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THS4521: THS4521 noise above 1MHz

user1930533
Prodigy 40 points
Part Number: THS4521
Other Parts Discussed in Thread: LMH6551, THS4551

Hello

In one of our product, we are using the THS4521 for ADC driving.

Our measurement shows that the THS4521 noise contribution is 1.4 time higher than predictions (theoretical calculations matches simulation results).

We have tested other FDAs and their noise measurement matches calculation and simulations.

So The ths4521 seems to be the only FDA we've tested that have this problem.

We're using the THS4521 with 300ohms feedback resistors, 1V/V gain, 10MHz lowpass filter at its output (2x100ohm + 78pF accross outputs)

I've noticed that the noise spectral density graph in the datasheet stops at 1MHz while a lot of other specs are characterized up to 100M or 1GHz.

Please could you confirm that the spectral noise density does not rise above 1MHz.

Thanks

  • 0
    TI__Genius 12631 points
    Hello,
    The noise of the THS4521 will stay flat up to the gain bandwidth product of the amplifier. Above this there could be a slight increase as the amplifier loses its open loop gain.

    I recently made noise measurements on the THS4521 and it matches the datasheet very well. Are you measuring noise spectral density or integrated noise?

    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 40 points
    We are measuring the integrated noise.
    As example, among other FDA, we have made the same measurement with the LMH6551 which has 6nV/sqrt(Hz) noise density and it performs better in our design than the THS4521 which has 4.6nV/sqrt(Hz) noise density.
  • 0 in reply to user1930533
    TI__Genius 12631 points
    Hello,

    I would highly recommend going to our latest FDA upgrade to the THS4521 - the THS4551. It has similar bandwidth but much improved distortion performance, dc precision specifications and more important for you -lower noise (3.3 nV/rTHz). The TINA Spice model has also been released for this.

    Is it possible that the THS4521 is oscillating which will increase the output noise?

    Are you comparing the noise performance of the THS4521 and LMH6551 using the same value of gain and feedback resistors and the same gain configuration? A 1kOhm resistor has an inherent noise of 4 nV/rTHz at 25C so the resistors themselves contribute noise at a level similar to the amplifier (and there are 4 of them in the system). Larger resistor also mean that the amplifiers current noise will be multiplied by the same factor so will lead to increased noise.

    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 40 points

    Hello,


    The comparison between the THS4521 and the LMH6551 has been done with the same circuit (same Rg, Rf...)


    The fact that the THS4521 might be oscillating may be an explanation but hard to see at theses voltage levels. The THS4521 simulation gives a stable output even with 2pf load from outputs to ground.

    We have just tried the THS4551 and is a good alternative for us. But it shows the same behavior than the THS4521: it shows 1.2 times more noise than the theory predict (but still better than the ths4521 which shows 1.4 time more noise) in our circuit.

  • 0 in reply to user1930533
    TI__Genius 12631 points
    Hello,
    If the THS4521 is oscillating that would explain the increased noise. Are you using an isolation resistor between the amplifiers output and the scope input (to isolate the capacitance of cable, scope probe, etc)?

    Please provide a drawing/schematic of your setup.

    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 40 points

    Hello,


    We are not using scope for our measurements.

    The noise is measured by injecting a very low noise source voltage source to the analog input and by processing the output data from the ADC. So no probes are present on the board when the noise measurement is done

    The ADC noise has been measured by shorting its inputs so we can subtract it from the total noise to get the FDA input contribution.

    I cannot provide any schematic as they are confidential.

    Thank you for your help.