Choosing a good opamp for a mic preamp

I'm looking for recommendations for a good opeamp to use in a simple microphone preamp circuit for ADI's MEMS microphones. The noise floor of these microphones is at -100 dBV (10 uV), A-weighted, with a 20 kHz bandwidth. The typical use of this preamp will apply between about 12-36 dB of gain, depending on the application. An example of a typical implementation of this non-inverting circuit is shown in Figure 8 of the ADMP404 datasheet.

Of course, the AD797 is a great audio opamp, but this may be overkill for some applications. I have used the OP275 and AD8606 in audio designs, but I've been looking at other opamps as well. The noise, distortion, and slew rate specs of the AD8597, ADA4841 (replacement for the OP275?), and ADA4075 (recommended in this thread) look good. Are there any good choices I'm missing here, or other specs I should be thinking of?

The MEMS microphone datasheets currently show an OP177 in a preamp circuit. Is this a good choice, or would another device be better as a general, standard recommendation?

  • Thank you for the suggestions. I should've mentioned something about the supplies in my original post; that certainly affects which device we would select. The MEMS microphones work with a 1.5-3.6 V supply. For many designs using these microphones, the only available supply may be 1.8 or 3.3 V. Which opamps would you suggest to use at these lower voltages?

    The MEMS mics have a lower -3 dB corner at 100 Hz, so I'd want to have a decent response down to that frequency.

    This isn't for a specific design right now. I'm making sure that our recommendations in the datasheet are up-to-date, and that I'm ready to answer the question when it's inevitably asked of me!

  • JeradL,

      A lot depends on what supply voltage(s) you have available.  The AD797 is on an older process than the AD8597 and is really

    high speed, so it requires a very clean four layer (or more) pc board layout with two bypasss caps on each supply pin.  (see d.s.)

    If you have 5V only, some of the older bipolar op amps need at least 6-8V to wake up.  Is this for a particular customer requirement

    or for an evaluation board?  The fact that you are taking a lot of gain says you can use a decompensated op amp such as the OP37.

    Bipolars in general have lower 1/f noise corners than CMOS, but if you only need to go to 200-300 Hz, they might be the best choice.

    We update the tables in AN-940 when we add newer op amps, so that would be a good place to look.

      For the general application in figure 8, I would update with an OP1177 or an OP07D.

    Harry

  • JeradL,

      The 1.8V requirement is tough.  There are not many op amps from any manufacturer that would be good for a mic preamp,

    the reason being they are focused on very low power, so they are usually CMOS and have high flatband noise.  For 2.7V

    or higher, the ADA4528-1 is a possibility.  Another idea would be the AD8651, AD8655 or AD8646 dual.

    Harry

  • I certainly understand that 1.8 V may be a bit too low for a good preamp. I don't think it's unreasonable to think that many or most designs with an analog preamp circuit will have at least 3.3 V available. Thank you again for the suggestions. I think there's a good selection of different parts mentioned here that could be used for this application.

  • Hi Jerad,

    I would recommend the ADA4841-1, -2 (single and dual version) in the low voltage application that you are discussing; it is spec'd down to 2.7 V. The ADA4841 has half the Input Noise (2.1 nV/rtHz) and is lower power than the other parts that have been discussed. Unfortunately, it is a little more expensive.

    BR,

    ColemanR