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LTC5586 : Anti-alias Lowpass Filter

Hello

I would like to have more informations about I/Q anti-alias Lowpass filter for LTC5586 IQ demodulator.

I don't understand how to calculate/simulate Low pass filter defined in datasheet page 24 ?

When I try to simulate, with LTspice this filter (page34), Bode diagram is not flat with several resonances.

Could you send me how this filter can be simulate with ltspice?

Thanks

Best regards

BP_FRANCE

  • Maybe the attached LTspice example would help you. Although it is for LTC5594, the LTC5586's IF output is the same. Note only half of the differential output is shown. The parallel resistance R3//R7 sets the output impedance seen by the ADC. In this case, 68.1//200 = 50 ohms, single-ended (we are interfacing to an ADC with 100 ohm differential input impedance. The anti-alias filter has asymmetrical impedance: 68.1 ohms at the input and 200//50 = 40 ohms at the output.

     LTC5594 Anti-alias filter 500MHz.asc

  • Thanks you for your response. I have an another question about ADC input impedance.

    Into simulation, there is 50ohms  resistor which is ADC input. Input impedance is usualy several kilo ohms so why there is this 50 ohms resistor ?

    Have you got an application note to compute anti-alias asymmetrical impedance low-pass filter ?

    I don't understand how low-pass filter are calculated ?

    Thanks

    BP_FRANCE

  • Many modern highspeed pipeline ADCs have buffered input with 100 to 400 ohm input impedance. In the LTspice example, we showed an ADC with 100 ohm differential input impedance. Even if you use an un-buffered ADC, it is recommended to double-terminate the anti-alias filter.

    The LTC5585 datasheet gives a tutorial on unequal termination filter design in its Appendix section.

    Filter designs for ADCs can be quite complex and tricky. I would suggest that you post in the ADC section to get support from our ADC experts.