I just ordered a current sensor based on the ACS720 for sensing mains power consumption. From what I understand, the voltage on the analog output will oscillate around 2.5V with 50Hz. Now, this causes two proplems for me: 1. I want to sense with a 3.3V device, so a peak of 5V is too much. 2. Even if I had a 5V device, I would halve my accuracy, since I would only sense over absolute values of 2.5V
So I came up with the idea of adding an appropriate absolute difference value op amp circuit. This should allow me to sense with a resolution of ~0.83W up to ~850W (240V*5A/sqrt(2)). Since I am a total noob with op amps, I googled and found this: http://www.edn.com/design/analog/4374814/Dual-op-amp-takes-absolute-difference It says it is for dc or slow circuits and I wasn't sure if 50Hz is exactly "slow", so I tried to simulate it. From the values I put in, I would have expected a rectified sine wave between 0V and 1.3V. However, it doesn't work at all, as it looks. I already tried to change the AC source to DC, but that also didn't work.
Any ideas? Error in my simulation? In the software? In the example`In my understanding? Any alternative suggestions?