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Differential Impedance Converter

Peter Baxter
Intellectual 380 points
Other Parts Discussed in Thread: PGA870, BUF634, OPA656, THS770006, THS4509, THS4520, THS4521, DRV1101, THS6204

A response to another of my questions on this forum, resulted in the TI moderator drawing my attention to the PGA870 Fully Differential Programmable Amplifier - which I wasn't aware of. It is possibly a better solution than what I had in mind.

My application is very similar to an oscilloscope - whose functional blocks can be used as an example. 

Between the Oscilloscope's "input attenuator" and the pre-ADC "differential gain stage" is an "Impedance Converter" (Source Follower), which is a single-ended, high-impedance input to low-impedance, low-drift differential output block. As the input attenuator network typically has 1M impedance, the Impedance Converter must have a very high input impedance. Discrete solutions often incorporate a high-speed and low-speed path. Another objective is to have low DC drift.

Using a FDA to convert single-ended to differential-ended has too low an input impedance.

I was interested to discover that TI has the BUF634 high speed buffer which is single-ended input to single-ended output. Might TI have a single-ended input to differential-ended output part that is specifically designed for that Impedance Converter function?

I'm also interested in a similar part oriented towards slower frequencies as well (say 10kHz).

Thanks

Peter 

 

  • 0
    TI__Genius 17905 points

    Peter,

    I am not aware of any TI part that performs a high-impedance single-ended input to differential buffer function. It sounds like you will need a FET input op amp to get a high enough input impedance.

    It is possible to use two or three high speed op amps to perform the function. For the two op amp implementation, you can use one op amp as a high input impedance buffer, then a second op amp as an inverter. The differential output would be taken as the buffered signal (+) and the inverted signal (-). Alternatively, you can use a third op amp to rebuffer the non-inverting signal so that the phase delay in the non-inverting and inverting signal paths are better aligned.

    In my example above, I chose the OPA656 which is a high-speed FET input op amp. Below I show the outputs with a 30MHz input tone. Notice that Vdiff2+, the rebuffered positive signal signal, is better aligned with the Vdiff- (i.e. the intersection of Vdiff2+ and Vdiff- is close to 0V) compared to directly taking the single buffer output as Vdiff+.

     

    OPA656 SE-Diff 2-16-2011.tsc
  • 0 in reply to Kris F
    Intellectual 380 points

    Might you expect a Fully Differential Amplifier to give a superior (or similar or worse?) differential output (phase delay and alignment) versus two individual opamps - as you have shown?

    TI's Differential parts include opamps and programmable gain amplifiers. For my interest only - are there any other differential parts in TI's product range that you can mention?

     

  • 0 in reply to Peter Baxter
    TI__Genius 17905 points

    Peter,

    Yes, the fully differential amplifier architecture is capable of generating a well-balanced output from a single-ended input and I would expect better balance than the two op amp approach I showed before. Thus, a better approach than I had proposed before might be to use a JFET op amp buffer followed by a fully differential amplifier to perform the single-ended to differential conversion. Depending upon the application, some good FDAs to consider would be the following, listed in order of increasing bandwidth:  THS4521, THS4520, THS4509, THS770006/12.

    The multiple single-ended output op amp approach works well at low frequencies where the amplifier phase response is still flat vs frequency in both non-inverting buffer and inverting configurations. It is necessary to use an op amp with much wider bandwidth than the signal bandwidth so that the phase is still flat over the operating frequency range and the differential balance is not subject to the different closed loop responses of the non-inverting buffer and inverter configurations.

    We have dual op amps suitable for differential line driving applications, for example, the THS6204, and a differential in-differential output driver DRV1101, though these parts are designed for differential inputs and may not be useful in your application.