UAF42AU

The UAF42AU is a universal active filter IC from Texas Instruments, a product originally developed by Burr-Brown Corporation (acquired by TI in 2000). It represents one of the most popular and widely-used state-variable filter ICs in the industry, offering designers a complete high-performance filter building block that requires only a few external resistors to implement virtually any filter transfer function.

The UAF42 implements a classic state-variable filter topology using two operational amplifiers, two matched 1000pF capacitors, and precision resistor networks – all integrated on a single chip. This architecture provides four simultaneous filter outputs: low-pass, high-pass, band-pass, and band-stop (notch). The center frequency (f0), quality factor (Q), and gain at each output are independently programmable via external resistors, offering exceptional design flexibility.

The internal capacitors are matched to within 0.5%, and the resistor networks are laser-trimmed for high accuracy, ensuring consistent filter performance across devices. The integrated op-amps feature low offset voltage (±1mV typical), low drift (±5µV/°C), and low total harmonic distortion (0.003% typical), making the UAF42 suitable for precision audio and instrumentation applications.

Designers can implement Butterworth (maximally flat passband), Chebyshev (steep roll-off with passband ripple), Bessel (maximally flat group delay), or custom filter responses by selecting appropriate resistor values. The UAF42 can be cascaded to create higher-order filters. The device operates from dual supplies of ±4.5V to ±18V or a single 9V to 36V supply, accommodating a wide range of system voltage requirements.

Available in a 14-pin SOIC package and specified over the industrial temperature range of -40°C to +85°C, the UAF42AU is a versatile building block for analog signal processing in instrumentation, audio, communications, and medical equipment.

The UAF42 universal active filter is based on the state-variable (KHN) filter topology, which operates as follows:

1. State-Variable Topology: The filter consists of two integrators and a summing amplifier arranged in a feedback loop. The first integrator produces the band-pass output, and the second integrator (in cascade) produces the low-pass output. By summing the input with the low-pass and band-pass signals, the high-pass output is derived. A notch output is obtained by summing the low-pass and high-pass outputs.

2. Center Frequency (f0): The natural or center frequency of the filter is determined by the two integrator time constants. With the internal 1000pF capacitors, f0 is set by external frequency-determining resistors (Rf). The formula is approximately: f0 = 1 / (2π × Rf × Cf), where Cf = 1000pF. Center frequencies from sub-Hertz up to 100kHz are practical.

3. Quality Factor (Q): The Q factor (selectivity) is set by the ratio of certain external resistors. High Q values (up to 500) are achievable, allowing very narrow band-pass or sharp notch filters. However, high Q designs require careful attention to component tolerance and stability.

4. Gain: Each output (low-pass, band-pass, high-pass) has its own gain, which is determined by the external resistor network. Gains from less than unity up to several hundred are possible, depending on the specific output and Q setting.

5. Notch Filter: The UAF42 includes a summing amplifier specifically designed to combine the low-pass and high-pass outputs to create a notch (band-reject) filter response. This eliminates the need for an external op-amp for notch generation.

6. Cascading: Multiple UAF42 devices can be cascaded to create higher-order filters (4th, 6th, 8th order, etc.). Each stage can be designed for a specific Q and frequency contribution to achieve the overall filter response (e.g., 4th-order Butterworth).

Note: Pinout is for the 14-pin SOIC package. The UAF42 is also available in PDIP-14 and DIP-14 packages. Refer to the datasheet for recommended connection diagrams for different filter types (Butterworth, Chebyshev, Bessel).

• Instrumentation and Measurement: Anti-aliasing filters for data acquisition systems, band-pass filters for lock-in amplifiers, and precision low-pass filters for DC measurement systems where the UAF42’s low offset and drift are critical.
• Audio Processing and Equalization: Active crossover networks for multi-way speaker systems, parametric equalizers, and tone control circuits in professional and consumer audio equipment, benefiting from the low THD of 0.003%.
• Medical and Biosignal Processing: ECG, EEG, and EMG signal conditioning where specific frequency bands must be precisely filtered, such as 60Hz notch filters for line noise rejection and band-pass filters for signal extraction.
• Communications Signal Conditioning: Channel select filters, IF filters, and tone detection circuits in telecommunications equipment, modems, and radio systems where precise band-pass or notch characteristics are needed.
• Industrial Sensor Signal Conditioning: Filtering of sensor outputs from strain gauges, pressure sensors, temperature sensors, and accelerometers to remove noise and isolate signals of interest in industrial control environments.
• Power Line Noise Rejection: 50Hz/60Hz notch filters for instrumentation and audio systems, where the UAF42’s high Q capability enables narrow, deep notch response with minimal effect on adjacent frequencies.
• Test and Measurement Equipment: Oscilloscopes, spectrum analyzers, and signal generators requiring selectable filter responses for bandwidth limiting, signal shaping, and noise reduction.