From: rcrowley@xp7rt.net   
      
   "Randy Yates"  wrote ...   
   > FYI, the reason for my question is that I am attempting to review the   
   > design of the front-end of a heart rate monitoring device. This design   
   > is based on the SSM2167 (e.g., from National).  My thinking is that this   
   > is the wrong part for the job since the sensor is fed to the preamp via   
   > a run of cable perhaps 4 feet in length, and there could therefore be   
   > lots of common-mode noise.   
      
   It does sound like a less than optimal design.  OTOH, if they are only   
   looking for cardiac sounds, they could make the case that everything   
   of interest is *below* the power mains frequency (50/60Hz) and can   
   be simply low-pass filtered.  So the kinds of things we do for wideband   
   audio are maybe not necessary for that special case?   
      
   > Also, I believe the AGC action is, in this stage, not appropriate.   
      
   I agree from an audio perspective. But maybe the full waveform is not   
   what they are looking for?  Maybe only the cardiac rate is desired here   
   and preserving the exact waveform is not important?  It would presumably   
   be different if they were running a full EKG.   
      
   > This appears to be an electret type of microphone in that DC power is   
   > fed to the device, so the poster who discussed how to feed phantom power   
   > and simultaneously receive the input differentially was right on the   
   > money.   
      
   There are some clever (and simple/elegant) circuits for using commodity   
   electret elements, feeding power, AND providing a fully balanced cable   
   run from the capsule to the preamp, etc.   
      
   When I went in for a sleep study, they taped a (disposable) sensor on   
   my finger with a SMD red LED over my fingernail, and what appears   
   to be a small photo-resistive element on the opposite side of my finger.   
   They said that they read both cardiac rate AND blood oxygenation   
   from the sensor.  I kept it as a souvenir. It is possible that there is also   
   an IR LED and/or sensor in there, but I haven't put it under the micro-   
   scope yet.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)