INPUT IMPEDANCE

The input impedance of a µPot is constant at a nominal

40 k

Ω. To eliminate any unwanted DC components from

propagating through the device it is common to use 1 µF in-

put coupling caps. This is not necessary, however, if the dc

offset from the previous stage is negligible. For higher per-

formance systems, input coupling caps are preferred.

OUTPUT IMPEDANCE

The output of a µPot varies typically between 25 k

Ω and

35 k

Ω and changes nonlinearly with step changes. Since a

µPot is made up of a resistor ladder network with a logarith-

mic attenuation, the output impedance is nonlinear. Due to

this configuration, a µPot cannot be considered as a linear

potentiometer, but can be considered only as a logarithmic

attenuator.

It should be noted that the linearity of a µPot cannot be mea-

sured directly without a buffer because the input impedance

of most measurement systems is not high enough to provide

the required accuracy. Due to the low impedance of the mea-

surement system, the output of the µPot would be loaded

down and an incorrect reading will result. To prevent loading

from occurring, a JFET input op amp should be used as the

buffer/amplifier. The performance of a µPot is limited only by

the performance of the external buffer/amplifier.

MUTE FUNCTION

One major feature of a µPot is its ability to mute the input sig-

nal to an attenuation level of 104dB as shown in

Figure 3.

This is accomplished internally by physically isolating the

output from the input while also grounding the output pin

through approximately 2 k

Ω.

The mute function is obtained during power-up of the device

or by sending any binary data of 01001111 and above (to

11111111) serially to the device. The device may be placed

into mute from a previous attenuation setting by sending any

of the above data. This allows the designer to place a mute

button onto his system which could cause a microcontroller

to send the appropriate data to a µPot and thus mute any or

all channels. Since this function is achieved through soft-

ware, the designer has a great amount of flexibility in config-

uring the system.

DC INPUTS

Although the µPot was designed to be used as an attenuator

for signals within the audio spectrum, the device is capable

of tracking an input DC voltage. The device will track DC

voltages to a diode drop above each 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. It should also be remembered that the

output buffer’s supply voltage does not have to be the same

as the µPot’s supply voltage. This could allow for more reso-

lution when DC tracking.

SERIAL DATA FORMAT

The LM1973 uses a 3-wire serial communication format that

is easily controlled by a microcontroller. The timing for the

3-wire set, comprised of DATA-IN, CLOCK, and LOAD/

SHIFT is shown in

Figure 2. Figure 9 exhibits in block dia-

gram form how the digital interface controls the tap switches

which select the appropriate attenuation level. As depicted in

Figure 2, the LOAD/SHIFT 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 each set of 16 data bits.

The serial data is comprised of 8 bits for channel selection

and 8 bits for attenuation setting. For both address data and

attenuation setting data, the MSB is sent first and the 8 bits

of address data are to be sent before the 8 bits of attenuation

data. Please refer to

Figure 7 to confirm the serial data for-

mat transfer process.

LM1973 Channel Attenuation

vs Digital Step Value

(Programmed 2.0dB Steps)

DS011958-15

FIGURE 5. LM1973 2.0dB Attenuation Step Scheme

DS011958-8

FIGURE 6. Mono Level Control with Panning Circuit

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