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ADC34J45: Input impedance matching

Raul Gracia
Prodigy 20 points
Part Number: ADC34J45
Other Parts Discussed in Thread: LMH6521

Hi,

We want to drive the input of the ADC34J45 with the Texas VGA LMH6521. We want to mount a passband filter centered in 45MHz between them:

LMH6521 --> 45MHz BPF --> ADC34J45

We observe that the LMH6521 has an output impedance of about 20 ohms at 45MHz, and that the input impedance of ADC34J45 at 45MHz is 4000 ohms aproximately.

Reading the datasheet of the ADC and looking its evaluation board, we see that a double 1:1 balun is employed at the input of the ADC. ¿Why is employed that double balun?

¿How can we introduce the passband filter between the  LMH6521 and the ADC34J45?. 

We are thinking in increasing the output impedance of the LMH from 20 ohms to 50 ohms, then mounting a 50 ohms bandpass filter and then match its output  to the input impedance of the ADC: ¿Is it correct? ¿What matching network can we use between the 50 ohm BPF and the ADC input?

 

Thanks in advance and best regards.

  • 0
    TI__Mastermind 19655 points

    Hi Raul,

    Yes, this is the correct approach. Terminating the amp for it's expected and desired output impedance is step 1. Then you can AC couple to ensure the amp output impedance is always satisfied before the 45MHz passband bandpass filter. Although the input impedance of the ADC will be k-ohm range, it's typical to drive with 100ohm differential impedance. This means the output of the BPF should be 100ohm differential. Regarding any kickback spurs from the unbuffered ADC, the BPF will absorb any kickback or glitches that occur from the discharging of the sampling capacitor. See below resource for proper back termination of amplifier to adc.

    Interfacing AMP to ADC.pdf

    Regarding the double balun configuration, if one balun is used for single ended to differential conversion, the second (must be the same exact balun) will tighten the phase imbalance from the first balun. 

    Thanks, Chase

  • +1
    TI__Mastermind 23150 points

    Hi Raul,

    There are two questions here to address:

    Q1: We observe that the LMH6521 has an output impedance of about 20 ohms at 45MHz, and that the input impedance of ADC34J45 at 45MHz is 4000 ohms approximately.

    Design the filter around the characterized impedances of the amp and ADC.

    These are the target impedances to shoot for.

    See attached for more information and details....

    Interfacing AMP to ADC-02072024.pdf

    Q2: Reading the datasheet of the ADC and looking its evaluation board, we see that a double 1:1 balun is employed at the input of the ADC. ¿Why is employed that double balun?

    Typically a balun is used to interface with the ADC to ensure lower noise as it is an passive device. This is also how the ADC is characterized as well. The double balun is used to help improve the phase imbalance of the balun which the ADC will manifest phase imbalance on the frontend/balun as worse HD2 or even order distortion.

    For more information regarding this and choosing between amps and balun frontends, see the link below.

    www.planetanalog.com/.../

    Regards,

    Rob

  • 0 in reply to Chase W
    Prodigy 20 points

    OK Chase. thank you very much for your help.

  • 0 in reply to Rob Reeder
    Prodigy 20 points

    OK Rob, thank you very much for yor help.

    Can be a problem to drive tha ADC with a typical 100ohms impedance although the ADC input impedance is 4000ohms?

  • 0 in reply to Raul Gracia
    TI__Mastermind 23150 points

    Hi Raul,

    Keep in mind the termination is set outside the converter. Even tho the input impedance of the ADC is 4kohms, you can terminate the ADC and AAF design, per the slides that were attached. This way you can design the filter around that impedance. This is the basis of the back termination resistor in the design.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 20 points

    OK Rob, thanks again.