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Maxim > Design
support > App notes
> Audio
Circuits > APP 3492

May 23, 2005

Keywords: Fading Ciruit, Audio, Fader, Volume,
Balance, Digital Pot, Digital Potentiometer
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APPLICATION NOTE 3492

Audio Fading Circuit Using the MAX5456/MAX5457

Abstract: This
application note describes a method for implementing volume,
balance, and fader control with a simple pushbutton interface
and three MAX5456/MAX5457 stereo logarithmic, taper digital
potentiometers.

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The
MAX5456/MAX5457 stereo, audio
taper potentiometers are dual, logarithmic
taper digital potentiometers with a simple pushbutton
interface that controls volume and balance in audio
applications. Each potentiometer has 32 tap points and
replaces mechanical potentiometers. The MAX5456/MAX5457 use
Maxim's proprietary SmartWiper™ control to eliminate the need
for a microcontroller to increase wiper transition rate.
Holding active-low DN / active-low BAL or active-low UP /
active-low BAL1 low for more than 1s advances the wiper at a
rate of 4Hz for the first 4s, then 16Hz thereafter. In
volume-control mode, the MAX5456/MAX5457 wipers move
simultaneously, adjusting the volume of both channels while
maintaining the balance separation between each channel. In
the balance-control mode, the MAX5456/MAX5457 adjust the
balance between channel 0 and channel 1, while maintaining the
set volume. Additional features include: single- or
dual-supply operation, clickless switching, mute function to
-90dB (typ), power-on reset to -12dB wiper position, (THD
+ N) = 0.01%, and small 16-pin QSOP/TQFN
packages.

While using digital potentiometers with AC
signals, certain considerations need to be addressed. Firstly,
it is important to understand how to bias an AC signal to
within the DC supply range. Secondly, while the digital pot
steps are usually designed to give equal-value resistive
increments, as a byproduct of the process variation, the total
end-to-end resistance
can vary widely from part to part—as much as ±30% in some
types of digital pots. (Refer to application note 161, "Using
a DS1802 Push-Button Digital Potentiometer to Create an Audio
Preamp with Attenuator" and application note 1828, "Audio
Gain using Digital Potentiometers" for more
information.)

For
Larger Image
Figure 1. Circuit block diagram for
implementing volume, balance, and fader control using three
MAX5456/MAX5457 ICs.

For applications requiring
fader control for multispeaker configurations, use additional
MAX5456/MAX5467's to implement this design.

The MAX5457
switches between volume- and balance-control modes by toggling
the active-low MODE input. An LED
output indicates when the device is in volume- or
balance-control mode. For the application circuit shown in
Figure 1, U1 controls the balance and volume of the
stereo input signal and outputs the left and right signals to
the fader control formed by U2 and U3. Change the master
volume with the active-low DN / active-low BAL0 or active-low
UP / active-low BAL1 buttons of U1 in volume mode. To change
the balance, toggle active-low MODE to switch
to balance mode and use the active-low DN / active-low BAL0 or
active-low UP / active-low BAL1.

U2 and U3 form the
fader control for the left and right channels respectively. By
adjusting the balance of U2, the left signal from U1 fades
between L0 and L1. Similarly, by adjusting the balance of U3,
the right signal from U1 fades between R0 and R1. Refer to the
MAX5456/MAX5457 data sheet for volume- and balance-control
operation. The volume-control function in U2 or U3 is
typically not used, because U1 handles the volume adjustment.
Adjusting volume in U2 and U3, however, can sometimes be
useful to compensate for mismatches in the audio
path.

The op amps
preceding U1 in the application circuit amplify line-level or
high-impedance input signals from a stereo audio source
to drive the MAX5456/MAX5457. Given that the minimum
end-to-end resistance of the MAX5456/MAX5457 ICs equal 7KΩ,
the worst-case load impedance
looking into the left or right input of U1 is 7KΩ (worst case)
and 10KΩ (nominal). The two amplifiers on the outputs of U1
are used as buffers and limit the load to minimize THD.
Finally, the four amplifiers on the outputs of U2 and U3
minimize wiper current
and amplify the signal to drive the speakers.

SmartWiper
is a trademark of Maxim Integrated Products, Inc.

Related parts

MAX5456

Stereo Audio Taper
Potentiometers with Pushbutton Interface

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samples

MAX5457

Stereo Audio Taper
Potentiometers with Pushbutton Interface

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Download,
PDF format (36kB)
APP 3492: May 23, 2005

AN3492, AN 3492, APP3492,
Appnote3492, Appnote 3492

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