Drift/Noise problems for an Instrumentation Amplifier

[Fish]

No, I don't need a fully differential amp. I'm not using one anymore. In my earlier post, I think I mentioned the phototransistor measures small motions in a lever arm. This displacement is porportional to the force exerted by the muscle over small displacements. The displacements have to be very small, because I want the msucle to be (approximately) isometric.

I actually have a force transducer that uses a photosensor as an analog device, although it operates a bit differently from what I'm doing. This is a commercial product that costs hundreds of dollars, so I'm trying to build a cheaper alternative for personal use.

A load cell/strain gauge would work, I admit, but as far as I can tell, there's no load cell small enough for my uses. I need to use a really tiny lever so that the force transducer can respond quickly to changes in force (hence, displacement), that's why I though an optical device would be a good idea. The magnitude of forces I'm talking about are from less than 1 g to about 20 g max. Things are working approximately, but I'm getting seven sorts of hell trying to work out the non-ideal problems =)

 

[Oznog]

No, I don't need a fully differential amp. I'm not using one anymore. In my earlier post, I think I mentioned the phototransistor measures small motions in a lever arm. This displacement is porportional to the force exerted by the muscle over small displacements. The displacements have to be very small, because I want the msucle to be (approximately) isometric.

I actually have a force transducer that uses a photosensor as an analog device, although it operates a bit differently from what I'm doing. This is a commercial product that costs hundreds of dollars, so I'm trying to build a cheaper alternative for personal use.

A load cell/strain gauge would work, I admit, but as far as I can tell, there's no load cell small enough for my uses. I need to use a really tiny lever so that the force transducer can respond quickly to changes in force (hence, displacement), that's why I though an optical device would be a good idea. The magnitude of forces I'm talking about are from less than 1 g to about 20 g max. Things are working approximately, but I'm getting seven sorts of hell trying to work out the non-ideal problems =)

Yeah, there are load cells that will do that. Your best best might be to use a simple electronic scale- there are some that go down to 1 g or 0.1 g increments. If you want to remove the load cell from it, then you're in a case where you actually do need a differential amp or a differential ADC.

Now that I think about it, not that it's really my business, but what possible use is an isometric measurement of such a muscle? It doesn't say much about it effectiveness- if the tiedown points are totally immobile, I could tie a piece of string or wire between them and if the temp or humidity changes it could create many grams of force. But it can't lift anything over a significant distance so the number is irrelevant. Nichrome muscle wire doesn't act as a muscle because the force varies, it is a muscle because its length varies.