OK, I'll do that. But I'd like to figure out why this circuit is not behaving like I expect. It bugs me that I can't even get 1.25 volts that this IC is supposed to maintain across those 2 pins. I'd like to see if I could get something like 1 mA or something from it.
If someone can tell me that the breadboard looks OK that would be very helpful. At least I would know if I'm understanding the circuit diagrams. Thanks for any help. I think I'll go read some of the general electronics post and see if I can feel comfortable with other circuits and their discussions.

Didn't realize the thread was continuing. I have been busy reading some books and trying to understand electrical circuits and how to do the current source.

I'm working with an LM317T, but I'm having trouble with the basics. I thought I understood what was going on, but I don't.
V=IR, so since the IC is maintaining 1.25 volts across the "output" and "adjust" pins, I can use a 12.5kOhm resistor.

Oddly, I never get a 1.25V reading across the output and adjust pins.  Obviously I'm missing something.
If anyone wants to take a look, this is my understanding of the circuit diagram that I'm trying to duplicate in the breadboard. Attached pdf.

But I am having many difficulties with the basics. The voltages at the output and adjust seem to be zero, with both being about a 0.5 volts less than the input with reference to ground.

I am also having difficulty measuring current. I setup an LED so that I make sure that the current was flowing thru the multimeter, but when I connect the multimeter in series the LED goes off.
My multimeter is very confusing in regard to units of current (uA/mA/A), and seems unstable. But that's probably cause I'm doing something wrong if I can't get current to go through it.

And when I plug the circuit into a higher voltage power source (2.5V, 3.3V) the LED gets brighter, which would seem to indicate more current and thus not working as a constant current source. As I think LEDs get brighter on current and not voltage. When I plug in a 5V power source it starts fading and getting erratic so I guess that voltage is nearly out of range for the LED.

I did manage to get IC fairly warm when I plugged stuff together in certain ways so I assume it does work.

Oh, and yes at some point I can calibrate the sensor. But I still likely need a constant current source so that the calibrations don't change over time.

Don't trimpots usually have much higher tolerances and produce noisier signals?

The signal is looking better with a better resistor for the instrumentation amplifier. I used a 10.2kOhm/0.1% tolerance. I attached a pdf of the pressure signals again.

The pictures are from 2 different trials.
Oddly, it does seem the device is altering the pressure a bit when it is detecting the systolic & diastolic blood pressure. Cause some noise-like signal appears where those pressures are in each trial.

You're right, I worded it wrong.

I meant the opposite. The good resistor I have is 0.05%, and the other one I'm using I don't know. But I'm guessing probably 5 or 10%.

I think part of my noise problem is the resistor I'm using now for the instrumentation amplifier.

I don't know the tolerances on the one I have but it's probably not good. The one I was expecting to use, until it was apparent the output of the sensor was 10x what the datasheet said, was very good. Now I'll just need to find one with better tolerances.

I hooked up the resistor with lower tolerances and the signal looks pretty clean with me just blowing pressure in the tube to the sensor.

I'm sure you're right that I won't comprehend it, but I can get some assistance from others who do. So I'll look into it.

I do see this which may be what you were talking about:
http://www.vishay.com/docs/70596/70596.pdf

I have a few 0.1uF caps. Will that work?

So where there is voltage output of the sensor, I bridge it to ground with a capacitor?
And where there is voltage output coming out of the opamp, I bridge that to ground too with a capacitor?

Here's the updated breadboard in case anyone wants to wire it up.


https://docs.google.com/file/d/0B_XDOJc4fAYxWk9ON1pLVTFDdHM/edit?usp=sharing

edit: The 2SMPP-O2 ic is upside down, so it probably looks wrong. It only comes in a SMD and I had to drill a hole in a DIP adapter for the pressure port, and put it upside down. So it goes like this in pic:

1 2 3 -
6 5 4 -

Also, do you consider this very noisy now that I've zoomed in? Attached pdf.

BTW, it's also sampled at 100Hz in the picture.

Thx. I didn't quite get find anything that was super clear if there was a single IC that I could get to do this for me.

It looks there's a typical op-amp setup to do this, but requires several resistors and an op-amp.
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=10&cad=rja&ved=0CHUQFjAJ&url=http%3A%2F%2Fwww.physics.byu.edu%2Ffaculty%2Fpetersonb%2Fphys240%2FConstantCurrentSource.pdf&ei=5EItUfvUB5HSiAKbooHIDQ&usg=AFQjCNEegrXZxq3JjrtV9dBqsFn3UaezcQ&sig2=o5itmazSvhixlP5Yr0JmiA&bvm=bv.42965579,d.cGE

http://michaelgellis.tripod.com/howland.html

I did find this:
http://www.reuk.co.uk/LM317-Current-Calculator.htm

Could I just use an LM317 with a single 33kOhm resistor?

Or what if I combined this with a voltage reference IC, or is that too demanding?

I haven't taken any measures to reduce noise except to use the internal teensy 3.0 feature to output the average of multiple samples (ie 16).

As it is, I think the noise is probably ok. The initial noise with the pump is the nature of the erratic piezo pump in getting to max pressure.

I haven't calibrated the values yet so the values are approximate, but the output looks pretty good. I changed the resistor to 10Kohm.
Attached is a picture of the graphed output and the indications of what is occurring.

Ok, finally had some time to sit down and check this out again.

It was definitely good to measure each item separately and check with a voltmeter.

I was probably wiring a couple of things wrong, and luckily it didn't seem to hurt anything.

The sensor was outputting -4mv with a null signal. Then the signal turns '+' for most of the output range. The datasheet mentions -40mv at resting, but in other places I've seen -4mv.

I only then connected up the INA122 and got up to nearly the 3.3V output. I was worried about the -4mv being amplified, but it seems to start from zero and go '+' from there. So, it looks like this combo works pretty well. Signal seems stable with me just blowing pressure into the tube.

Now, I'll pneumatically connect up to a blood pressure cuff and I'll post a pic of the pressure signal when I do. Cool!

I'll also update with my breadboard, I think I had some things wrong.

Thanks for your help.

I bought several more sensors when I ordered (somewhat expecting something to go wrong) and my reluctance to pay more for shipping than items. I'll try and figure out as much as possible with the potentially broken one. It should be a good learning experience.

I'm only using 3.3V, not 5V. I don't know how much current would come from that so I may have destroyed the sensor. So, I should use an op-amp to get a constant current source? I'll look over the example again, as I didn't realize that. I'm pretty naive to the electronics but I'm slowly getting there. It was confusing on the datasheet, cause I think it wants 2.5V for power, but the mouser page had it listed as 5V. And then it mentions current, so I assume it's somewhere in that range but current needs to stay constant.

Good point, I should check output directly from the sensor.

You are suggesting that I put a potentiometer and take the power source and divide the voltage and read the inputs? I think I checked the analog inputs on another sensor, but I should double-check that cause I can't quite remember. I'm pretty sure I get 0 connected to A-REF, and a 65535 connecting to 3.3V power pin.

Currently my analog input is hovering around 20,000, which should be at about 1V ((65536/2000) / 3.3).

Thanks.