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Topic: Problem with amplifying a pressure sensor for measuring blood pressure (Read 6 times) previous topic - next topic


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?


If you are looking for a 2 terminal current source try a CC100 diode. It's a specially constructed Jfet that is a 2 terminal constant current diode (@ 100uA). It can be done with a Jfet and a resistor but I suspect that this is a little beyond your current grasp of theory. There are extensive design notes available in the App note but I doubt they are available any longer... Siliconix was one Mfr of CC diodes...
The app notes I refer to are from the National Semiconductor analog design app notes, Vol 2.

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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:


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.


Words can be such a pain. Lower tolerance is the opposite of +/- smaller percent. Did you really mean that using a lower tolerance resistor made it work when the higher tolerance resistor did not? If so then the resistance you really want is not the one calculated but closer to what the LT resistor actually is.

A 5% tolerance resistor is higher tolerance than a 10% tolerance resistor. The higher the tolerance, the higher the accuracy must be to pass. It's intuitive if you make things to tolerances but otherwise not.

I think that they test the things every so many and if within 10% then that part of the run goes in the 10% bin even as the process is tweaked for higher accuracy. Still there's nothing saying that +/- 10% can't be within 1%, just that's not likely given consistent manufacture.

I find it harder to express logic in English than in Code.
Sometimes an example says more than many times as many words.

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