LM1971
LM1971 Overture Audio Attenuator Series Digitally Controlled 62 dB Audio
Attenuator with/Mute
Literature Number: SNAS104A
LM1971Overture™Audio Attenuator Series
Digitally Controlled 62 dB Audio Attenuator with/Mute
General Description
The LM1971 is a digitally controlled single channel audio
attenuator fabricated on a CMOS process. Attenuation is
variable in 1 dB steps from 0 dB to −62 dB. A mute function
disconnects the input from the output, providing over 100 dB
of attenuation.
The performance of the device is exhibited by its ability to
change attenuation levels without audible clicks or pops. In
addition, the LM1971 features a low Total Harmonic Distor-
tion (THD) of 0.0008%, and a Dynamic Range of 115 dB,
making it suitable for digital audio needs. The LM1971 is
available in both 8-pin plastic DIP or SO packages.
The LM1971 is controlled by a TTL/CMOS compatible 3-wire
serial digital interface. The active low LOAD line enables the
data input registers while the CLOCK line provides system
timing. Its DATA pin receives serial data on the rising edge of
each CLOCK pulse, allowing the desired attenuation setting
to be selected.
Key Specifications
nTotal harmonic distortion 0.0008% (typ)
nFrequency response >200 kHz (−3 dB) (typ)
nAttenuation range (excluding mute) 62 dB (typ)
nDynamic range 115 dB (typ)
nMute attenuation 102 dB (typ)
Features
n3-wire serial interface
nMute function
nClick and pop free attenuation changes
n8-pin plastic DIP and SO packages available
Applications
nCommunication systems
nCellular Phones and Pagers
nPersonal computer audio control
nElectronic music (MIDI)
nSound reinforcement systems
nAudio mixing automation
Typical Application
Overture™is a trademark of National Semiconductor Corporation.
01235301
FIGURE 1. Typical Audio Attenuator Application Circuit
September 2002
LM1971 Overture Audio Attenuator Series Digitally Controlled 62 dB Audio Attenuator with Mute
© 2002 National Semiconductor Corporation DS012353 www.national.com
Connection Diagram
Dual-In-Line Plastic or Surface Mount Package
01235302
Top View
Order Number LM1971M or LM1971N
See NS Package Number M08A or N08E
LM1971
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Absolute Maximum Ratings (Notes 1,
2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage, V
DD
15V
Voltage at any pin (GND −0.2V) to (V
DD
+0.2V)
ESD Susceptibility (Note 4) 3000V
Soldering Information
N Package (10s)
M Package
Vapor Phase (60s)
Infrared (15s)
260˚C
215˚C
220˚C
Power Dissipation (Note 3) 150 mW
Junction Temperature 150˚C
Storage Temperature −65˚C to +150˚C
Operating Ratings (Notes 1, 2)
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤+85˚C
Thermal Resistance
M08A Package, θ
JA
167˚C/W
N08E Package, θ
JA
102˚C/W
Supply Voltage 4.5V to 12V
Electrical Characteristics (Notes 1, 2)
The following specifications apply for V
DD
= +12V (V
REF
IN = +6V), V
IN
= 5.5 V
pk
, and f = 1 kHz, unless otherwse specified.
Limits apply for T
A
= 25˚C. Digital inputs are TTL and CMOS compatible.
