What input resistance for AD7960?

I would like to know what would be the maximum recommended resistances to place between an FDA and AD7960 inputs (R3 and R6 on the figure below). I would like to reduce the capacitor value to reduce power consumption of the LTC6363 and increase the resistances accordingly to keep cutoff frequency. I am thinking of 200R or 300R instead of 100R. The input signal is a amplitude modulated sine wave, 125kHz.

Thank you in advance for your inputs.

  • Hello LLoben, I cannot give many details about the application. It is non-muxed though. The signal is AM and we demodulate it to retrieve the AC component of the AM component. We do not need an absolute precision. I believe we may live with increased THD if it is not too high and it is not noisy. Any idea how much the THD increases when I use 200 or 300 Ohms before each input? Also what is the input capacitance of the AD (I could not find the info in the data sheet).

    I may (will) measure SNR/THD under both conditions (lower/higher resistances) but knowing what to expect based on the AD design would be helpful because figures based on measurements may change should some change (e.g. process update) be applied to the AD  in the future.

    Thank you,

    Elder

  • 0
    •  Analog Employees 
    on Nov 3, 2017 4:40 AM

    Hi,

    You should  expect  at least 5-10dB drop in THD, if not worse with increased value of source impedance(200 Ω  or 300Ω) at input. The internal DAC capacitance is 26pF(see figure 30 in the datasheet). ADI don't change the part or process without notifying our customers about a formal process change notification.

     

    Regards