I'd like some help understanding this previous thread about a current transformer input to ADC. The part I don't understand is how the reference of the input circuit is tied to the reference of my ADC. It seems to me like the rectified AC input needs to somehow share the same reference as the one being used by my ADC in order to get a consistently accurate reading. I can't quite figure out how to do this while protecting my MCU from the negative of the AC signal. Any help would be greatly appreciated.

Here are the requirements for my system, if it matters at all: The MCU is powered by a truck battery at 9-32 Vdc. This part of my system is required to measure the current provided by a generator, which can be 50-400Hz, 120 or 240 VAC, up to 400A. My system is provided a signal from the secondary of a 150:5A current transformer, which I must use to measure the current with an accuracy of 1%.

  • \$\begingroup\$ You choose the CT and burden R for I max then determine that Vout and then choose gain of precision rectifier to get Vout max 3.3 while 0A=0V. Which choice was unclear? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 24 at 18:46
  • \$\begingroup\$ @Sunnyskyguy EE75 I understand that, the part that confuses me about the example circuit is whether the ground is common with the Vdc system ground, and whether this can potentially damage the system components such as the MCU. It seems like a bad idea to expose the MCU's system ground to the influences of a VAC signal, but I haven't been able to find any good information explaining how to avoid having the conditioning circuit floating with respect to system ground. \$\endgroup\$ – datapanel Jun 24 at 19:07
  • \$\begingroup\$ The great thing about a CT is that it's a transformer, the secondary is isolated and you're free to hook it to whatever ground reference you want. You don't want your MCU to have any connection to the AC system. (If I understand what you're asking correctly.) \$\endgroup\$ – John D Jun 24 at 19:20
  • \$\begingroup\$ So the secondary of the transformer can be connected to the MCU ground? That makes me uneasy to expose the system ground to an AC signal. Otherwise if the secondary is floating with respect to the MCU, then it seems like you won't be able to get accurate and consistent readings. That's my dilemma. Thank you for helping. \$\endgroup\$ – datapanel Jun 24 at 19:41
  • \$\begingroup\$ Yes, the secondary of the CT has to be connected to you MCU ground if you want to convert the signal. An A/D is designed to digitize AC signals, and if your CT is properly designed your voltage across the secondary will be well within the rating of the A/D input. If you're worried about transient surges put a bidirectional TVS across the secondary. \$\endgroup\$ – John D Jun 24 at 19:43

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