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Advanced Beginner interested in learning more especially abt analog synthesizers
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rstofer:
I decided to practice what I talked about with the 30V supply and virtual ground.  I have attached a .pdf of an LTspice schematic.

Important things to note:

* The virtual ground is created by a voltage divider consisting of R1 and R2
* Everything is referenced to this virtual ground.  Note the signal input and scope output connections
* I drew the power supply as a simple 30V floating (ungrounded) supply as this is what you will be using
* I forgot to show the power connections at the op amp.  Connect 0V to the V- pin and +30V to the V+ pin.
* I have also omitted the decoupling capacitor across the op amp power pins.  0.1 ufd 50V
* It may be useful to add a capacitor between virtual ground and one of the power rails.  0.1 ufd 50V
* I have indicated the leads from the Analog Discovery 2 by color
* Given the matching values of R3 and R4, we expect a gain of 1
* I actually built the circuit and it works fine, with a gain of 1
* IMPORTANT: The black wire from the signal input will bring an Earth Ground to the virtual ground point.  The significance of this may seem nebulous but it's vitally important to understand how USB and the computer Earth Ground mess with this kind of thing.
* Depending on the op amp, 30V is right at the upper voltage limit.  If the circuit works with the PS cranked back to 24V there would be more margin against the Max Voltage spec.  Nevertheless, a bazillion op amps are running at 30V.
There are some excellent tutorials and a forum on the AD2 over at Digilent.

Make absolutely sure the virtual ground works before hooking up the AD2.  You are close to the point where the ESD protection kicks in if you should somehow get the entire supply across the AD2 inputs.  Nominally, the limit is +-25V.  Under the exact wrong circumstances, you can get as high as 30V if the black lead is on 0V and a signal lead is on +30V.  There's not much that can go wrong with a virtual ground as long as the circuit isn't shorted.  This would be a good thing to check with a DMM.  The virtual ground should be 15V away from the rail voltages.  Whether it is + or - 15V depends on which way you hold the probes.  Just make sure the virtual ground is centered between the rails.  Then hook up the AD2.  They also make rail-splitter chips...

BTW, that's why people use the BNC adapter.  Yes, it earth grounds everything you connect to the probe ground (it would connect to virtual ground in this case) but the 10X on the probe leaves a lot of voltage margin.  I didn't use my adapter for this experiment.

There's nothing magic about the TL081, it just happens to be the first op amp I picked up.  It has very high input impedance because of the JFET input.  This is critical for analog computers because input current upsets Kirchhoff's Current Law and the integration is less than perfect.

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It is vitally important that your power supply be floating.  There can be no electrical connection between the (-) terminal or the (+) terminal and the earth ground terminal.  Most power supplies are floating.  Turn the supply off and test continuity between each output terminal and earth ground.  My DMM reads OL on all outputs - as I expect.

That virtual ground is the 'ground' for the entire system and will be earth grounded the moment you hook up the black wire from the AD2.  It wouldn't do to short the power supply which could occur if the power supply wasn't floating.

This is so much more straightforward with dual 15V supplies.  Then we ground the center point to earth ground and everything works fine.
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