Symbol Parameter Conditions
LM1971 Units
(Limits)
Typical
(Note 5)
Limit
(Note 6)
I
S
Supply Current Digital Inputs Tied to 6V 1.8 3 mA (max)
THD Total Harmonic Distortion V
IN
= 0.5V
pk
@0 dB Attenuation 0.0008 0.003 % (max)
e
IN
Noise Input is AC Grounded
@−12 dB Attenuation
A-Weighted (Note 7)
4.0 µV
DR Dynamic Range Referenced to Full Scale = +6 V
pk
115 dB
A
M
Mute Attenuation 102 96 dB (min)
Attenuation Step Size Error 0 dB to −62 dB 0.009 0.2 dB (max)
Absolute Attenuation Attenuation @0dB
Attenuation @−20 dB
Attenuation @−40 dB
Attenuation @−60 dB
Attenuation @−62 dB
0.1
−20.3
−40.5
−60.6
−62.6
0.5
−19.0
−38.0
−57.0
−59.0
dB (min)
dB (min)
dB (min)
dB (min)
dB (min)
I
LEAK
Analog Input Leakage Current Input is AC Grounded 5.8 100 nA (max)
Frequency Response 20 Hz–100 kHz ±0.1 dB
R
IN
AC Input Impedance Pin 8, V
IN
= 1.0 V
pk
, f = 1 kHz 40 20
60
kΩ(min)
kΩ(max)
I
IN
Input Current @Pins 4, 5, 6 @0V <V
IN
<5V 1.0 100 nA (max)
f
CLK
Clock Frequency 3 2 MHz (max)
V
IH
High-Level Input Voltage @Pins 4, 5, 6 2.0 V (min)
V
IL
Low-Level Input Voltage @Pins 4, 5, 6 0.8 V (max)
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which
guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit
is given, however, the typical value is a good indication of device performance.
Note 2: All voltages are measured with respect to the GND pin (pin 3), unless otherwise specified.
Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX,θJA, and the ambient temperature TA. The maximum
allowable power dissipation is PD=(T
JMAX –T
A)/θJA or the number given in the Absolute Maximum Ratings, whichever is lower. For the LM1971N and LM1971M,
TJMAX = +150˚C, and the typical junction-to-ambient thermal resistance, θJA, when board mounted is 102˚ C/W and 167˚ C/W, respectively.
Note 4: Human body model, 100 pF discharged through a 1.5 kΩresistor.
Note 5: Typicals are measured at 25˚C and represent the parametric norm.
Note 6: Limits are guarantees that all parts are tested in production to meet the stated values.
Note 7: Due to production test limitations, there is no limit for the Noise test. Please refer to the noise measurements in the Typical Performance Characteristics
section.
LM1971
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Pin Descriptions
V
REF
IN (1): The V
REF
IN pin provides the reference for the
analog input signal. This pin should be biased at half of the
supply voltage, V
DD
, as shown in Figure 1 and Figure 6.
OUT (2): The attenuated analog output signal comes from
this pin.
GND (3): The GND pin references the digital input signals
and is the lower voltage reference for the IC. Typically this
pin would be labeled “V
SS
” but the ground reference for the
digital logic input control is tied to this same point. With a
higher pin-count there would generally be separate pins for
these functions; V
SS
and Logic Ground. It is intended that
the LM1971 always be operated using a single voltage sup-
ply configuration, for which pin 3 (GND) should always be at
system ground. If a bipolar or split-supply configuration are
desired, level shifting circuitry is needed for the digital logic
control pins as they would be referenced through pin 3 which
would be at the negative supply. It is highly recommended,
however, that the LM1971 be used in a unipolar or
single-supply configuration.
LOAD (4): The LOAD input accepts a TTL or CMOS level
signal. This is the enable pin of the device, allowing data to
be clocked in while this input is low (0V). The GND pin is the
reference for this signal.
DATA (5): The DATA input accepts a TTL or CMOS level
signal. This pin is used to accept serial data from a micro-
controller that will be latched and decoded to change the
channel’s attenuation level. The GND pin is the reference for
this signal.
CLOCK (6): The CLOCK input accepts a TTL or CMOS level
signal. The clock input is used to load data into the internal
shift register on the rising edge of the input clock waveform.
The GND pin is the reference for this signal.
V
DD
(7): The positive voltage supply should be placed to this
pin.
IN (8): The analog input signal should be placed to this pin.
