Op Amp Virtual Ground Questions.

I need to create a virtual ground for a single supply (24V) amplifier circuit that has 8 opamps for active filters and four LM3886 amplifier ICs.

At the moment I am using an LM324 op amp set up as a voltage follower with a VCC/2 divider connected to the input. When I first connected this up to the circuits virtual ground the op amp output (virtual ground) oscillated at about 2MHz. I have since added a 47ohm resistor and a 100nf Cap to the output of the follower set up as a low pass filter and it has stopped the oscillations.

Can I also add a 68uF Cap to the output of the follower for better regulation of the virtual ground? I am worried about overloading the opamp (capacitive loading) or is it ok in this case as the op amp output is not swinging? What is the best way to filter/regulate the virtual ground in this case? Should I also connect caps from V.Gnd to VCC rail? Any other input is appreciated. Thanks.

 

[cbarn24050]

Hi,ther is nothing to be gained by using an op amp to provide your mid rail voltage, a simple resistive or zener divider will do fine.

 

[melone]

CBARN24050, you MIGHT be right. Does the "virtual ground" have to be exactly mid-rail or does it have to be a constant that happens to fall mid-rail of a fully charged circuit? Do this thing have to work over tepmerature? How accurate does this voltage have to be? How much current is being supplied / sunk by this voltage level? What is this voltage level trying to drive? How strong is the "VCC" that is being supplied? Will a simple regulator work, or will you have to go to a temperature compensated voltage circuit?

Just a few questions. Good luck and keep us posted!

 

[cbarn24050]

Hi, its not correct to call it a virtual ground as its not at zero volts. Mid rail voltage gives the widest signal range.In this application the amps are ac coupled so the absolute voltage is not critical. There should be no load applied to the voltage as it should only be used as a refence.

 

[melone]

CBARB24050, what is DoiNk! trying to accomplish with his circuit? Is he/she trying to create a positive and negative negative voltage by simply splitting up the output of a regulator and referencing ground to some arbitrary point "in the middle", or is DoiNk! trying to create a high precision reference value that will be used for A/D readings, or how about a secondary voltage level that is used to power up some seperate circuits? I must be missing something because I just don't get what DoiNk! is trying to so, because each of those scenarios (plus factoring in some of the other factors I mentioned in my previos post), would tend to lead DoiNk in a different directions.

I agree that a simple resistive divider will work wonderfully if a ball-park, dynamic mid-rail voltage circuit is required, but I would think twice about using a zener clamp if this must be held constant over temperature (not too mention that the voltage is no longer "guarenteed" to be mid-rail). Even worse, would be to use a op-amp circuit if you intend to drive tens of amps.

I don't want to argue whether your solution will work, because it probably will, but I just want to make sure that DoiNk! (and the rest of the people reading this post), get to understand HOW you came up with the circuit. I have read several of your posts, and have no doubts in your abilities, but I still don't think that we have enough information to give accurate advice. Perhaps if DoiNk! could elaboarte a little more on his/her problem statement, then we determine which solution will work the best.

 

[cbarn24050]

Hi, DoiNk has stated that he is using some opamps as active filters, he likely got a circuit from a text book. Most text book circuits use dual suppply op amps so that the classical virtual earth can be used.DoiNk has decided to save himself some cash by only using 1 supply which means he needs a refence voltage midway between vcc and 0v to replace the missing reference.

 

Gentlemen,

Sorry for any confusion. What I am basically trying to do is create a voltage reference that is equal to half the supply voltage (ie, Supply/2). My supply is 24V in this case, so a steady 12V reference is what im after. This voltage will then be used as a midpoint reference for a couple of op-amp (active) filter circuits. High output current is not necessary as the reference is only used to supply small bias currents to the op amps.

Currently, I am using a voltage follower almost identical to what is described on page 2 of this document:

http://www.analogzone.com/avt_0806.pdf

I realise there are supply splitter chips available, but i am looking for a cheap/simple solution. I also had a spare opamp available on the board, thats why I decided to go for the follower instead of the basic 2 resistor voltage divider.

What I was originally asking was weather or not I could add a large 68uF electro capacitor also to the output of the follower for better filtering/regulation? I was worried about loading on the op amp.

If you could suggest any further methods of filtering or a better way of creating the half supply reference, it would be most appreciated. Thanks in Advance...

 

[melone]

Does the voltage have to be 12V or mid-rail? How accurate does it have to be? What kind of ripple will you see on the rails?

 

Mid-Rail and as accurate as possible. I want to filter out as much ripple and oscillations as possible otherwise this ripple voltage will be seen on the output of the opamps using it as a reference.

 

How do engineers usually create this mid-rail voltage reference on single supply opamp/amplifier circuits?

 

[melone]

Why don't you filter your upper supply and then perform your voltage translation?