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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Dr. Slack wrote ...
Sulaiman wrote ...
The common way to achieve what you want is this
Your circuit will give Av.(Va-Vb) PLUS Va ...
i.e. if Va = Vb, the output will be Va (=Vb) ... NOT Av(Va-Vb) = 0
I've captured the drawing at that link and very slightly redrawn it
to show the symmetry and emphasise what it is doing. The op-amp will work to keep both its inputs at the same voltage. That means that as the top pair and bottom pair of resistors have the same ratio, the *difference* between the output terminals will be -g times the difference between the input terminals. Now you can put the output ref terminal to where you like. Often it goes to ground and you have a floating to single-ended (that is - referenced to ground) converter. You could also regard the circuit as adding the output reference to -g * the input difference to give the output voltage. Note that the ratio g can be bigger or smaller than 1.
Just my halfpen'orth anyway. I can never remember configurations, so whenever I come to want to use any sort of differential amplifier, I first draw this symmetrical form, then move it around until it does what I want.
I've created a differential opamp as described here and in your drawing above.
I have a reference voltage between 0 and 1v that goes to V1 and another voltage, between 0 and 4v going to V2. The output is V2-V1. This works fine until I start lowering V2. I set V1 to 0.2v. Originally, V2 is 1v and the output is 0.8v. As I lower V2, the output drops. When I get close to V1 the output stops dropping, plateaus and then starts to rise. Does this have to do with either the opamp offset not being low enough, or the fact that my resistors, which are all the same value, have a 1% tolerance and not 0.1% tolerance?
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I fixed the problem by using an opamp with a lower voltage offset. The original was 3mv. This one is 0.1mv. I can now get the differential down to 1.4mv.
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op amps
