Dual matched filter is unmatched for performance

As challenging as designing low-noise filters can be, the tolerances of even the most expensive reactive components make realizing a tightly matched filter pair well-nigh impossible in a production environment. Yet, some filter applications, such as antialiasing and reconstruction for I/Q or other quadrature-related channels, depend on good offset, amplitude, and group-delay agreement between filter pairs.

Such circuits depend not only on the match between passive networks, but also on the amplifier GBWs (gain-bandwidth products). According to Linear Technology Design Manager Nello Seva-stopolous, a good heuristic for filter design is GBW≥500 f_CQ. For example, a filter with a Q of 3 and a cutoff frequency of 1 MHz requires amplifiers with 1.5-GHz GBW to keep the amplifier's dominant pole from affecting the filter's behavior. For single-channel filters, you can calculate and compensate for limited GBW on the filter's passive-component values and take the part-to-part GBW scatter as a performance tolerance. Until recently, however, tightly matched filter pairs required extremely fast amplifiers and had to suffer their high cost and power dissipation.

The problem for filter pairs is that the op-amp manufacturers specify only the minimum GBWs; the actual performance can vary substantially from amplifier to amplifier and between amplifiers over time and temperature. GBW mismatches between channels result in group-delay differences that appear well into the passband. In quadrature-related channel pairs, the group-delay errors result in signal attenuation and a consequent SNR degradation.

Linear Technology addresses this problem with its LT1568, which integrates a closely matched, low-noise, two-pole filter pair that can implement Bessel, Butterworth, Chebyshev, or elliptic responses for cutoff frequencies of 0.2 to 10 MHz (Picture). The $4.25 (1000) SSOP-16 costs less than you might pay for the op amps in a discrete design with similar performance but integrates the filter capacitors, which the manufacturer trims to ±0.75%—a tolerance simply not available in off-the-shelf parts. The LT1568's amplifiers are trimmed to match and track their GBW over time and temperature. Part-to-part GBW agreement allows for consistent filter behavior without external trims. The dual filter requires only three external resistors per channel to set the cutoff frequency and response shape. You can use the LT1568 to attain a unity-gain Butterworth response with all same-value resistors. Both second-order, lowpass, Chebyshev sections with 0.25-dB passband ripple and second-order Bessel sections require two resistor values among the three resistors per channel. The chip maker provides the resistor values that properly compensate for the trimmed amplifier GBWs for a given cutoff frequency. The tightly trimmed GBWs and capacitor values also allow you to cascade two-pole sections to implement matched, four-pole filter pairs.

The 1568 operates from supply voltages of 2.7 to 11V. The maximum quiescent current is 35 mA at 3V and is 38 mA with ±5V rails. The outputs can typically drive ±80 mA. The gain mismatch between channels is less than 0.3 dB with a 10-MHz cutoff frequency. The wideband output noise is 34 µV rms in a 20-MHz bandwidth with a 10-MHz cutoff, corresponding to an SNR of 84 dB with a 2V p-p signal. Under similar conditions of input amplitude, the SNR reaches 92 dB with a 1-MHz signal in a 2-MHz bandwidth. The filter's typical THD is –84 dB for a 1-MHz, two-pole Butterworth section, rising to –69 dB for a 10-MHz cutoff.

Linear Technology, 1-408-428-2050, www.linear.com.