AD9656 front end for narrow-band

Hello

We would like to use AD9656 for narrow-band application (circa 90MHz - 110MHz). Problem is that AD9656 has high fullscale for our application.

We have good experience with interfacing ADc with buffered inputs where transformers with 1:4 impedance ratio are used to achieve lower fullscale (see MT-006, MT-228 and AN-935).

The AD9656 has switched-capacitor input so this solution is more challenging. Do you think it is solvable? If so can you please provide input impedance parameter for band of interest?

Thanks and regards

Daniel

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  • +1
    •  Analog Employees 
    on Oct 17, 2019 4:45 PM

    Hi Daniel,

    Thank you for considering the AD9656.

    The input impedance information is on the AD9656 product page, in the Tools & Simulations section https://www.analog.com/en/products/ad9656.html#product-tools . This shows the impedance in the "track" phase" of the switched cap input, which is the phase of interest because it is the mode that leads up to the sampling instant. The file shows two equivalent simplified representations of the input impedance. The series RL representation is 20Ohms in series with 7pF. This is a good representation across a wide frequency range. For narrow band applications some users find it helpful to use the parallel equivalent representation. You choose your frequency of interest in the AD9656_AINDIFFZ tab of the impedance Excel file, and find the equivalent R and C at your frequency of interest. For example 100MHz corresponds to 2.6kOhms in parallel with 6.95pF. Then it is easier as a first pass to calculate a parallel inductance at the input, that counters the equivalent parallel capacitance.

    If the input full-scale is too high for your application, you can use Register 0x18 Bits[2:0] to adjust the full scale range downward. This (Bits[2:0]) is a back end digital adjustment and does not affect the analog portion. For example, writing Register 0x18 = 0x00 results in an input full scale voltage of 1Vpp_differential. Please see Table 19 in the AD9656 datasheet for more information.

    Thank you.

    Doug

Reply
  • +1
    •  Analog Employees 
    on Oct 17, 2019 4:45 PM

    Hi Daniel,

    Thank you for considering the AD9656.

    The input impedance information is on the AD9656 product page, in the Tools & Simulations section https://www.analog.com/en/products/ad9656.html#product-tools . This shows the impedance in the "track" phase" of the switched cap input, which is the phase of interest because it is the mode that leads up to the sampling instant. The file shows two equivalent simplified representations of the input impedance. The series RL representation is 20Ohms in series with 7pF. This is a good representation across a wide frequency range. For narrow band applications some users find it helpful to use the parallel equivalent representation. You choose your frequency of interest in the AD9656_AINDIFFZ tab of the impedance Excel file, and find the equivalent R and C at your frequency of interest. For example 100MHz corresponds to 2.6kOhms in parallel with 6.95pF. Then it is easier as a first pass to calculate a parallel inductance at the input, that counters the equivalent parallel capacitance.

    If the input full-scale is too high for your application, you can use Register 0x18 Bits[2:0] to adjust the full scale range downward. This (Bits[2:0]) is a back end digital adjustment and does not affect the analog portion. For example, writing Register 0x18 = 0x00 results in an input full scale voltage of 1Vpp_differential. Please see Table 19 in the AD9656 datasheet for more information.

    Thank you.

    Doug

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