# Need some help understanding this AC power measurement schematic

Discussion in 'Homework Help' started by electrophile, Apr 22, 2016.

1. ### electrophile Thread Starter Member

Aug 30, 2013
100
1
Hi, this is a reference schematic from TI for one of their smart plugs. My questions are specifically around the Metrology section on page 1 that measures AC mains voltage and current. I understand that U1B,C,D and U7B,C,D form an instrumentation amplifier. I'm not quite sure what is happening in the stage after this. The non-inv terminals of U1A and U7A seem to be tied to Vcc? Also is that combination of resistors and capacitors dividing Vcc to a lower value? How do I determine what voltage value is at the non-inv terminals? Also how is this aiding in determining voltage and current?

Second part: What are these metrology analog filters? Any help deciphering this would be really appreciated.

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,990
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No, the non-inv terminal is connetrd to the voltage divider form by R39, R40, R52, R53. And the voltage at non-inv input is equal to:
V"-" = Vcc * R53/(R39 + R40 + R52 + R53) = Vcc * 5.1kΩ/(22kΩ + 12kΩ + 6.8kΩ + 5.1kΩ) = Vcc * 1/9 ≈ Vcc*0.11 ≈ 0.367V
I do not see nothing special here. Ordinary voltage divider to measure the voltage, shunt resistor to measure the current andsome RC filtering and circuit protection.
Try read this
http://www.ti.com/lit/ug/tidu983/tidu983.pdf

electrophile likes this.

Aug 30, 2013
100
1
4. ### electrophile Thread Starter Member

Aug 30, 2013
100
1
So would I be able to use a single supply here instead of the dual one they've used? That would reduce some complexity from the power supply.