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LMH6505: Noise analysis for LMH6505/THS4131 OP

kent gu
Intellectual 685 points
Part Number: LMH6505
Other Parts Discussed in Thread: THS4131, VCA820

Hi,

We are using LMH6505 & THS4131(using these 2 chips for differential function, just use their noise data for analysis, not compare apple and apple)。

Seems that we found the LMH6505 would bring more noise. So we studied chip's noise character. We are newer for noise analysis, want to ask for help:

  • Why LMH6505 noise(about 1000 @10HZ)bigger than THS4131(<10 @HZ)? Since LMH6505 special function decide the big noise?(Left picture is LMH6505, right picture is THS4131)

  • In the spec there are input noise and output noise, how to calculate total noise? Total noise power = input noise power + output noise power?
  • If total noise power don't equal input noise + output noise summary, how to connect the relationship of input noise power and output noise power data?
  • OP chip, there are input current noise and input voltage noise, which noise for our analysis? Such as TIA, use input current noise?
  • If my circuit noise is 10M low pass circuit with LMH6505, the noise power of the chip calculation forum as below?  (Total noise power = A1 + A2 +...+ An-1+ An, Ax is the noise power during a special frequency range, it is （eno * eno） * Hf？Hf is frequency width supposed that eno is same during one Hf)

Best Regards,

Kent

  • 0
    TI__Genius 15555 points

    Hello Kent, 

    I have a few questions i'd like to address first:

    1) Could you share the schematic LMH6505 & THS4131? The easiest way to confirm what you are seeing is expected is to run these circuits in sim.

    2) What gain are you using for the LMH6505? You will notice that the output noise figure differs depending on the VG used.

    Now to address some of your points:

    • Output noise is the going to be the noise at your input multiplied by the noise gain (along with any other passive noise sources), this explains why the figure for the LMH6505 changes with Vg.  The figure you posted for the THS4131 refers to the input noise, and so the gain of the device must be considered to try a make a comparison of both plots.
    • Both current noise and voltage noise can factor into your total output noise. Current noise is typically more relevant when working with larger resistances (such as in a TIA application). 

    I would recommend watching the following precision labs series on noise, which will explain to you the relationship between input noise density, output noise density and total noise.

    https://training.ti.com/ti-precision-labs-op-amps-noise-1?context=1139747-1139745-14685-1138803-13232

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Intellectual 685 points

    Hi Hasan,

    Sorry to reply late since I am studying the document that you recommend. It is helpful.

    I have the simple question, 

    Why can't place a capacitor across Rf? My understanding is that if I can put capacitor across Rf, it can limit bandwidth, and reduce total noise

    Best Regards,

    Kent

  • 0 in reply to kent gu
    Guru 48395 points

    Well the LMH6505 is the VIP10 migration of the much earlier CBICU CLC520. I spent quite a lot of time in that data sheet trying to generate a working noise model, not easy as it turns out. 

    Anyway, one of the odd things hiding inside these parts (all successors to the CLC520 and CLC522, like the VCA820 series) is the output amplifier is a current feedback device. That was because the guys doing the CLC520 were the original current feedback designers. Not necessarily a good choice for noise, but very good for slew rate. CFA will oscillate if you put a cap across the feedback R.