Typical Performance Characteristics
Supply Current vs
Supply Voltage
Supply Current vs
Temperature
01235310 01235311
Noise Floor
Analog Measurement THD+NvsFreq and Amp
01235312 01235313
LM1971
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Typical Performance Characteristics (Continued)
THD+NvsFreq and Amp Noise Floor Spectrum by FFT
01235314 01235315
THD+NvsAmplitude THD+NvsAmplitude
01235316 01235317
Mute Attenuation
vs Frequency THD vs Freq by FFT
01235318 01235319
LM1971
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Typical Performance Characteristics (Continued)
THD vs Freq by FFT
Output Impedance vs
Attenuation Level
01235320 01235321
Application Information
SERIAL DATA FORMAT
The LM1971 uses a 3-wire serial communication format that
is easily controlled by a microcontroller. The timing for the
3-wire set, comprised of DATA, CLOCK, and LOAD is shown
in Figure 2. As depicted in Figure 2, the LOAD line is to go
low at least 150 ns before the rising edge of the first clock
pulse and is to remain low throughout the transmission of the
16 data bits. The serial data is composed of an 8-bit address,
which must always be set to 0000 0000 to select the single
audio channel, and 8 bits for attenuation setting. For both
address data and attenuation setting data, the MSB is sent
first with the address data preceding the attenuation data.
Please refer to Figure 3 to confirm the serial data format
transfer process.
Table 1 shows the various Address and Data byte values for
different attenuation settings. Note that Address bytes other
than 0000 0000 are ignored.
µPOT SYSTEM ARCHITECTURE
The µPot’s digital interface is essentially a shift register
where serial data is shifted in, latched, and then decoded.
Once new data is shifted in, the LOAD line goes high,
latching in the new data. The data is then decoded and the
appropriate switch is activated to set the desired attenuation
level. This process is continued each and every time an
attenuation change is made. When the µPot is powered up,
it is placed into the Mute mode.
µPOT DIGITAL COMPATIBILITY
The µPot’s digital interface section is compatible with TTL or
CMOS logic. The shift register inputs act upon a threshold of
two diode drops above the ground level (Pin 3) or approxi-
mately 1.4V.
TABLE 1. Attenuator Register Set Description
Address Register (Byte 0)
MSB LSB
A7–A0
0000 0000 Channel 1
0000 0001 Ignored
0000 0010 Ignored
Data Register (Byte 1)
Contents Attenuation (dB)
MSB LSB
D7–D0
0000 0000 0.0
0000 0001 1.0
0000 0010 2.0
0000 0011 3.0
::::: ::
0001 0000 16.0
0001 0001 17.0
0001 0010 18.0
0001 0011 19.0
::::: ::
0011 1101 61.0
0011 1110 62.0
0011 1111 96 (Mute)
0100 0000 96 (Mute)
::::: ::
1111 1110 96 (Mute)
1111 1111 96 (Mute)
LM1971
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Application Information (Continued)
µPOT LADDER ARCHITECTURE
The µPot contains a chain of R1/R2 resistor dividers in a
ladder form, as shown in Figure 4. Each R1 is actually a
series of 8 resistors, with a CMOS switch that taps into the
resistor chain according to the attenuation level chosen. For
any given attenuation setting, there is only one CMOS switch
closed (no paralleling of ladders). The input impedance
therefore remains constant, while the output impedance
changes as the attenuation level changes. It is important to
note that the architecture is a series of resistor dividers, and
not a straight, tapped resistor, so the µPot is not a variable
resistor; it is a variable voltage divider.
01235303
*Note: Load and clock falling edges can be coincident, however, the clock falling edge cannot be delayed more than 20 ns from the falling edge of load. It is
preferrable that the falling edge of clock occurs before the falling edge of load.
FIGURE 2. Timing Diagram
01235304
FIGURE 3. Serial Data Format Transfer Process
01235305
FIGURE 4. Resistor Ladder Architecture
LM1971
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Application Information (Continued)
ATTENUATION STEP SCHEME
The fundamental attenuation step scheme for the LM1971 is
shown in Figure 5. It is also possible to obtain any integer
value attenuation step through programming, in addition to
the 2 dB and 4 dB steps shown in Figure 5. All higher
attenuation step schemes can have clickless and popless
performance. Although it is possible to “skip” attenuation
points by not sending all of the data, clickless and popless
performance will suffer. It is highly recommended that all of
the data points should be sent for each attenuation level.
This ensures flawless operation and performance when
making steps larger than 1 dB.
