To build active filters for high-frequency RF applications, designers may construct discrete designs using op amps and passives. When matched filters are required, this can become costly, as it requires precision resistors, capacitors, and high-bandwidth op amps. The LT1568 from Linear Technology offers a lower-cost, monolithic alternative to such discrete filters.
Aimed at channel anti-aliasing or reconstruction applications, this active-filter building-block IC integrates four low-noise op amps and four laser-trimmed capacitors. Combined with external programming resistors, these op amps can be configured as a pair of matched two-pole Butterworth low-pass filters suitable for filtering I/Q signals in RF applications (see the figure). In this configuration, the gain mismatch between channels is 0.02 dB typ. and 0.25 dB max. The op amps also may be cascaded to create a single four-pole low-pass filter.
When a single value of a resistor is used, the filters may be programmed for a cutoff frequency as high as 5 MHz. With unequal resistor values, cutoff frequencies as high as 10 MHz are possible, as are different low-pass or bandpass filter responses and values of gain.
Because the amplifiers are fabricated on a single piece of silicon, amplifier characteristics are inherently matched. This eliminates the need for multiple, high-bandwidth amplifiers. Moreover, the filter's four on-chip capacitors are trimmed using a proprietary technique to tolerances better than 1%. This combination of matched amplifiers and capacitors allows for tightly matched filters within each chip, as well as repeatable performance from part to part.
Signal-to-noise ratios greater than 90 dB are possible. The chip provides either single-ended or differential output. Housed in a 16-pin SSOP, it operates from 2.7- to ±5-V supplies. Cost is $4.25 per 1000.
Linear Technology Corp.