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LMH6521EVAL: 50 Ohm Output Impedance in Single-Ended Operation

Darryn Jordan
Prodigy 130 points

Part Number: LMH6521EVAL
Other Parts Discussed in Thread: LMH6521

Hi All,

Could anyone please suggest how to alter the evaluation board to achieve 50 ohm output impedance in single-ended operation.   

I want to use it to drive a Red Pitaya (1 mega-ohm input impedance ADC) with a 50 ohm tee for matching. 

If anyone has better suggestions they would be appreciated.

Darryn

  • 0
    TI__Genius 15975 points
    Hi Darryn,

    If you look the LMH6521EVAL schematic, the board does have single-ended 50-ohm matched output coming from the JTX-2-10t+ transformer. So, would it be possible for you to use that while connecting the ADC?

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 130 points

    Hi Rohit,

    I must be confused with the wording of the datasheet then? It seems to state that after the transformer the output impedance is 200 ohms.

    Are you suggesting I take the output from before the transformer?

    Kind Regards,

    Darryn

  • 0 in reply to Darryn Jordan
    TI__Genius 15975 points
    Darryn,

    The output transformers have a 2:1 turns ratio presenting a 100-ohm on the secondary side and 50-ohm on the primary side. The two on-board 40.2-ohms + two 8-ohms LMH6521 output impedance along-with the 100-ohm on the secondary side of the transformer give ~ 200-ohm output load to the LMH6521. The 50-ohms on the primary side of the output transfromer is still connected to the output SMA, which by default can be connected to a 50-ohm load or instrument input.

    If you are planning to connect the LMH6521 output deferentially to the ADC, then you would have to remove the output transformer and use a short across the primary and secondary side to reach the the two output SMAs. So for channel A on the LMH6521EVAL, you would have to short across T2, remove R34, make C6 and C7 as 0-ohms. Also, since the ADC input impedance is 1Mohms, you would need to place a differential 100-ohm load across the ADC inputs primarily to bring the ADC input impedance down such that it is easier to drive for the LMH6521 with a known 200-ohm load.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 130 points

    Thank you for your help Rohit!

    Its not often you get such well written and clear answers in forums!

    I had originally though the reason I was seeing a maximum gain of 20 dB in single-ended operation was because of impedance mismatches.

    But, using your answer and this one it seems that the maximum gain achievable with the evaluation board is 20 dB with no modifications in single-ended mode?

    Can you confirm this?

    Kind Regards

  • 0 in reply to Darryn Jordan
    TI__Genius 15975 points
    Darryn,

    Yes, the maximum gain in single-ended operation without any loss in the output transformer should be around 20-dB. However, you will see some additional voltage loss due to the non-ideal transformer and get about ~18 to 19dB maximum gain.

    Best Regards,
    Rohit