Application Information
(Continued)
current op amp to keep DC shifts inaudible. Additionally, the
output of 碌Pot needs to see a high impedance to keep linear-
ity errors low.
Attenuation level changes cause changes in the output im-
pedance of a 碌Pot. 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, a 5 k鈩?impedance change
times a 1 碌A(chǔ) bias current results in a 5 mV DC 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
5 k鈩?change results in an inaudible 1 碌V DC shift. Since the
worst case output impedance changes are on the order of
several k鈩? a bias current much less than 1 碌A(chǔ) 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.
Without the use of a high input impedance (
>
1 M鈩? op amp
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
碌Pot 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 碌Pot鈥檚 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鈥?5.5V and also comes in a surface mount
package.
碌POT HALF-SUPPLY REFERENCING
The LM1971 operates off of a single supply, with half-supply
biasing supplied at the V
REF
IN terminal (Pin 1). The easiest
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 鈥渉olding鈥?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 碌F 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 碌F鈥?.1 碌F) could be added in parallel to the 10 碌F
capacitor.
DS012353-7
FIGURE 6. Higher Performance
Active Half-Supply Buffering
DS012353-8
FIGURE 7. Active Reference with Active Gain Buffering
LOGARITHMIC GAIN AMPLIFIER
The 碌Pot 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.
DS012353-9
FIGURE 8. Logarithmic Gain Amplifier Circuit
www.national.com
8
1
2
3
4
5
6
7
8
9
10
11
