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INA155: Sampling AC high voltage

Emmanouil Tsachalidis
Prodigy 40 points
Part Number: INA155
Other Parts Discussed in Thread: INA149, OPA392

Hi everyone,

I am trying to design a high voltage sampling system as part of a larger project.

The sampling front end will eventually drive a AVR128DA 12 bit ADC.

ADC reference will be 1024mV.

Given the chaos of the IC shortage and the need for lowering cost per project unit i am searching for a solution in measuring the 230VAC with the uC ADC above.

So, using Tina i was able to put together the circuit given below:

Everything looks great but i have some concerns regarding the GND of the AC.

What if the system ground is connected to the AC GND?

I grounded the Neutral (if i can call it that) of the AC source as you can see below:

But the simulation is not looking good:

Should i be using one of those high common voltage opamps like INA149? Still INA149 has a CMV of +/- 275V. Not sure what applies in this case, that is, high voltage sampling especially is the sys ground is connected to AC GND.

Can you pls help?

Maybe propose an better topology for high voltage sampling?

Thanks

Manos

  • 0
    TI__Genius 10850 points

    Hi Manos,

    Thanks for your post. I can help you.

    The input signal is pushing the input of the INA to a value beyond its spec:

    I can help you put together a circuit, I just want to make sure I understand your requirements. Please answer / confirm the following bullets:

    • Power supply: 5V single supply
    • reference voltage: 0.5V
    • Input signal:
      • what is the input differential voltage?
      • what is the input common mode voltage? 
    • Output signal:
      • What is your expected output signal? 

    Are you trying to attenuate a 230V signal down to 5V?

  • 0 in reply to Tamara M Alani
    Prodigy 40 points

    Hi Tamara,

    Thank you so much for your response.

    I am guessing what you re saying is that grounding one of the AC source ends violates the CMV of the opamp hence the square-like output wave.

    The voltages that you see in my first post pretty much define the requirements.

    1. ADC reference used: 1024mV
    2. Power supply: +5V (single supply)
    3. Reference voltage: ADC reference / 2 or simply 512mV
    4. Max measurable AC RMS value: 250V (353.5V peak to peak)
    5. The attenuation ratio (for each waveform - pos or neg) is therefore: 353.5V / 0.5V = 707x (57dB)
    6. Expected signal (peak to peak) is marginally 1024mV with the reference at 512mV. Since opamps cannot reach 100% their Rails, i would say very close to GND depending on the opamp capability if it is a Rail to Rail etc.

    Looking forward to your response!

    Regards

    Manos

  • 0 in reply to Emmanouil Tsachalidis
    TI__Genius 10850 points

    Hi Manos,

    Your circuit requires a very high attenuation. We don't have an integrated solution that can provide that. I have put together a discrete difference amplifier with our high precision OPA392. Is this what you are looking for?:

    Here is my TINA file attached:OPA392_Manos.TSC