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I'm building a blood pressure measuring system (passive). I'm plugging inline to a portable blood pressure cuff system, to passively measure the pressure in the cuff with a teensy 3.0.

I've identified an inexpensive sensor that should work.

The specs on the mouser page say that the sensor is powered by 5V, but I don't see any mention of that in the datasheet. The teensy operates at 3.3V.

The output of the sensor is from 0 to 4mV, so it would ideally be amplified 825x to get the full range of 3.3V of the ADC on the teensy 3.0, although considerably less amplification is probably fine. Especially since the teensy 3 has 16-bit ADC.

Can I use the 5V from the USB power driving the teensy? I'm not sure if I can get the USB 5V off of one of the teensy 3 pins. And is that a good thing to do because it should be more amplification? I would imagine it's better to use the 3.3V since then it would be the same voltage as a reference for the ADC, as well as the sensor.

But most importantly, what is a simple amplification circuit that I can use? I would prefer buying a single chip to get the voltage amplification but any other pointers to get something working would be much appreciated. Obviously I just need the voltage scaled, I don't need any more current. I'm still a novice at the electronics so the simpler, the better.

Mouser page: http://www.mouser.com/Search/ProductDetail.aspx?R=2SMPP-02virtualkey65300000virtualkey653-2SMPP-02
Datasheet: http://www.components.omron.com/components/web/PDFLIB.nsf/0/DEFC0948FE2A5C91862577A7005A8099/$file/2SMPP_1112.pdf

teensy 3: http://www.pjrc.com/teensy/schematic.html

Thanks in advance,
Oscar
« Last Edit: January 30, 2013, 06:37:02 pm by oscarcar » Logged

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Have a look at op-amps for your voltage amplification requirements.

It'll be a little bit of a learning curve but that will point you in the right direction.

From the blood pressure point of view, I'm curious to know how you will figure out the systolic and diastolic pressures - are you hoping to look at the pulse variations?

I don't suppose the reference voltage issues will matter, because provided your amplifier only goes up to 3.3v, then that will be the highest you can read (provided the op amp supply voltage is the same 3.3 volt supply that is fed into the AREF).

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I'm thinking an instrumentation amplifier would be easier. I'm trying to read up on it, but it may take awhile.

Something like this might suffice: http://www.mtmscientific.com/dcamp.html

Yes, I should be able to see it with the loss and re-emergence of pulse variations. The portable unit will tell me what the systolic and diastolic that it determined, so I can work backwards from there.
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See if you can make it work with an irregular heartbeat, the ones I work with don't manage that very well!

Another way to get the info of course, is to hack into your existing monitor, if that's an option ;-)
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I'd rather amplify a sensor, since it seems I need to do this for so many projects that I have in mind.

It looks like if I use an INA122, and use a 250ohm resistor, I'll get 803.3x gain.

But it's unclear to me on a couple of pins on where it should be wired. I know that if I entirely understood the basics here I would realize it.

INA122        |  
--------------------------------------------
pin3 Vin+    =    Vout+ 2SMP-02
pin2Vin-      =    Vout- 2SMP-02
pins1->8     =    250ohm resistor
pin6 Vo       =    teensy GND
pin4 V-        =     ? (Is it GND also)
Pin7 V+       =    A0 on teensy
Pin5 Ref      =     ? (seems to me that it should be GND again because I don't want voltage added to my signal)
« Last Edit: January 30, 2013, 08:01:19 pm by oscarcar » Logged

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