# A problem regarding connecting the grounds within two seperate circuits

I am trying to design the above circuit for my project to create zero crossing detector as shown in the above figure. This configuration was proposed originally by the member named "Transistor" in this community.

But the problem is I cannot connect the grounds at the Point A and Point B as shown in the above diagram due to the nature of my full circuitry.

My full circuit is shown below

Figure 2 simulate this circuit – Schematic created using CircuitLab

1) The red squared portion is a different section of my full circuit and its functionality does not need to be understood and it has nothing to with my zero crossing detector circuit. But it plays a significant role in preventing me from implementing the circuit shown in the previous image

2) I have a +12V and -12V power supply with the common point at the middle acting as ground. (POINT B)

3) My AC source is 3V 50Hz providing AC signals to the red squared portion of the circuit. But I am also using that particular AC source to supply power to my rectifier circuit as shown in the above image.

4) I have connected my AC source to the common point of the two +-12V supply (I have to do it to provide reference)

5) Now comes the problem, please compare the figures 1 and 2.

• In figure 1 , my point A has been connected to the point B (common point of the two +-12V sources).

• But in figure 2, point A cannot be connected to the point B since
point C has already been connected to point B to provide a return path for
the AC current.

How can I change my circuitry in figure 2 so that the results in figure 1 can be achieved?

• One option is to use a long-tailed pair to generate differential signals then feed them to the rectifier just like in the 1st image. Another and the most simplest (at least to me) is to put an opamp-based rectifier instead of a buffer like in the 2nd image. – Rohat Kılıç Jul 4 '17 at 6:44

One means of solving the problem is to replace OA3 with a precision full-wave rectifier. Figure 1. For negative inputs, the first section operates as a closed-loop inverter (A=-1) and the second stage is simply a buffer for the positive output. When the input signal is positive, the first opamp output remains saturated near ground and the diode becomes high-impedance, allowing the signal to pass directly to the buffer stage non inverted. The composite effect is a full-wave rectified waveform at the output of the buffer. Source: Linear.

Another would be to add a second LM339 comparitor. The outputs can be paralleled as they are open-collector type (as far as I remember). simulate this circuit – Schematic created using CircuitLab

Figure 2. Dual comparitor option. The two pots could be replaced by a resistor chain.

• Your precision full wave rectifier is a great idea , if you are interested, I have found the best video which describes the steps to implementing that circuit. youtube.com/watch?v=dCojRDwoFaI I am going to implement this circuit tomorrow and post my results here. – Amy Jul 4 '17 at 16:20
• Adding a precision full wave rectifier worked for me. Thanks for your help regarding this problem. – Amy Jul 6 '17 at 1:21