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THD of THS4271 amplifier in differential to single ended conversion configuration

Anoop Joshi
Expert 1125 points
Other Parts Discussed in Thread: THS4271, OPA695, THS3091

Hi,


I was considering using the THS4271  opamp in a differential to single ended (D2S) configuration as shown in the attached snapshot to convert 0.5Vpp differential signals at 30MHz to a single ended one. Can you please say if I can hope to meet the <-90dBc THD spec quoted for the non-inverting mode of operation in the D2S mode too, or if significant degradation may be expected, esp in the even order harmonics which may normally not be expected for differential signals. I was especially curious about the possible degradation in the even order harmonics in this configuration if any

 

Thanks,

Anoop

  • 0
    TI__Genius 12631 points

    Hello Anoop,

     The amplifier will be in a noise gain of 2 so ideally it would match the distortion performance of the amplifier in a gain of 2. The impedance seen from the previous stage will be different for VG2 vs. VG1. VG1 will see an impedance of 500Ohm (250+250). However because the inverting pin of the amplifier is moving, the load seen by VG2 will be 167Ohms. This may have a minor effect if the previous stage is not meant to handle such heavy loads. If you make R3 = R4 =750Ohms then both sides will see approximately the same load.

    -Samir

  • 0 in reply to Samir Cherian
    Expert 1125 points

    Hi Samir,

    Thanks for the quick response. Actually I had used this configuration for a high speed DAC testing application(differential output DAC) for measuring the DAC output SINAD previously. There were 2 THS4271 opamps in the unity gain buffer configuration for either of the differential outputs, followed by a D2S stage using another THS4271 opamp as discussed. When the performance was measured with a 150 ohm load for the D2S(DAC op peak to peak was around 2V), there was a strange observation that HD2 was dominating compared to HD3, which was not expected for the differential output DAC(I'm afraid I don't have the FFT plots to attach at the moment, will try sharing the same by Monday once I get back to the office)

    We had tried equalising the load seen by the source at either input of the D2S etc, but still didn't help. Do you think the usage of the unity gain buffers at each differential output may possibly have contributed to a minor phase imbalance between the differential outputs due to unequal phase shifts on the 2 paths, resulting in HD2? If 2 well balanced differential inputs are applied at the D2S input we'd expect the even order harmonics like HD2 to be typically lesser right?

    Thanks,

    Anoop

  • 0 in reply to Anoop Joshi
    TI__Genius 12631 points

    Hello Anoop,

      I dont think the buffers would necessarily be the issue. HD2 requires an extremely symmetric board layout for best performance so that is one area to focus on. Symmetry in the power supplies relative to the device is also extremely important. Here is a useful article for the future when designing for DAC buffer applications:

    http://www.ti.com/lit/an/sbaa135/sbaa135.pdf

    While the THS4271 is good wide bandwidth amplifier with excellent slew-rate, it is recommended to use Current Feedback Amplifiers like the OPA695 or the THS3091 for best power to performance ratio, and also for good performance at large output swings. Nonetheless, the 4271 should have also given you good results.

    -Samir

  • 0 in reply to Samir Cherian
    Expert 1125 points
    Thanks Samir for the details

    Regards,
    Anoop