• Resolved

LME49710NABD: LME49710

Part Number: LME49710NABD

Good morning to you all.

What I ask is to know if possible the reason why LME49710 inserted as a buffer in the filter of which I enclose the wiring diagram, produces noise in the low pass section of the filter.

In my opinion it is the classic 50/100 Hz noise but I can not be sure because I do not have an analyzer.

The strange fact is that by putting back into the OP27 circuit of AD the noise disappears completely, even with an OPA134PA the noise is non-existent.

In the wiring diagram there are no 10 mF electrolytic capacitors that you suggest to use in the LME49720 datasheet that I have inserted.

Looking for a solution to the problem I noticed that many integrated require measures for their use as a buffer, this also applies to LME49710?

I inserted a double diode bridge in the power supply which I enclose the wiring diagram, and I also replaced the BD139 with a 2SC2660 the noise with OP27 is always inaudible but returns when I replace it with LME49710.

I want to clarify that the other integratiti have always been and are currently the LME49720, also also with the "noise" the sound quality that gives LME49710 is fantastic, the detail is something "super".

I have owned LM325 for years but I think it is time to change, so what can I use to get better nutrition than the current one. As usual, I would like to use only your components that I think are unparalleled.

The LME49710s were bought by me by Mouser.

Thanks for your help.

Sincerely, I greet you

Guido Bullo

  • Hi Guido,

    Welcome to e2e.
    We'll take a look at your questions and provide some comments during this week.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators
  • In reply to Ivan Salazar:

    First of all thanks for the interest in my problem.

    I add the filter scheme with all the values of the components the cut of the filter is 1500/5000 Hz.

    In addition, the power supplies are two for each filter.

    Needless to say, I thank you in advance.

    Guido Bullo

  • In reply to guido bullo48:

    Hi Guido,

    OP27 and LME49720 are two different amplifiers. OP27 is a precision and high speed amp, while LME49720 is mainly focused on audio bandwidth.
    OP27 has a very low frequency 1/f corner compared to LME49720, around 10 times lower. So this looks like a part-to-part related difference.
    I would just recommend you to use the device that best fits your application.

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators
  • In reply to Ivan Salazar:

    grazie 

  • In reply to Ivan Salazar:

    Greetings and thanks to all of you.
    At this point I ask you which combination to use in the filter that replaces LME49710 / 49720, and that is higher quality, keeping in mind that I need 8PDIP components.
    As I said, I want to use your products.
    I forgot to ask you which components to use for a positive and negative feed.
    Do I have to continue using the LM317 / 337 pair or are there better components?
    Thanks, see you soon
    Guido Bullo
  • In reply to guido bullo48:

    Ciao Guido,

    I know this circuit. It's from the article "uniphase active" published in the Elektor 5/88, right?

    The circuit contains lots of active filters. The resistors in the active filters are rather high ohmic (10k...69k6), which makes it a lot easier to find the appropriate precision filter caps.

    In order to keep the noise performance low you must not only use OPAmps with very low voltage noise but with very low current noise as well!! That's why in this circuit the TL072 was originally chosen.

    Go into the datasheets and search for the "input noise". You will find the "input voltage noise density" and the "input current noise densitiy" . A current noise density of 1pA/SQR(Hz) flowing across a 10k resistor converts to an additional voltage density of 1pA/SQR(Hz) x 10k = 10nV/SQRT(Hz)!

    So, to find a replacement for the TL072 I would search for a very low noise FET-OPAmp. A FET-OPAmp has the lowest input current noise.

    Kai
  • In reply to kai klaas69:

    Hi Guido again,

    the LM317 and LM337 are still state-of-the-art voltage regulators! There's no reason to choose something different for your circuit.

    By the way, many modern OPAmps have such a high PSRR (power supply rejection ratio) that the supply noise doesn't have any noticable influence in the signal noise any more. Only in applications using very high gain supply noise can become an issue. But in your application the gain is very low and the signal is rather high.

    Kai
  • In reply to kai klaas69:

    Hi Kai

    The scheme is taken from Progetto Elektor in June 1989, which was the version translated into Italian.

    Logically, like all the things that are done in Italy, it was devoid of the value of the components of the filters.

    The formulas for its realization I found them randomly on the WEB about ten years ago.

    The first thing I want to clarify is the one concerning the noise produced by the integrated ones.

    I remembered that I had LME49720 purchased from Digikey, I put them in a channel and to my amazement, the noise was almost imperceptible, so those bought by other suppliers about three years ago were tarots that made me furious for months.

    Now I pass to the first question I ask you and it's the one about the value of the resistors!

    If the value of the capacitors is triplet, the value of the resistances drops considerably, so the noise could go down a little.

    I state that I use Panasonic polypropylene capacitors, exactly the ECW series, what do you think?

    For the resistors which can I use?

    I currently use the Vishay MRS25000, which ones could I use in the circuit to improve performance.

    For the integrated my choice would fall on the OPA134 / OPA2132 series because the OPA1602 series are SMD, they are too small and the printed is almost impossible to make for us car manufacturers.

    One thing that I consider very important in light of the facts is to ask Texas to publish photos, or data that make the originals of their components distinguishable, as the copy market is scary and harmful for everyone.

    I await your precious advice.

    Guido Bullo

  • In reply to guido bullo48:

    Hi Guido,

    regarding the noise of LME49720 you should read this

    e2e.ti.com/.../415907

    e2e.ti.com/.../725471

    Kai
  • In reply to guido bullo48:

    Hi Guido,

    OPA134 and OPA2132 look good. But I would decrease the resistors in the circuit a bit because these OPAmps are very fast. Yes, the cap values in the filters must be increased by the same factor then. The overall effect is not only a better stability but also a much lower resistor noise.

    The ECW series looks good. Nevertheless, I would select the caps from a handful to get +/-1% tolerance.

    Metal film (thin film) resistors are rather uncritical. The MRS2500 should be ok.

    Kai