains and

speaker sensitivities, the audibility of a DC shift is dependent

upon the output impedance change times the required input

bias current. As an example,a5k

Ω impedance change

timesa1µA bias current results ina5mVDC shift; a level

that is barely audible without any music material in the sys-

tem. An op amp with a bias current of 200 pA for the same

5k

Ω change results in an inaudible 1 µV DC shift. Since the

worst case output impedance changes are on the order of

several k

Ω, a bias current much less than 1 µA is required for

highest performance. In order to further quantify DC shifts,

please refer to the Output Impedance vs Attenuation graph

in the Typical Performance Characteristics section and re-

late worst case impedance changes to the selected buffer/

amplifier input bias current.

Ω)opamp

for the buffer/amplifier, loading will occur that causes linearity

errors in the signal. To ensure the highest level of perfor-

mance, a JFET or CMOS input high input impedance op

amp is required.

One common application that requires gain at the output of a

µPot is input signal volume control. Depending upon the in-

put source material, the LM1971 provides a means of con-

trolling the input signal level. With a supply voltage range of

4.5V to 12V, the LM1971 has the ability of controlling fairly

inconsistent input source signal levels. Using an op amp with

gain at the µPot’s output, as shown in

Figure 7, will also al-

low the system dynamic range to be increased. JFET op

amps like the LF351 and the LF411 are well suited for this

application. If active half-supply buffering is also desired,

dual op amps like the LF353 and the LF412 could be used.

For low voltage supply applications, op amps like the CMOS

LMC6041 are preferred. This part has a supply operating

range from 4.5V–15.5V and also comes in a surface mount

package.

µPOT HALF-SUPPLY REFERENCING

The LM1971 operates off of a single supply, with half-supply

biasing supplied at the V

and most cost effective method of providing this half-supply

is a simple resistor divider and bypass capacitor network

shown in

Figure 1. The capacitor not only stabilizes the

half-supply node by “holding” the voltage nearly constant,

but also decouples high frequency signals on the supply to

ground. Signal feedthrough, power supply ripple and fluctua-

tions that are not properly filtered could cause the perfor-

mance of the LM1971 to be degraded.

A more stable half-supply node can be obtained by actively

buffering the resistor divider network with a voltage follower

as shown in

Figure 6. Supply fluctuations are then isolated

by the high input impedance/low output impedance mis-

match associated with effective filtering. Since the LM1971 is

a single channel device, using a dual JFET input op amp is

optimum for both output buffering and half-supply biasing.

A 10 µF capacitor or larger is recommended for better

half-supply stabilization. For added rejection of higher fre-

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

Fig-

ure 8. In this configuration the attenuation levels from Table

1 become gain levels with the largest possible gain value be-

ing 62 dB. For most applications, 62 dB of gain will cause

signal clipping to occur. However, this can be controlled

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 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 imped-

ance, low input bias current op amp should be used.

DS012353-7

FIGURE 6. Higher Performance

Active Half-Supply Buffering

DS012353-8

FIGURE 7. Active Reference with Active Gain Buffering

DS012353-9

FIGURE 8. Logarithmic Gain Amplifier Circuit

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