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 (
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
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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