Texas Instruments LM1972MX/NOPB

Overview

LM1972MX/NOPB from Texas Instruments (formerly National Semiconductor) is a digitally controlled 2-channel CMOS audio attenuator with logarithmic 0.5 dB/1.0 dB step resolution, 78 dB attenuation range (excluding mute), 104 dB mute isolation, and TTL/CMOS-compatible 3-wire serial interface. It serves as a precision volume control in professional audio signal paths requiring pop-free transitions and high channel separation.

For engineers reviewing the LM1972MX/NOPB datasheet, LM1972MX/NOPB pinout, LM1972MX/NOPB application, or LM1972MX/NOPB equivalent, key selection criteria include its dual-channel mute capability, daisy-chainable serial interface, ±0.25 dB differential attenuation accuracy, 110 dB SNR (A-weighted), and compatibility with ±6 V dual-supply or +12 V single-supply configurations.

Technical Context

The LM1972MX/NOPB implements two independent resistor ladder networks - one per channel - each with digitally addressed tap switches that select attenuation levels per Table 1 of the datasheet. Its architecture maintains constant 40 kΩ input impedance across all steps while output impedance varies nonlinearly (25–35 kΩ), necessitating JFET-input buffering to preserve linearity and prevent loading-induced errors.

Control logic uses an 8-bit channel address followed by 8-bit attenuation data over a rising-edge clocked 3-wire interface (CLOCK, LOAD/SHIFT, DATA-IN); DATA-OUT enables daisy-chaining up to n devices with synchronized 16n-bit shift-and-latch operation. Mute is triggered by any data word ≥ 0x7F (127), physically disconnecting input from output and grounding the output through ~2 kΩ.

Key Specifications

Pinout & Package

LM1972MX/NOPB is housed in a 20-pin plastic dual-in-line package (DIP), NS package number N20A, with through-hole mounting and 0.300-inch body width. Pin 1 is index-marked; pins 1, 3, 5, 6, 14, 16 are AC ground connections; pins 4 and 20 are analog inputs (IN1, IN2); pins 2 and 17 are analog outputs (OUT1, OUT2); pins 7 and 18 are negative supply (VSS1, VSS2); pins 13 and 15 are positive supply (VDD1, VDD2); pin 8 is logic ground; pins 9–12 handle serial interface (CLOCK, LOAD/SHIFT, DATA-IN, DATA-OUT).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar digitally controlled audio attenuator applications.

Compared with PGA2311U and DS1669S+, the LM1972MX/NOPB uniquely delivers dual-channel 78 dB logarithmic attenuation with hardware mute, daisy-chainable 3-wire interface, and 100 dB channel separation - making it optimal for professional audio routing where channel independence and silent muting are mandatory.

Availability

LM1972MX/NOPB is available at Aetrix Electronics and suitable for automated studio mixing consoles, music reproduction systems, and sound reinforcement systems requiring stable component supply, long-term lifecycle support, and traceable sourcing for industrial audio equipment manufacturing.

Supply support for LM1972MX/NOPB includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments acquired National Semiconductor in 2011 and maintains its legacy precision analog portfolio, including high-performance audio ICs designed for studio, broadcast, and pro-audio applications.

The LM1972MX/NOPB belongs to National Semiconductor's μPot™ family of digitally controlled analog attenuators, engineered specifically for noise-critical, multi-channel audio signal conditioning where mechanical potentiometers fail to meet reliability or precision requirements.

FAQ

What is the maximum clock frequency supported by the LM1972MX/NOPB serial interface?

The LM1972MX/NOPB supports a maximum clock frequency of 2 MHz, as specified in its Electrical Characteristics table. This allows rapid attenuation updates - up to 2 million steps per second - though practical audio control rates typically operate well below this limit to ensure reliable data capture and minimize EMI. The LM1972MX/NOPB's timing diagram confirms setup/hold requirements relative to the rising clock edge.

Does the LM1972MX/NOPB require external op-amps for proper operation?

Yes, the LM1972MX/NOPB requires an external JFET-input op-amp (e.g., LF412ACN) at each output to prevent loading-induced linearity errors, eliminate clicks/pops during attenuation changes, and provide necessary drive capability. Its output impedance varies nonlinearly (25–35 kΩ), and direct connection to low-impedance loads degrades attenuation accuracy and SNR. The LM1972MX/NOPB datasheet explicitly mandates this buffer configuration.

Can the LM1972MX/NOPB operate from a single +12 V supply?

Yes, the LM1972MX/NOPB supports single-supply operation: tie pins 7 and 18 (VSS1/VSS2) to ground and apply +12 V to pins 13 and 15 (VDD1/VDD2). Its Absolute Maximum Ratings specify VDD–VSS up to 15 V, and Operating Ratings confirm 4.5 V to 12 V operation. In single-supply mode, input signals must be biased within the common-mode range, and AC coupling is recommended to block DC offsets.

How does the LM1972MX/NOPB achieve 104 dB mute attenuation?

The LM1972MX/NOPB achieves 104 dB mute attenuation by physically disconnecting the analog input from the output path and simultaneously grounding the output pin through an internal ~2 kΩ resistor. This dual-action method - signal isolation plus active grounding - exceeds standard switch-off attenuation and prevents residual leakage or capacitive coupling. Mute is activated by sending any serial data word ≥ 0x7F (127) to the LM1972MX/NOPB.

Is the LM1972MX/NOPB pin-compatible with other μPot™ devices like the LM1971?

No, the LM1972MX/NOPB is not pin-compatible with the LM1971 or other μPot™ variants. The LM1972MX/NOPB uses a 20-pin DIP package (N20A) with dedicated dual supplies, dual analog I/O, and 3-wire serial interface pins; the LM1971 is a 16-pin device with different pin assignments and functional allocation. Substitution requires PCB layout revision and firmware adaptation due to distinct pinout, register map, and channel count.