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LM833-N: Input noise at +/-5v

John Linsley
Intellectual 510 points
Part Number: LM833-N
Other Parts Discussed in Thread: LM833

Hi,

I just built a x1000 low  noise amplifier based on Fig.2 of "Design of ultra low noise amplifiers" by Vojtěch Janásek, using an LM833.

It works perfectly but the noise is 100 times what I expected based on calcs and Spice simulations, output noise is -88dBv. After trying many things I finally measured the noise at the output of the spare amp which is connected as a voltage follower (pins 6 & 7 linked and pin 5 connected to 0v). It was about 500nV/root-Hz (-125dBv). This was measured using the amplifier of which it is a part, sadly. The noise on 0v is unmeasurable (with my equipment) and the power supplies are pretty quiet as well, on the order of -140dBv.

Does the noise spec of LM833 degrade at lower supply voltages? It is only specified at +/-15v.

Regards, John.

  • 0
    TI__Guru** 100965 points
    Hi John,

    I'll look for data at lower supply voltages. I'm not completely sure if supply voltage could degrade the noise performance but I wouldn't expect a high impact due to this.
    500nV/root-Hz is what you measured on the system set to x1000?

    Best regards,
    -Ivan Salazar
    Applications Engineer - Low Power Audio & Actuators
  • 0 in reply to Ivan Salazar
    Intellectual 510 points

    Hi Ivan,

    Yes, I measured 500nV/root-Hz with the gain at x1000. I agree about the degradation but I had to ask the question to be sure.

    I'm going to build a discrete version tomorrow so that I have a second opinion. I have another theory but I need that second opinion to confirm it.

    Thank you for replying and taking the trouble to investigate.

    Best regards, John.

  • 0 in reply to John Linsley
    Intellectual 510 points

    Hi Ivan,

    The discrete amp confirmed the noise levels so I'm now sure that the device (and the others that I bought - I measured them all) have been degraded by poor handling. I'm out of Australia in an almost developed country where the electronics shop ships it's ICs with the legs stuck in polystyrene foam (no alfoil, even) and this is then sealed in a cellophane packet. I guess they think that 90% humidity makes them immune to static charges.

    Best regards, John.