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Specifying an op-amp

So you need an op amp…and you can’t use just any generic op amp because of certain requirements. For example, a low supply voltage (

  • Has to run on a 3V “single supply” and we’ll have our DC offset at 1.5V
  • Input offset voltage (Vos) must be
  • Since we need to handle such a large swing, it should be rail-to-rail to within .2V of the rails to give us space.
  • 1000 gain at 20KHz means the gain bandwidth (GBW) must be > 20MHz (in reality it can be much lower because the tape probably isnt good enough to record past 12KHz)
  • Since our input signal is 10uV then (at worst) we want to have less than 5% noise (which isn’t so good but we’re willing to have poor quality audio). 5% of 10uV = 500nV. 500nV/sqrt(20000) = 3.5nVrtHz noise figure.
  • Package should be SOIC for easy soldering
  • Low power is nice but not -that- necessary, maybe
  • Low price! Must be under $2 at quantity 100, the lower the better
  • Stuff we don’t care about:

    • Operating temperature
    • Slew rate (at 10mV and 20KHz, it can be really slow)

    The OP37, for comparison, has 12MHz GBW, requires 8V power supply, not rail-to-rail, Vos = 0.03mV, and noise figure of 3.2nVrtHz. So: great offset, great noise, so-so bandwidth and incompatible power requirements.

    Lets go to TI and see what they have to offer. Select >=16MHz GBW, rail-to-rail and 8-SOIC package. The only chip they’ve got is the OPA350.

    Now lets try National. Their system is a little tougher to use: click on 1mA offset first, which will pare down the options to 5 items, none of which are SOIC. You’ll notice national doesnt have a very good selection of low noise, ~20MHz op amps.

    Next, we go to Analog Devices. Enter in package = SOIC, Vcc-Vee = 3V, noise STMicro (click on “low noise”) they don’t have anything that runs on 3V.

    Linear Technologies has a bunch of incredibly low noise op amps, at extraordinary bandwidths, but they’re rather expensive, at least $3 which is outside of our budget.

    Maxim has a few good options (sort by noise, then compare all of the ones 20MHz GBW, that leaves the MAX4488.

    Part Noise (nVrtHz) GBW (MHz) Vos (mV) Iq (mA) $/qty 100
    OP37 3.2 12 0.03 5.5 $1.07
    OPA350 5 38 0.5 7.5 $1.73
    AD8655 4 29 0.05 4.5 $0.88
    MAX4488 4.5 42 0.75 2.5 $0.72

    There are a couple other manufacturers but we’ve covered the most common. So let’s stop here. The OPA350 is clearly not a good choice, it’s the most expensive, noisiest and most power-hungry. So we’ll just ignore that, leaving the AD8655 and MAX4488. Now its just a comparison between price, power and noise. Since our noise figure is already higher than we’d like, I’ll place priority on that: the price difference is pretty small and power isn’t a huge priority.

    Finally, we have chosen the AD8655. Yay Analog Devices!


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    2 Comments

    1. This is a nice example and I suspect you’re aware of the following but I wanted to mention a couple things for the potential edification of other readers:

      1. The Art of Electronics also contains some good information on op-amp selection.
      2. In a somewhat demanding amplifier design such as the one in your example, it is sometimes also worth considering a two-stage design. Sometimes two op-amps of somewhat lesser specs (purchased as a dual monolithic device) cascaded appropriately work as well as and cost less than a single fancier one. You generally wouldn’t be able to get out of whatever noise requirement you have (for the first stage anyways) but the requirement for gain-bandwidth product of each op-amp is reduced appreciably.

    2. I would like to mention that the AD8655 actually has a thermal noise floor of 2.7nV/rtHz.

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