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ADS58J63: ADS58J63 matching netwok

Part Number: ADS58J63

Hi TI Team,

Greetings for the day.

We are using ADS58J63 in our receiver system.

In DATA sheet Differential input resistance 0.5kE and Differential input capacitance 2.5pF has given. Down converted IF Differential 100E signal i am giving to ADS58J63 by using matching network which is given in datasheet as shown below.

I have simulated this matching network by taking source as Differential 100E and Load is 500E. I am getting 11 dB insertion loss and bad return lose.

In my receiver can i use this matching network? If yes could you please explain the source to load matching network how it will work in my design?

Thanks.

  • Greetings:

    The recommended match is shown in the datasheet in section 7.3.2. Your match network is almost the same (your series element is in a slightly different place). The ADC has an input buffer with specified impedance (at 370 MHz) of 500 ohms and 2.5 pF as you have stated.

    Let's breakdown the elements in the match. The series 10 ohm resistor (or 5 ohms) is a recommendation to impede any kickback noise from the switching elements in the ADC. As this device is buffered, kickback should be minimal. The shunt R-C-R is a filter. At low frequency the C is open and the network does nothing. At high frequencies the resistor is a short and effectively there is another 25+25 = 50 ohm load. Let's put that to the side for the moment. The key match components is the two 25 ohm resistors (your R15, R14). These provide a 50 ohm differential load. You are specifying a 100 ohm differential source so a large mismatch is expected.

    If your source is truly 100 ohm differential, then I would change your R14, R15 to 50 ohms instead. You may need to go a bit higher to account for the parallel device load resistor. I would also change the resistors in the RCR filter to 50 ohms as well.

    --RJH
  • Hi Hopper and TI team,

    Greetings for the day.

    Thanks for your quick reply. As you suggested i have done simulations in two cases.

    1.Placed 50R resistors at input side so that i am able to match my input source. S11 is good but insertion loss i am getting 8-11dB loss. S22 is very bad i hope because of 500R load.

    2.I have tuned the resistor values to get good insertion loss. I got 3.5dB IL with 1KE resistors with bad return losses.

    My query and request is:

    1. I am feeding my Differential IF filter(100R impedance) to ADC ADS58J63 (500R input impedance). Can i use this matching ckt in between My Diff filter and ADC or not? My IF bandwidth is 1-500MHZ.

    2. If it not works Could you please suggest the correct values in this case to get optimum performance.

    3. May i Know the function of R12,R13 and C12?

    4. Can i use LC filter instead of RC. Will it help me?

    5. Why S22 is bad. Could you explain me?

    Case 1:

    Case 2:

    If my quires are not good ignore my ignorance.

    Waiting for your Acknowledgement.

    Regards.

    Sreehari.K

  • Sreehari:

    1.Placed 50R resistors at input side so that i am able to match my input source. S11 is good but insertion loss i am getting 8-11dB loss. S22 is very bad i hope because of 500R load.

    2.I have tuned the resistor values to get good insertion loss. I got 3.5dB IL with 1KE resistors with bad return losses.

    RJH>> Running these types of simulations may be a bit misleading in insertion loss as the ADC is not a truly passive termination.  It is similar concept to that of an op-amp.  The op-amp has a high impedance and "monitors" input voltage swing.  The ADC input in this converter is buffered and behaves similarly with a high impedance input and "monitors" input voltage swing.

    My query and request is:

    1. I am feeding my Differential IF filter(100R impedance) to ADC ADS58J63 (500R input impedance). Can i use this matching ckt in between My Diff filter and ADC or not? My IF bandwidth is 1-500MHZ.

    RJH>> If the goal is to design a passive differential filter, then you can design an unequal termination filter with output load at 500 ohms diff and input source impedance at 100 ohms diff.  This is an effective way to include the filter and impedance transformation in one circuit.

    3. May i Know the function of R12,R13 and C12?

    RJH>> Those components are a high frequency filter/load adjustment that kicks in a higher frequencies to account for impedance variation of ADC input.  I would keep those components since you are operating at high IF frequencies.

    4. Can i use LC filter instead of RC. Will it help me?

    RJH>> If you are referring to the R12,R13,C12 then no, keep it as a RC network.

    5. Why S22 is bad. Could you explain me?

    RJH>> S22 is poor because the network is not impedance transformed.  Looking from the 500 ohm load it sees 100 ohms so it is not matched.  That in itself is OK.  The ADC is monitoring voltage swing and is not transferring power.