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Message started by mender on 11/17/10 at 21:42:47

Title: gain of amplifier for heart rate count
Post by mender on 11/17/10 at 21:42:47
i am using ir led and qsd 123 ir phototransistor for sensing heart pulse rate using sensor in finger. what should b the biasing of qsd transistor and more importantly, what should be the gain of amplifer for achieving good signal?

Title: Re: gain of amplifier for heart rate count
Post by barragan on 11/19/10 at 05:58:48
I am not an expert in such theme, anybody? I will forward your consultation to a friend.

Title: Re: gain of amplifier for heart rate count
Post by barragan on 11/19/10 at 14:19:49
My friend Sebastian suggest this:
I assume you're using the light level transmitted through the  tissue of the fingertip to determine the heart pulse rate. In such set-up, light intensities recorded at the fototransistor will have variations corresponding to the blood volume changes in the tissue. Even when the infrared LED is well aligned with the fototransistor, light is diffusely scattered through the fingertip and so the signal-to-noise ratio SNR will be very low. I would firstly recommend using a instrumentation amplifier to reject white noise (http://focus.ti.com/docs/prod/folders/print/ina114.html) with a gain between 1 and 10, and then, assuming you want to record and process the signal using Wiring, an additional subtracting stage would be necessary to offset and  scale the signal to a 0-5V domain. Regarding your question itself, the blood signal will probably be in the range of tenths to hundreds of milivolts, then the total gain of the instrumentation chain would probably be between 50 and 100.

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