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Looking for a DC to 4GHz GBP opamp with high input impedance

Jan Volkering
Expert 8190 points
Other Parts Discussed in Thread: THS4304, THS4302, THS4509, LMH5401

Hi,

For a customer I am looking for a high speed opamp that can handle including DC up to AC with a GBP up to 4GHz and a voltage noise density spec of <2nVrtHz.

Drift is less important since the environment is temp stable and can be compensated.

Another requirement is that the ampifier has to be placed in a non inverting topology, gain +2.25 with the input signal at the + input.

The + input impedance has to be high compared to the source impedance that can vary between 40 and 100Ohms. Gain should have no influence on the source impedance.

The input signal can go between zero and +0.4V, so the input has to be able to cope with signals to ground (single supply is preferred)

Ideally the output has to be differential to drive a shielded cable.

Do we have such a solution available?

  • 0
    TI__Genius 12631 points
    Hello Jan,
    So with 4GHz GBP and a gain of 2.25, it sounds like your desired signal bandwidth is around 1.82 GHz.

    We do not have a solution for an amp that goes single ended to differential and has high input impedance. I am assuming by high Zin you would like something > 1k(10x the source impedance)???

    If using a fully differential amplifier, a high input resistor will mean a high feedback resistor to achieve the gain which then interferes with the amplifiers input capacitance and makes stabilizing it difficult. Also note, that is using resistors >= 1kOhm, the noise contribution of the resistor itself will be > 4 nV/rTHz.

    Its best to do this as a 2-stage solution with a noninverting amplifier in the 1st stage and a fully-differential amplifier in the 2nd stage.

    For the 1st stage, you could use the THS4304 as a buffer. The THS4304 has a unity gain bandwidth of 3GHz. Another option is to use the THS4302 which has a bandwidth of 2.4GHz in a fixed gain of 5. The THS4302 is not Rail to Rail in or out so lets restrict our self to the THS4304. This is RRI but not RROut, so if you have an input that goes from 0 to 0.4V, then the output in a buffer config will go from 0V to 0.4Vwhich is not supported...the solution to this is to have a level shift voltage at the Rg (input resistor on inverting terminal) of the THS4304. However that level shift signal will also have to be a negative voltage and also have very wide bandwidth to be able to drive the THS4304. As you can see this is getting quite complicated if not impossible. So the 1st thing is to see if we can get a bipolar supply.

    Now for the 2nd stage going single-ended to differential - here you could use the LMH5401 or THS4509. I would recommend the LMH5401...it has wider GBP. You can always configure it in a higher gain, say 4V/V like recommended in the datasheet and use termination at the other end of the line to attenuate the signal back to the needed range.

    This 2nd stage design will become clearer once we can anil down the design of the 1st stage.

    -Samir