OPA1622: opa1622

Hello Sagar, 

To start the discussion on your project I will make some device and circuit suggestions to see if we can identify products for your design to narrow down devices. From there we can start to look at technical details as they arise. There isn't a chip that would provide all the functionality in one product. 

For the microphone you will need a pre-amp stage. The input signal level produced by the microphone will be dependent upon the microphone sensitivity. Nonetheless a pre-amp stage will be required followed by mixing stages and the headphone driver. 

The OPA1641 can operate at a minimum supply of 4.5V, and has a high input impedance, JFET front end. It has very low noise and also provides ultralow distortion due to its stable input capacitance. I have linked the product page below. If you scroll down on the page you will also find additional resources that may be of interest regarding noise, headroom and headphones. 

https://www.ti.com/product/OPA1641

I assume you have already seen the application circuit for the OPA1622 driving headphones. I would like to also suggest looking at the INA1620. Specifically the application note on using parallel amplifiers for higher output power. Both the OPA1622 and the INA1620 can drive 32 ohm headphones. The biggest consideration will be how much power is needed. The following application note gives a design option that could be used with any amplifier, however showcases the INA1620 and its capabilities. 

https://www.ti.com/lit/an/sboa352/sboa352.pdf?ts=1670626985952&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FINA1620

Some questions I have regarding your design:

• What is the sensitivity of your microphone? Do you have specifications for the mic?
• Does your application require analog or digital mixing?

We don't provide software on our devices to perform mixing and volume control. For volume control this could be done by adjusting gain of the headphone amplifier using a potentiometer as an example. If analog mixing is desired, this could be accomplished using op amp equalizing circuits. Let me know if you have questions with any of my suggestions above. 

I hope this information helps guide your design. 

Best Regards, 

Chris Featherstone

Hey Chris,

 Thank you for the info. Let me ask one more question before going down this path. We are a hearing aid manufacturer, so essentially I am looking for a hearing aid type amplifier. My current situation is the circuits I buy are somewhat band-limited in the lower frequencies and don't give me enough headroom to process the high input levels like rock concerts or clubs where the performer wants to hear the ambient levels without clipping/distortion. So, my concern is the power and voltage levels because my battery source will likely be a NiMH 1.3 V DC rechargeable battery. I also use your TPS613225A to provide a boost for the microphone power which allows me to power it at 5V DC which is supposed to help achieve input of 135dB SPL  without distortion. I need a better amplifier, so just more info for you to see if you still recommend this path.

Regards,

Sagar

Hello Sagar, 

I am currently looking into this and will respond by end of business day tomorrow. 

Best Regards, 

Chris Featherstone

Hello Sagar, 

Based on the details provided it is a bit difficult to recommend a specific amplifier in particular. I believe you are looking for a very low power device in the 10uA range based on the Iq specification of the TPS61322. The OPA1622 is fantastic for audio applications however is much higher in supply current and won't satisfy the minimum voltage supply of 1.5V. The OPA1622 has an Iq of 2.5 mA per channel. This is significantly higher than the TPS61322. Texas Instruments does provide low power amplifiers for microphones such as the LMV1015 or the LMV721. Both have minimum supplies of 2V and 2.2V respectively. 

I had some questions based on the application. 

• What is the max acceptable supply current for your application
• How much power needs to be delivered to the 32 ohm load? This will help me determine the output current drive needed from one of our amplifiers. 
• Do you know the input signal level in volts peak to peak that the microphone will provide to the input of the amplifier providing the pre-amp function?

If you know the sensitivity of the microphone we can determine the voltage produced. This would help determine the input voltage range provided to the amplifier to find a suitable amp. As an example:

1 Pascal = 94 dB

112.5 Pascal = 135 dB

If the microphone has a rated sensitivity of -50 dBv or 3.16mVrms/Pa we could say that the microphone produces 3.16 mVrms when exposed to a 1 kHz tone of 94 dB-SPL at the capsule. For 112.5 pascals produced the microphone would output 355.5mVrms of signal level. This would translate into roughly 1Vpp.  

I believe you may be looking for a fully customized chip for this application which unfortunately we do not have. The power requirements would be hard to meet with our amplifier selection for audio. 

Best Regards, 

Chris Featherstone

Great ?. I need the current consumption to be under 7 mA max, and averaging no more than 5mA. Need about 6 hour usage on a 27mAH NiMH cell. Also, the balanced armature speaker we are using is more efficient than a dynamic speaker. We want to limit the output to 110dB SPL: Hope this info is enough....

Thanks Chris!!

Hello Sagar, 

Do either the OPA1622 or OPA1641 look appealing for your application? The OPA1622 can operate at a minimum supply of +/-2V. We can put together a simulation if it would help of a pre-amp configuration. 

Best Regards, 

Chris Featherstone

Hey Chris,

 So, just use it as a pre-amp for my hearing aid sound processor? Here is a spec sheet on the hearing aid processor, can it handle the output of 1622 or 1641?Ethos_datasheet_22945_A.pdf.

Regards,

Sagar

Hello Sagar, 

I am looking over the datasheet for the Ethos device. It appears to be intended such that the microphone plugs directly into the inputs of the Ethos device itself. It has pre-amplifiers built in although it suffers from limited input noise performance. Unfortunately the datasheet is lacking information in regards to what the input signal range can be in order to determine whether or not a stage of amplification prior would be acceptable. The Ethos chip has built in programmatical tone shaping that can be used to filter the frequency spectrum or in other words mix the tones at different frequency bands as desired. It is a very specific customized chip suited for hearing aids. 

I could recommend a circuit that would take the signal from the microphone and act as a low noise pre-amplifier to achieve better SNR performance prior to the Ethos stage. Based on the provided datasheet I can't determine how high of a gain is acceptable such that I do not violate the input signal range of the Ethos. The input impedance of the Ethos chip doesn't appear to be available either. I wonder if both pieces of information are available?

As an additional resource to glance over, we have the reference design below that takes an Electret Microphone similar to what you are using and creates a pre-amplifier stage. The output signal could be fed into the Ethos device however the same question applies as to what the input signal range is of the Ethos. The other limitation is the minimum amplifier supply of 4V total of the OPA1641. The OPA172 minimum supply is 4.5V total and this is the amplifier used in the reference design. The intention of this reference design is to amplify signals from an electret microphone to line level. Even if this doesn't fit your application, the reference design document does provide valuable information that you would find of use in regards to using the specification of the electret mic and converting the signal levels accordingly. 

https://www.ti.com/lit/ug/tidu765/tidu765.pdf?ts=1671174091562&ref_url=https%253A%252F%252Fwww.google.com%252F

Best Regards, 

Chris Featherstone