Stereo Line-Input & Microphone Mux

[SeattleChris]

Hello everyone,

I need to share the stereo line inputs to a codec with dual electret microphone inputs. My plan is to have a 2-channel analog switch under CPU control and feed Left Mic & Line to one channel and Right Mic and Line to the other. The Mic inputs would of course be fed through pre-amps first.

Am I on the correct path here? I'm specifically concerned about mic-pre selection and whether or not I need a PGA. I've got the LMV1031 earmarked but I'm interested in other suggestions, better approaches, etc.

The on-chip PGAs for the line inputs have a gain of +12 to -34.5dB while the single on-chip mic pre can provide 0 to 20dB gain. So I don't think I can get away with no pre-amp in the mic chain. Part of the problem is that we will be evaluating different mics and sensitivities so I don't know in advance exactly how much gain I'm going to need. Since I need dual mics it makes the most sense to share the line inputs.

And no, I can't change the codec... sorry... :O(

Thank you in advance for all your help.

-SeattleChris

 

[VEBill]

Sometimes one can use the type of input jacks with the built-in switching mechanism so that plugging in an external connector disconnects the internal mics.

 

[SeattleChris]

I do not have any flexibility with respect to the inputs. I need to solve the problem as stated.

The line and mic inputs are always simultaneously present and the CPU (embedded application) decides which pair are active at any given time, depending upon device state.

I've solved the analog switch problem and the only thing remaining is identifying a suitable op-amp that is also 1.8V characterized (the hard part), small (BGA or CSP), wide-band, low-input offset voltage, low-noise, and less than $0.50 for a pair of them in high (200K piece) volume.

If anyone has any suggestions in this regard I would appreciate it. I've scoured all the amplifier suppliers I know of and can't find a fit.

Thanks,

-Chris

 

[VEBill]

I thought I was addressing your first question (starting with "Hello..." and ending with the first "?").

Sorry.

 

[jimkirk]

Do you have actual specifications for offset, noise, bandwidth, slew rate, supply current, et cetera. That would help.

Analog Devices has plenty of op amps specified to 1.8 volts that may suite your needs.

Easy to get specs in the range of 5 to 7 MHz small signal bandwidth, 0.8 to 1.3 mV offset, 15 to 23 nV/rtHz voltage noise... and down to 23 cents per channel in 1k to 5k quantities.

I'd guess Linear Technologies and plenty of others have a similar set of offerings.

 

[SeattleChris]

The specs you mention should be sufficent with the additional need for Input Bias Current <= 100pA for electret mics.

ADI has lousy packaging selections and Linear Tech is notoriously expensive (though good). National seems to be the best bet for 1.8V parts in small packages.

I've got some additional feedback from Robert Pease at National as well. I'm going to follow up on that lead now and get back to you all.

Thanks,

Chris