INPUT IMPEDANCE
The input impedance of a µPot is constant at a nominal
40 kΩ. Since the LM1971 is a single-supply operating de-
vice, it is necessary to have both input and output coupling
caps as shown in Figure 1. To ensure full low-frequency
response,a1µFcoupling cap should be used.
OUTPUT IMPEDANCE
The output impedance of a µPot varies typically between
25 kΩand 35 kΩand changes nonlinearly with step
changes. Since a µPot is made up of a resistor ladder
network with logarithmic attenuation, the output impedance
is nonlinear. Due to this configuration, a µPot cannot be
considered as a linear potentiometer; it is a logarithmic
attenuator.
The linearity of a µPot cannot be measured directly without a
buffer because the input impedance of most measurement
systems is not high enough to provide the required accuracy.
The lower impedance of the measurement system would
load down the output and an incorrect reading would result.
To prevent loading, a JFET input op amp should be used as
the buffer/amplifier.
OUTPUT BUFFERING
There are two performance issues to be aware of that are
related to a µPot’s output stage. The first concern is to
prevent audible clicks with attenuation changes, while the
second is to prevent loading and subsequent linearity errors.
The output stage of a µPot needs to be buffered with a low
input bias current op amp to keep DC shifts inaudible. Addi-
tionally, the output of µPot needs to see a high impedance to
keep linearity 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, a 5 kΩimpedance change
timesa1µAbias current results ina5mVDCshift; a level
that is barely audible without any music material in the
system. An op amp with a bias current of 200 pA for the
same 5 kΩchange results in an inaudible 1 µV DC shift.
Since the worst case output impedance changes are on the
order of several kΩ, 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 Attenu-
ation graph in the Typical Performance Characteristics
section and relate worst case impedance changes to the
selected buffer/amplifier input bias current.
Without the use of a high input impedance (>1MΩ)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
input source material, the LM1971 provides a means of
controlling 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
allow 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
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 “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-
LM 1971 Channel Attenuation
vs Digital Step Value
(1 dB, 2 dB, and 4 dB Steps)
01235306
FIGURE 5. LM1971 Attenuation Step Scheme
LM1971
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Application Information (Continued)
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
Figure 8. In this configuration the attenuation levels from
Table 1 become gain levels with the largest possible gain
value being 62 dB. For most applications, 62 dB of gain will
cause signal clipping to occur. However, this can be con-
trolled 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 impedance, low input bias current op amp should
be used.
MUTE FUNCTION
A major feature of the LM1971 is its ability to mute the input
signal to an attenuation level of 102 dB. This is accom-
plished internally by physically disconnecting the output from
the input while also grounding the output pin through ap-
proximately 2 kΩ.
The mute function is obtained during power-up of the device
or by sending any binary data of 0011 1111 and above
serially to the device. The device may be placed into mute at
any time during operation, allowing the designer to make the
mute command accessible to the end-user.
DC INPUTS
Although the µPot was designed to be used as an attenuator
for signals within the audio spectrum, it is also capable of
tracking and attenuating an input DC voltage. The device will
track voltages to either supply rail.
One point to remember about DC tracking is that with a
buffer at the output of the µPot, the resolution of DC tracking
will depend upon the gain configuration of that output buffer
and its supply voltage. Also, the output buffer’s supply volt-
age does not have to be the same as the µPot’s supply
voltage. Giving the buffer some gain can provide more reso-
lution when tracking small DC voltages.
01235307
FIGURE 6. Higher Performance
Active Half-Supply Buffering
01235308
FIGURE 7. Active Reference with Active Gain Buffering
01235309
FIGURE 8. Logarithmic Gain Amplifier Circuit
LM1971
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Physical Dimensions inches (millimeters)
unless otherwise noted
Order Number LM1971M
8-Lead (0.150" Wide) Molded Small Outline Package, JEDEC
NS Package Number M08A
LM1971
www.national.com 10
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Order Number LM1971N
8-Lead (0.300" Wide) Molded Dual-In-Line Package
NS Package Number N08E
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www.national.com
LM1971 Overture Audio Attenuator Series Digitally Controlled 62 dB Audio Attenuator with Mute
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