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Thread: board MCP3911 (ADM00398)

  1. #1
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    board MCP3911 (ADM00398)

    I want to measure the voltage and current of 220V using MCP3911 2-channel analog input 3.3V. How measuring circuit for analog input of MCP3911? thank all

  2. #2
    Senior Member Constantin's Avatar
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    The circuit you're looking for is described in the various application notes.

    The first question you need to answer is whether you want a isolated rig or not. MCP designs generally assume that the chip will be used in a isolated enclosure (electrical meter, for example). For cost reasons, they assume direct measurements (i.e. shunt and resistor ladder for current and voltage, respectively). There are ways to connect that sort of rig to a computer safely (i.e. use isolation chips like the ones that Analog Devices makes) but any such design takes a lot of careful thought in layout because clean power is needed for the analog section for excellent measurement quality and isolation chips are not known to produce it. Remember, most electrical meters communicate solely with a small LED.

    For hobbyists, you are likely better off with an inherently isolated design, i.e. use a voltage transformer and a current transformer. The benefit is 100% isolation, the downside is that the the two signals will be very slightly out of sync depending on the influence the transformers yield in the respective signals. This rig will also cost more since resistors and shunts are relatively cheap and transformers are not.

    MCP publishes fairly useful schematics with the exception of the load capacitors used on crystals. For whatever reason, multiple schematics list said capacitors with 0.1uF values instead of perhaps 10pF, or less. Repeated inquiries to MCP tech support elicited yawns and no desire to fix obviously-wrong schematics. I'd consider using a PWM output from the Teensy as a CLK in signal, it saves some chips / real estate but limits you realistically to about 12MHz instead of the datasheet max (16MHz).

    Getting the chips to do what you want is fairly straightforward, though a logic analyzer helps to track down any issues in SPI communications. I wrote a library but work restrictions currently prevent me from publishing it.
    Last edited by Constantin; 03-24-2017 at 02:22 PM.

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