Microphones in parallel to reduce noise
in [Design]
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From: osr on 16 Dec 2009 18:28 My boss laid a unit on my desk today. Its got ten electret mics fed into one low noise opamp. Any ideas how they did this without a ton of noise. All I can see from the board layout is it is NOT the classic op amp summer. Any suggestions for papers on low noise signal combining? Steve
From: Jim Thompson on 16 Dec 2009 18:44 On Wed, 16 Dec 2009 15:28:27 -0800 (PST), osr(a)uakron.edu wrote: >My boss laid a unit on my desk today. Its got ten electret mics fed >into one low noise opamp. Any ideas how they did this without a ton >of noise. All I can see from the board layout is it is NOT the classic >op amp summer. > >Any suggestions for papers on low noise signal combining? > >Steve Assuming they are the bufferless version, aren't they all simply in parallel? ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Help save the environment! Please dispose of socialism properly!
From: Phil Allison on 16 Dec 2009 18:50 <osr(a)uakron.edu> > > My boss laid a unit on my desk today. Its got ten electret mics fed > into one low noise opamp. Any ideas how they did this without a ton > of noise. All I can see from the board layout is it is NOT the classic > op amp summer. > > Any suggestions for papers on low noise signal combining? > ** Likely all the mics are simply wired in series and supplied with power from a DC current source and the signal taken from the top most mic. The s/n advantage comes from *correlation*. The 10 noise signals ( from the internal JFET pre-amps) are un-correlated while the sound arriving at each co-sited mic will be heavily correlated. So the output signal will be 10 times that of one mic, while the self noise will be only 3.16 times - so a 10 dB improvement is possible. I suspect the mics are arranged to create a directional pick up pattern too. BTW Dynamic mics have much less self noise than typical electrets - cos there is no internal pre-amp. Means it is easy to get a 10dB or more improvement, just by using one instead. ...... Phil
From: Vladimir Vassilevsky on 16 Dec 2009 18:58 osr(a)uakron.edu wrote: > My boss laid a unit on my desk today. Its got ten electret mics fed > into one low noise opamp. Any ideas how they did this without a ton > of noise. All I can see from the board layout is it is NOT the classic > op amp summer. Electret mikes usually have built-in FET stage. Are you sure they are combined in parallel, not in series? Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Jim Thompson on 16 Dec 2009 19:01
On Wed, 16 Dec 2009 17:58:19 -0600, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > > >osr(a)uakron.edu wrote: >> My boss laid a unit on my desk today. Its got ten electret mics fed >> into one low noise opamp. Any ideas how they did this without a ton >> of noise. All I can see from the board layout is it is NOT the classic >> op amp summer. > >Electret mikes usually have built-in FET stage. Are you sure they are >combined in parallel, not in series? > > >Vladimir Vassilevsky >DSP and Mixed Signal Design Consultant >http://www.abvolt.com > They are available WITHOUT the FET. I have an RFQ on my desk requesting a complex chip that will use Electrets sans FET's. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Help save the environment! Please dispose of socialism properly! |