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THS4541: Signal loss after LPF mentioned in datasheet

saritha nalluri
Prodigy 50 points
Part Number: THS4541
Other Parts Discussed in Thread: ADC3644

Tool/software:

I'm using THS4541 Diff amplifier with 1  gain and LPF with 60Mhz cutoff . After Ths4541 out at LPF my signal loss is almost 6-7 dB.the filter values are according to the datasheet. 

Please provide any alternative values  to avoid that loss after filtering.

Please see the attached sch for reference 

  • 0
    TI__Mastermind 22794 points

    Hello,

    The filter in the THS4541 application & detailed design procedure section should present an insertion loss of 0.85 V/V.  This pairs with the >2V/V gain on the THS4541 FDA to result in an net signal-chain gain from inputs to ADC of 2V/V.

    If you are using the FDA in unity-gain, you will not gain up the input signal enough to match the ADC full-scale range as shown in the IC datasheet.

    You would need to change your gain & feedback resistors as well as change the filter component values to adjust for your design needs.  You cannot use the datasheet LPF as-is if you wish to raise the cutoff frequency to 60MHz.  The LPF shown in Figure 9-4 " DC-Coupled, Bipolar Input Gain of 2 V/V Single-Ended to Differential Interface to ADC" is a third order, 20MHz Bessel filter.

    For your active filter considerations, you would need to choose between an active or passive filter, pick your desired response type (ie Butterworth), and share your desired gain & signal frequencies.

    I recommend checking your filter design with TI's online filter tool: https://webench.ti.com/filter-design-tool/filter-type

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 50 points

    Adc3644 we are using and my design is freezed.the only requirement is to lower the loss in this configuration my cutoff is 45MHz

  • 0 in reply to saritha nalluri
    Prodigy 50 points

    We designed filter according to the Adc evaluation board and after that simulated with TINa software freezed those values

  • 0 in reply to saritha nalluri
    TI__Mastermind 22794 points

    Hello Saritha,

    Let me take some time and propose a fix to the circuit for you.  I will consider ADC3644 and passive LPF in my proposal.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Mastermind 22794 points

    Hello Saritha,

    Please find the attached PDF of 3rd-Order Bessel Low Pass Filter calculations.  I have provided 45MHz and 60MHz cutoff frequency options as well as 50-Ohm and 500-Ohm load resistances for both frequency cases.

    THS4541_passive_LPF_options.pdf

    Overall, driving the THS4541 output into 50 Ohms differential may result in more loss than driving 500-Ohms, depending on how you filter and what your output network is composed of.

    Please use this as a starting point for your component value exchanges.

    Best,

    Alec