Hey Guys,

I am a recent grad student and really want to build an analog EC meter for a summer project I am working on. I have a good foundation on basic circuitry and arduino interfaces but when it comes to Virtual grounds and power rails I get really lost. The EC schematic I am working off of (link below / img 2) demands for a 5kHz sine wave oscillator. I was thinking about using the single op amp phase shift oscillator (img 1) which uses a TLV2471 op amp and three RC cells. Following that the EC meter schematic uses four LF351 op amps (img 2). After sifting threw the forms I found this article explaining virtual ground circuits pretty well (link 1), I dont really know which one to use. I was thinking of going with the buffered based virtual ground driver utilizing BUF624 (img 3) due to the fact that it can handle voltage offset pretty well as long as the power draw on this circuit is evenly balanced... this is were I get mixed up, again my circuitry knowledge is limited. I guess what I am asking is what is a practical way to properly power the TLV2471 and the four LF351 op amps. I have been using LTspice, which has been fun but even when I am just implementing the single op amp phase shift oscillator and the power supply the power is not evenly drawn from + and - side (img 4). If someone could point me in the right direction that would be awesome, any criticism will do, I really want this to come together and willing to barrel threw articles in order to get this done. I have been researching for a while but I think due to my lack of conceptual knowledge on circuitry I have been in a standstill.

Thanks again for reading images and links are below

Link one (http://tangentsoft.net/elec/vgrounds.html)

 

Your problem is that you have mixed two versions of the virtual ground circuit in img 4.
The virtual ground from U1 is your new signal ground.
Thus it replaces the 2.5V virtual ground point on U2.
And the minus power for U2 goes to V-, not the virtual ground.

 

Your problem is that you have mixed two versions of the virtual ground circuit in img 4.
The virtual ground from U1 is your new signal ground.
Thus it replaces the 2.5V virtual ground point on U2.
And the minus power for U2 goes to V-, not the virtual ground.

Thanks for the reply so I fixed what you said and it seems to work fine, I am just confused why the + (~2.4V) and - (-4.8V) voltages are different. So would I follow this same logic when wiring up the four LF351's? Can this virtual power setup be all I need for the rest of the schematic?

Thanks again for your time

 

Creating a Virtual Power Supply Ground


[URL='http://forum.allaboutcircuits.com/threads/output-power-boost-for-op-amps.117595/']Output Power Boost for Op Amps [/URL]

 

The bridge in img-2 is driven by not one sinewave source, but two. Also, they need to be low impedance sources and out of phase. The point in the phase shift oscillator marked as the output is a relatively high output impedance point. From that point you need to add two circuits, one unity gain non-inverting buffer (voltage follower), and one unity gain inverting amplifier. Those two outputs are low impedance sources and 180 degrees out of phase, correct for your bridge circuit.

ak

 

You might do better with a real Instrumentation amp. You can generally level shift the output. Most, though, require that that pin be driven by a low Z force.

 

Thanks for the reply so I fixed what you said and it seems to work fine

Still not correct.
The U2 (+) input should be connected to common, not V+.