Input impedance matching

Question

Hi. I am using LTC6409 to level shift an IQ demodulator that requires 3.5V common mode voltage with a vector modulator that requires 0.5V common mode voltage. From the components datasheet it can be seen that it uses some resistors and capacitors to differentially match the component with an IQ demodulator of 130&Omega;||2.5pF impedance. In my case the Demodulator has 200 &Omega;||3pF differential impedance. The diagram has little explanation as to why it uses two 75ohm resistors or any of the capacitor values. What should i change to accommodate it for my own design. Thank you in advance.

GLADION · Accepted Answer

Hi EDSANA , 
 Good day. The circuit shown shows a typical configuration for a DC-coupled DC shifting. The capacitor and resistor combination does not exactly match each other. But it was designed to work with minimal error. I think the capacitor combination will still work with your modulator. The resistor values should match your gain setting. You can use 100 ohms as a starting value for the input resistors to substitute the 75 ohms resistors network. Ensure that both input terminals have matched impedances to avoid additional error caused by mismatch. Thank you. Regards, Gilbeys)

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Input impedance matching

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