virtual gnd with opamp

Thread Starter

barg

Joined Dec 23, 2015
55
Hi,

I am trying to make a virtual ground with an opamp for battery cut off circuite to supply power to the mcu while switch cuttoff.

Attched is a scheme that describes the comoponents, i am not sure if all correct? also i am trying to figuer how to determine the R1 and R2_REF (maybee also to understand how this realy gives a virtual gnd), appreciate for anyone assitance?

In addtion, could also advice for a common opamp single ic?

Thanks in advance!
 

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crutschow

Joined Mar 14, 2008
25,250
Your circuit does not give you a virtual ground.
For that you need to configure the op amp as a gain of 1 non-inverting follower.
Here's a discussion on various ways to make a virtual ground.
 

ErnieM

Joined Apr 24, 2011
8,053
May we forget all about virtual ground or any specific circuits for the moment?

Instead, let me ask just what is this circuit intended to do? What is the given input and what is the desired output?

(A wise man on this forum once stated it is easier to solve your problem than to fix your solution.)
 

Thread Starter

barg

Joined Dec 23, 2015
55
Hi,

Purpose of this opamp is to provide a virtual ground as part of battery under voltage cut off protecting circuit, using a mcu to controll a switch - mosfet that connected to a battery and gnd, in order to keep the mcu "alive" when an under/over voltage cuttoff incident ocure - for that i believe i need one battery and two grounds.

Battery 12v, motor load - apx 3A, desired output=virtual ground.

If i didnt put exactly the details is because i'm in learning stage of this issue...

Appreciate your guidness to move on with this project i have.

Thanks
 

atferrari

Joined Jan 6, 2004
4,069
May we forget all about virtual ground or any specific circuits for the moment?

Instead, let me ask just what is this circuit intended to do? What is the given input and what is the desired output?

(A wise man on this forum once stated it is easier to solve your problem than to fix your solution.)

In another forum, one of the most frequent contributors (if not the top one) instead of replying questions about "unusual " solutions, he asks most of the times "what do you intend to accomplish?". In the beginning that used to bother me but ended accepting it is the best.

Myself, sure I came too, with unviable solutions instead of simpler/reasonable ones. Now I tend to ask about possible solutions.
 
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Thread Starter

barg

Joined Dec 23, 2015
55
Hi Gopher,

Nice quotation but the thread stoped... i have noted the missing details, Can you please advice? Or any of you to solve this ground issue?

attached also a revised scheme as noted by crutschhow.

Thank you!
 

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#12

Joined Nov 30, 2010
18,210
That schematic is not revised. It still has the same mistake. You have used an inverting configuration with something called R2 REF in the gain equation instead of non-inverting with a gain of 1.
 

crutschow

Joined Mar 14, 2008
25,250
You make a non-inverting follower by connecting the voltage from the R3-R4 resistor divider to the (+) op amp input and connect the (-) input to the op amp output.
 

atferrari

Joined Jan 6, 2004
4,069
What voltage you put at the non-inverting input, is the key here. I suggest you keep a copy of the image I posted here below. It was prepared by audioguru, who happens to be a former member of this forum.

Take your time to understand it.

Polarizing op amps with single & dual supplies.gif
 
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