LM1973M/NOPB National Semiconductor, LM1973M/NOPB Datasheet - Page 9

• Manufacturer Part Number: LM1973M/NOPB
• Description: IC AUDIO ATTENUATR W/MUTE 20SOIC
• Manufacturer: National Semiconductor
• Type: Audio Attenuatorr

Specifications of LM1973M/NOPB

• Applications: Consoles, MIDI
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (7.5mm Width)
• Pin Count: 20
• Screening Level: Commercial
• Package Type: SOIC W
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM1973M; *LM1973M/NOPB; LM1973M

Application Information
DAISY-CHAIN CAPABILITY
Since the µPot’s digital interface is essentially a shift register,
multiple µPots can be programmed utilizing the same data
and load/shift lines. As shown in Figure 11, for an n-µPot
daisy-chain, there are 16n bits to be shifted and loaded for
the chain. The data loading sequence is the same for
n-µPots as it is for one µPot. First the LOAD/SHIFT line goes
low, then the data is clocked in sequentially while the pre-
ceding data in each µPot is shifted out the DATA-OUT pin to
the next µPot in the chain or to ground if it is the last µPot in
the chain. Then the LOAD/SHIFT line goes high; latching the
data into each of their corresponding µPots. The data is then
decoded according to the address (channel selection) and
the appropriate tap switch controlling the attenuation level is
selected.
CROSSTALK MEASUREMENTS
The crosstalk of a µPot as shown in the Typical Perfor-
mance Characteristics section was obtained by placing a
signal on one channel and measuring the level at the output
of another channel of the same frequency. It is important to
be sure that the signal level being measured is of the same
frequency such that a true indication of crosstalk may be
obtained. Also, to ensure an accurate measurement, the
measured channel’s input should be AC grounded through a
1 µF capacitor.
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 opera-
tion would cause a DC voltage “pop” when the output imped-
ance of the µPot changes. Again, this phenomenon is due to
the fact that the output impedance of the µPot is changing
(Continued)
FIGURE 9. µPot System Architecture
9
with step changes and a bipolar amplifier requires a finite
amount of DC bias current for its operation. As the imped-
ance 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 ocur-
ring. 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 AMPLIFIER
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 10, a µPot used in the feedback loop creates a
logarithmic gain amplifier. In this configuration the attenua-
tion levels from Table 1, now become gain levels with the
largest possible gain value being 76dB. For most applica-
tions 76dB of gain will cause signal clipping to occur, how-
ever, because of the µPot’s versatility the gain can be con-
trolled through programming such that the clipping level of
the system is never obtained. An important point to remem-
ber 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 perfor-
mance, a JFET input amplifier should be used.
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