ViaDesigner Pulse Oximeter

[lgbeno]

So I'm all setup with a ViaDesigner license from the amp hour and I think that I'm going to try my hand at designing a pulse oximeter circuit.  For now ill just use a red led and photo diode to measure a waveform.

Basic architecture is to use a opamp and pnp to create a constant current led drive.  Then another opamp as a TIA.  The output of the TIA will be ac coupled to a gain stage followed by a low pass filter and the a buffer amp to drive a ADC.

Anyone have experience with such a circuit or any words if wisdom?

I'll use this thread as sort of a project log.

[babysitter]

make the led current controllable, not fixed. so you can make it brighter with big thumbs and dim it when a lot of light reaches the receiver. pulse the led to discern from ambient light. its not a pulseoxi, its a photoplethysmograph (if you plot optical attenuation over time) or pulse sensor.

written with baby on my knees

[lgbeno]

Thanks for the advice!  Sorry I haven't been on the forum for awhile but have been working in this project.

I did notice exactly what you are talking about wrt the variability of Rx light through the thumb vs finger.  If I use the same led current through a finger, it saturates the diode however works great through the thumb.

To address this, I made my current reg variable and placed an op amp to regulate the DC common mode of the Tia output (filtered through LPf of 2meg ohm w/ 2uf).  This still presents a problem because that regulator also cancels out my pulse signal because this as well is very low frequency!  I thought that time constant would crush it but I can still see my pulse leak through on the current output.

It might be time to setup a dac and some logic instead...

[lgbeno]

By the way, like the dedication of staying on the forum while caring for kids.  When my two year old sits on my lap I have the hardest time keeping her away from pressing keys in the keyboard.  "Super letters" as she calls them.

[babysitter]

OK, you talked about a DAC taking care of the measured signal, so I guess there is a Microcontroller around, too. Use a PWM output and a low pass filter to make a little DAC to control the light intensity. In the µc, you can play with the time constant of the LED intensity regulator however you like it.. even measuring it once, keep it on a fixed value until the fixed value doesnt work anymore because the ADC is over/underfed.

Fixed intensity with a resting test person can tell you a bit about the tendency of the blood pressure rising or falling.

Yeah, kids are great - mine always wants too see what I am doing, but at nine months still quite easy to keep off of the devices i am playing with