Using AD1938 ADC1P/ADC1LN as MIC‑In

Hello Avnendra,

I would not try to add a lot more gain on the single ended to differential amplifier. Let that work well to develop a differential signal. 

So I suggest you use a mic preamp before this stage. There are so many ways to do this. It is hard for me to tell you what to do because it so depends on your application. Out linear amplifier group can help you with pre amp designs. 

There are a lot of variables. What is your power available? How much expense or sound quality do you want? Transformer based? How small does it need to be? Is the application battery operated where power is super important? 

This is what I mean that there are so many options. 

Dave T

Hi Dave,

Thank you for the guidance. We’ll keep the single‑ended‑to‑differential stage at minimal gain and add a dedicated mic preamp ahead of it.

We’re looking for a compact solution that works with common earphone/headset microphones. PCB area is tight (80 mm × 65 mm), so size and simplicity are priorities. We have 12 V and 3.3 V rails available, and the system is not battery‑powered. We’d prefer to avoid transformer‑based options due to space.

Could you recommend a basic preamp circuit we can place ahead of ADC_IN (which comes from our TRRS jack), ideally including the required mic biasing and AC coupling? Any reference schematics or app notes from your linear amplifier group would be very helpful.

To help us converge quickly, a few quick questions:

• Would a single‑supply 3.3 V preamp be sufficient, or should we use 12 V for additional headroom and lower noise?
• What gain range would you suggest for typical headset electret mics?
• If feasible within our area constraints, do you recommend a simple method for optional line‑in/MIC‑in switching on the same jack?

Thanks again for your support.

Regards,
Avnendra

Hi Dave,

I wanted to follow up on the request Above. If possible, could you connect us with your linear amplifier group or point us to suitable reference designs for a compact headset MIC preamp? That guidance would be a great help.

We’re happy to provide any additional details you need on our constraints and existing input stage.

Regards,
Avnendra

Hi Dave,

I’m following up on my earlier query. Could you please advise whether a simple RC network (bias resistor and coupling capacitor) as shown in the image below would be sufficient for the headset MIC input to work, or if we should still plan on a dedicated preamp stage?

If the RC approach is viable, any guidance on recommended values, expected input levels, and potential limitations (noise, impedance, headroom) would be very helpful.

Regards,
Avnendra

Hi Dave,

I’m following up on my earlier query. Could you please advise whether a simple RC network (bias resistor and coupling capacitor) as shown in the image below would be sufficient for the headset MIC input to work, or if we should still plan on a dedicated preamp stage?

If the RC approach is viable, any guidance on recommended values, expected input levels, and potential limitations (noise, impedance, headroom) would be very helpful.

Regards,
Avnendra