Catalog
A smattering of hobby demodulators
These are just a few more that seem to function well. Some better than others.
Or just use a switch
As ridiculously simple as this seems, it actually works alright. There are a myriad of synchronous switch type versions. Of course, this one needs improvement if your after an actual position measurement. But if the question is just, are you at null, then this might do (at least in the garage any way).
This is especially clever
This is very much easier to put together then it would appear. In fact only the A2A and C1 op amps are really necessary. Don't except super mic precision but with some tweaking this version is fine. The clever part here is all in the A2A op amp. It is non-inverting and inverting, depending on the state of its non-inverting pin. This way you've got yourself a full wave rectifier. Add in C1 to get a square wave from the excitation signal and it's phase sensitive. This one really is about $5 to put together. But of course, all values need to be 1% or better or you'll have different gains from inverting to non-inverting.
A demodulator for five lead LVDTs
This one is stolen from www.mikesflightdeck.com. It relies on having access to the five lead version. This is all much easier if the center tap wire is present. If it is then you can simple rectify the output of each secondary and sum the two. And thats what this circuit does, only Mike is using precision rectifiers instead of just diode. This is because, usually the most valuable information about position takes place very close to null. Simple diode rectifiers would loose the signal below a certain point.
