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Noisest Opamp?

Stephen Tomporowski
Prodigy 210 points
Other Parts Discussed in Thread: TL062, TLV2402, OPA2137, INA128, OPA2170, INA121

Yes, this is really a need and a valid question!

My company wants to be able to sell to China without waiting months for an export license.  We can do that by degrading resolution (adding noise).  We currently use dual opamps in an SOIC-8 package and I'm looking to replace them with noiser opamps.  Espcecially noisier in the band below 100 Hz.

I know FET opamps tend to have more noise in that region, but any experience with anything else.  I'm asking because I don't think I can find any 741s....;-)

  • 0
    TI__Genius 12325 points
    Stephen, you definitely get the award for the most fun question in recent memory! I've actually had this come up previously as well and it can be educational to other forum members in that they can get some insight into the design trade-offs in op amps. First, regardless of the input device type (BJT, JFET, MOSFET) noise is always inversely proportional to the current in the input differential pair. So the first criteria we're looking for in our search for a noisy op amp is a low power op amp. Next, you mention something very critical, you want high noise at low frequencies. And this is where CMOS really shines, especially old CMOS process technologies. After that JFETs would be the next noisy-est at low frequencies and BJTs would be the quietest.

    You don't mention what power supplies you're running on, or what bandwidth you need so I'll list a few options here. The TLV2402 offers a really solid showing with 500nV/rtHz at 100Hz! But it's also a very low bandwidth part and can only be used with supply voltages totaling 16V. The OPA2137 and TL062 are more useful in general applications and both have noise spectral densities above 40nV/rtHz.
  • 0 in reply to John Caldwell
    Prodigy 210 points
    John,

    Thanks! I just saw a video on Youtube where they measured the noise on a TL062 and it looked horrendous, especially with a pretty high noise corner. Suddenly I'm beginning to think I might end up with too much noise. Anyways, I have two systems I have to degrade, one will be operating at +11/-7 and the other at ±15. On the latter, the architecture is different, so I need to find a noisy instrumentation amp. Right now there is an INA128 in that spot. The previous stage is an AD630 which has no pin-for-pin replacement that I know of. There is a pair of opamps at the back end, but they are high-C drivers and that's not the best place to inject noise. As for bandwidth, the first product is 50kHz currently, although I think we may be able to get along with less (we're degrading after all), while the 2nd is a 20kHz bandwidth. The rather ironic part of this is that one of the products is my original design from 5 years ago which won an award at the Sensors show for resolution at bandwidth.

    Steve
  • 0 in reply to Stephen Tomporowski
    Prodigy 210 points
    Sorry, typo there, it's an AD830 Video receiver.
  • 0 in reply to Stephen Tomporowski
    TI__Genius 12325 points
    I wondered if we might run into the issue of too much noise! For the op amp spots I think the OPA2137 or and OPA2170 might be good options. The OPA2170 is a low power CMOS part thats about 23nV/rtHz at 100Hz and a gain bandwidth of 1.2MHz (you don't mention the gain your parts are in). The instrumentation amplifier might be a bit more difficult to narrow down, the INA121 should be slightly noisier than the INA128.
  • 0 in reply to John Caldwell
    Prodigy 210 points
    John,

    Thanks, this is all good stuff. For the instrumentation amp, I think I have the option of replacing some resistors with carbon comp resistors. If I can find them in surface mount. Carbon comp is kept around because of its resiliancy to high voltage spikes.

    Steve