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LMC6482: For AC sense circuit the opamp output is saturating

Part Number: LMC6482
Other Parts Discussed in Thread: LMC6034, , TINA-TI

Hi E2E forum,

I am working on AC sense circuit with 320Vp AC and 60Hz signal. I am facing a issue with the LMC6482 Opamp IC and LMC6034 both. The Input starting from 0 to 320V is saturating the opamp and giving me a square wave signal from 10V to 0V. A +5V Vref at positive pin is given. Circuit attached below. kindly help me in this. 

I want to know why is this happening. As per formula i am not getting saturation for low input AC voltage.

Regards,

  • Hi Rajat,

    works for me in TINA-TI:

    rajat_lmc6482.TSC

    Could be an issue of your simulator. Try to power the LMC6482 by bipolar supply voltage and shift the other potentials accordingly. This does the trick sometimes.

    Kai

  • Hi Rajat,

    Kai is correct. Somehow your simulation tool may issue.  

    I derived the transfer function and it agrees with Tina simulation. 

    rajat_lmc6482 03032022.TSC

    If you have additional questions, please let us know. 

    Best,

    Raymond

  • Hello Kai and Raymond for the reply. I found the issue in simulation and resolved it.

    I have another doubt in output voltage due to capacitor at the positive input of Opamp is affecting my output of the opamp. The below circuit is the following circuit to the above one.

    Is it possible for you to help me in this. I am not able to create a formula for the output at 'H'.

    I have played around with charge and discharge of the capacitor voltage.

    The main input is 640VAC pk to pk 60Hz. In simulation final output H is around 3V. 

    I tried using the formula  . But what I got is 1.83V in analysis.

    Kindly help me in this.

  • Hi Rajat,

    there's no simple formula. It would need to contain information about the complex input signal shape. So either you carry out a Laplace Transformation (which I wouldn't do because of complexity) or you just trust your simulations and measurements.

    Kai

  • Hi Rajat,

    As Kai indicated, you have to trust the simulation tool. Your transfer function has issues.

    With 124k||249k and C1, LPF filter is formed. The cutoff frequency is approx. 1.9Hz. On its own, it will attenuate 60Hz signal. 

    In addition, the thevenin resistance is 124k||249k and you need to consider thevenin voltage as well. On top of that, the output has a phase shift from input. It is good to understand how the simulation is came about, but this one is a bit complicated. I calculated with one data point, and the simulation is doing correctly. 

    If you have additional questions, please let us know. 

    Best,

    Raymond

  • Hi Rajat,

    you could do it this way:

    rajat_lmc6482_1.TSC

    Kai