# Which low voltage single supply opamp for ultrasound mic?

I have a problem choosing right opamp to amplify signal from 10KHz-65KHz mic SPM0204UD5. According to microphone's datasheet, it has max output impedance of 300Ω, and supply voltage from 1.5 to 3.6 V.

Datasheet also has recommended interface circuit as follows:

More precisely, my question is: should I use voltage divider to get 3.6v max on the mic? Can opamp be connected with single-supply or do I need two (and thus 2 voltage dividers to put the mic "around" the Vref, or just up the ground)? Of course, I'd preferred single supply.

Are there single-supply opamps for 3.6v? (I have not found), which are capable to amplify up to 65kHz signals?

I guess, there is some implicit knowledge required to interface the mic.

The output will be some circuit, which will convert part of the bandwidth to audible domain or maybe just measuring the volume. Actually, at first I just want to check if the device work in the audible range, covered by the mic (10KHz-20kHz).

UPDATE: I get it to work with Vcc=3.3v, C1=10uF, R1=1k, R2=100k, Vref obtained by voltage divider with 100k and 100k resistors to Vcc and ground, Load 820 ohm to the ground. U1 - LM4562 device. Pls note, that pin numbers on schematic are different ones than LM4562 uses.

• You have not stated details about your circuit. What is your power budget for the op-amp? What is the voltage of your project's power supply, or what voltages are available if more than one?
– Kaz
Jul 26, 2013 at 14:38
• How to power the mic is a separate question from the shopping question about the op-amp; there are actually two questions here. Whether or not a voltage divider is appropriate depends on the driving power supply: how big is the drop from the supply voltage to the mic voltage, and how much power can you afford to waste in the divider.
– Kaz
Jul 26, 2013 at 14:38
• Power budget not restricted (i.e. not going to be continuously operated on battery). Power from 3v to 5V (that is, just 5v), but up to 9v (single) is fine, if it improves precision. Also, I have the following parts readily: UA741CN, TS271CN, RC4558P. Also, I have +/-12v dual power supply, but I do not like to use it in this project. Jul 26, 2013 at 15:10

The microphone's output impedance is irrelevant to the choice of op-amp, because you "program" that aspect by a suitable op-amp circuit.

The low impedance of the mic means that the amplifier can have a low input impedance, in the thousands of ohms. But if the connection from the mic to the amplifier is short (we don't have to worry about stray capacitance of a cable), it doesn't have to. You can build the amplifier to have a relatively high input impedance, like 50 kOhms and up.

If you plan on using a coupling capacitor, like in the recommended circuit, a low input impedance will work against you: a low R means you will need a large C to maintain frequency response, which is linked to the RC product. (Since you give the audible range as 10 Hz (!) to 20 kHz, it can be assumed that you care about low frequency response).

The choice of op-amp depends on various parameters. This is a shopping question that is generally considered off-topic (on most stackexchange sites). You probably want it to be a low-noise unit suitable for audio, which has published distortion figures which are low. Then you have to consider your power supply: would a dual op-amp IC that drains up to 16 mA of current be acceptable? Or how about one that needs a minimum of 10V across its power rails to work properly: would that work? Cost: is it okay if the op-amp costs ten dollars? Or is fifty cents more appropriate? Output: does the op-amp have to produce output that goes almost all the way to the power rails? Or is it okay if it only goes to within a few volts of either rail before clipping? Manufacturing: are you comfortable with small, surface-mounted IC's, or would it be better to have a classic through-hole part with 0.1" pin spacing?

Whether or not a voltage divider is the best approach to power the mic depends on how much wattage will be wasted, and whether you can afford it.

• Thanks for the input impedance hint. Microphone has range from 10KHz, but I guess, it's just less effective in the lower frequencies. Jul 26, 2013 at 15:27
• This is not really a shopping question. It's rather general direction, what to seek for. Re manufacturing: DIP case is best. Low noise, less distortion - yes. Opamp should not be rare. Power is not a concern. Single supply is better, but divider can be afforded too. Jul 26, 2013 at 15:31
• How about one of these. Dual-supply op-amps can be used single supply; just some of their performance characteristics are degraded, like PSRR and perhaps CMRR.
– Kaz
Jul 26, 2013 at 19:20
• Look suitable for the purpose, and also important, is available from places I get parts. I guess, I will need to try the recommended schematics once I get the part. BTW, I've checked the mic with oscilloscope, and it seems to work even for low frequencies (I had worries as it is my first surface-mounted device). Jul 26, 2013 at 20:06

Regarding biasing of the input: With the circuit configuration given in the datasheet, you should not need any biasing of the microphone output to get it to within the op-amp's common mode range. Ideally, simply power both mic and op-amp from a single 3.3 Volt regulator, and you're good.

For the op-amp's input indicated as Vref, either use a voltage divider between the 3.3. Volt supply and ground, use a spare op-amp as a virtual ground circuit if you plan to use a dual package, or use a dedicated rail splitter IC specified down to 3.3 Volts or so, if stability of the reference is paramount.

There are actually a plethora of suitable single-supply op-amps. Most low-voltage op-amps available today are optimized for single-supply battery operated designs, and also, many support rail-to-rail output.

For instance, by going to the Texas Instruments site, using the parametric search for low voltage op-amps, and narrowing down to GBW > 20 MHz, Vsupply_min <= 2.7 VOlts, slew rate > 35V/uSec, these 4 options show up:

• OPA2835 - Dual, Ultra Low Power, Rail to Rail Out, Negative Rail In, VFB Amplifier
• OPA2889 - Dual, Low Power, Wideband, Voltage Feedback Operational Amplifier with Disable
• OPA835 - Ultra Low Power, Rail to Rail Out, Negative Rail In, VFB Amplifier
• THS4531 - Ultra low power 0.25mA, RRO, fully differential amplifier

Ignoring the differential amplifier, that still leaves 3 options, all far exceeding the specified requirements, with part pricing down to under \$1 in cases.

While a GBW of 20 MHz might be overkill for the purpose, it's best to have a GBW significantly higher than the bandwidth times the gain (determined by R1 and R2) it will be used for.

• Thanks for the search hint. Why fully differential is not good here? Are there other pitfalls to avoid? Jul 26, 2013 at 15:37
• @RomanSusi Because you don't need it, and because a full diff-amp has a Vout+ and a Vout-, so your next question would be what to do with those :-) Jul 26, 2013 at 15:41