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Using INA225 for CT measurement

Jeroen3
Prodigy 60 points

Other Parts Discussed in Thread: INA225

Hi,

I'm exploring if an INA225 would be suitable as amplifier in a CT measurement circuit, this should work with AC up to 400 Hz. Since it is bi-directional and features programmable gain (to support different types of CT) it would fit nice in an mcu controlled environment. However, it does seem to defeat the purpose the INA family was intended for.

There are a few things I'm still puzzling with.

- The CT output will be floating, should one leg be connected to GND, or some other potential?

- A group of current sensors resides in an electrically isolated domain. Is common mode transient protection possible with such a circuit?
Since there is virtually no chance of a transient on 1 wire of the CT as long as the shunt is intact. Which will be approx 100 mOhm to 10 mOhm.


I would appreciate feedback.

  • 0
    TI__Expert 4595 points
    Hi Jeroen,
    Can you please provide details like common mode voltage, load current, suppy voltage that you are planning to use.
  • 0 in reply to Rabab Itarsiwala
    Prodigy 60 points

    As I said, I'm exploring possibilities. Nothing is decided yet, but this is what I'm thinking of right now.

    I'll have a 500 mArms to 5Arms external current transformer running trough the shunt. The goal is to get the full current "sine" into an ADC.
    Normally you'd use a higher burden voltage referenced to Vref/2 and a fixed gain amplifier, but I'm looking for ways to use a programmable amplifier to have 1 pair of connectors for different types of CT's. Or rectify it, and turn it into DC amplitude value and a square wave at the base frequency.

    The circuit will be powered using an isolated dc/dc brick followed by a 5V LDO. Vcc/2 is made using an opamp, the ADC communicates using an digital isolator to the MCU power domain.

    The common mode voltage is decided by the leg of the CT I reference at. I can connect one to Vcc, or GND or Vref. The CT is floating in normal situations. I'm not sure what to do here. If I connect one to ground, the shunt voltage will go below ground. If I connect one to Vcc, the shunt voltage will go above Vcc.
    I can also connect to Vref, which I believe would make most sense when using a regular amplifier circuit. But I'm not sure what impact this has on the INA225.

    Since the circuit is electrically isolated, I'm not sure how, and if, to add common mode transient protection.

    The supply is 5V with Vref at 2.5. And the current of the shunt is up to +/- 10A, from an external source. I'm looking at 10 mOhm-range shunts to limit heat.

    Attached is the circuit I've drawn up now.

    I'm not very experienced in analog circuits. My background is in embedded system engineering focused on the digital domain. But I'm learning a lot doing these projects.

  • 0 in reply to Jeroen3
    TI__Expert 4595 points

    Hi,

    The common mode voltage and the device supply are independant of each other. So as long as the common mode voltage ((VIN+ + VIN-) / 2) and the input differential voltage (VIN+ - VIN-) does not exceed the absolute maximum ratings, you may be able to float it. But normal operation will need a common mode voltage within the specified limit. The common mode voltage can be greater or less than the VCC as long as it is within it's specified limit.

    If you don't expect transients above the input ratings then you may not need transient protection. If you do then please refer to section 7.4.3 "Using the Device with Common-Mode Transients Above 36 V" in the datasheet. Hope this helps.