Tiny 1MHz Lowpass Filter Uses No Inductors

  
A simple circuit for evaluating the performance of the fi lter. The LTC1560-1 offers a pinselectable cutoff frequency of either 500kHz or 1MHz. The fi lter gain response is shown in Figure 2. In the 1MHz mode, the passband gain is fl at up to (0.55)(fC) with a typical ripple of ±0.2dB, increasing to ±0.3dB for input frequencies up to (0.9)(fC). The stopband attenuation is 63dB starting from (2.43)(fC) and remains at least 60dB for input frequencies up to 10MHz.
Tiny 1MHz Lowpass Filter Uses No Inductors - schematic

The elliptic transfer function of the LTC1560-1 was chosen as a compromise between selectivity and transient response. Figure 3a shows the 2-level eye diagram of the fi lter. The size of the “eye” opening shows that the fi lter is suitable for data communications applications. Additional phase equalization can be performed with the help of an external dual op amp and a few passive components. This is shown in Figure 4, where a 2nd order allpass equalizer is cascaded with the IC. The allpass function is achieved through traditional techniques, namely, passing a signal through a low Q inverting bandpass fi lter and then performing summation with the appropriate gain factors. Figure 3b shows the eye diagram of the equalized fi lter. The input amplifi er stores the DC offset of the IC across its feedback capacitor. The total output DC offset is the input DC offset of the 1/2 LT1364 plus its offset current times the 10k resistor (less than 1.85mV). Upon powerup, the initial settling time of the circuit is dominated by the RC time constant of the DC correcting feedback path; once the DC offset of the LTC1560-1 is stored, the transient behavior of the circuit is dictated by the elliptic fi lter.




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