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| Joined: | Sep 22nd, 2008 |
| Location: | Doylestown, Pennsylvania USA |
| Posts: | 130 |
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The diagram is correct since the coil is floating and current flowing in the coil will appear as a differential input to the opamp. Common mode is referred to ground and cancels between differential inputs.
The opamp details are not included regarding gain or ground reference.
joecioppi
Greetings, Stephan. Welcome back! Hello Mark!
Gentlemen, I must chime in here. Stephan, you have created a diagram that shows what I was posting about! Thanks for that!
However, unless we are looking for a common-mode disturbance, we would not connect both outputs of the coil to the inverting and uninverting inputs to the opamp.
I think we would rather ground the inverting input to that opamp; connect one of the leads of the coil to that ground point; and connect, via a stabilizing resistor, in series, the other lead of the coil to the uninverting input to the opamp.
Then, set the opamp gain to a known value, such as 100,000, by feedback, with a medium value capacitor, such as a 0.0147 microfarad disc, and a 100kOhm resistor in parallel with the capacitor (this is only a starting suggestion). This would stabilize the amp altogether.
At this point, while the opamp is quite stabilized, we must consider the mechanical physical relationships. The length of hair is quite short, and will resonate at a very high frequency; far above what we, as humans can possibly hear. In addition, springs change their shape; and resistance to change, based on temperature, and humidity; as will the hair.
So we don't have a base to compare to, unless the system is placed in a known environment of such temperature, and such humidity. When we know that, we can place the system under test.
I only offer these thoughts to further help in design of the system.
Blessings!
Jeff