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OPA4374: what spec need to focus when gain smaller than 1

Part Number: OPA4374

Hi team,

if I am going to sample 220V/50Hz voltage signal, could I directly using a differential circuit with gain around 0.003 and have a 1.65V as bias voltage for the non-inverting port. The 0.1V resolution is okay.

If I have this 0.003 gain circuit, what spec of amplifier should I focus most?

I think input offset could be ignored, since the gain is so small, as well as the input bias current. 

Right now I am thinking of the input voltage density may cause some problem here because of the 6.5MHz GBW of opa4374. Even though the gain is 0.003, the noise gain is 1+0.003 here. And the En_in=en_in*square[1.57*fc], fc here could be 6.5MHz*1.003 also around 6.5MHz, then we have En_in=15nV*square[1.57*6.5M]=~0.048mV on the input, and we have output En=0.048*1.003=0.048mV. And it seems okay with the resolution since I have  LSB= 0.1*0.003=3mV. Could you please help to check if my calculation is correct?

Besides the noise density, do you have other concerns for this circuit?

James

  • James,

    Offset voltage gets the same gain is noise,  1+0.003.  Noise can be reduced with a simple RC Low Pass Filter on the output.    

    High voltage requires attention to layout and tracework and multiple series input resistors reduce voltage on resistors which also reduces the voltage modulation on those resistors. Sampling the line introduces some leakage current which can cause issues.

     

  • Hi Ronald,

    Thanks for the information here.

    Do I need to pay attention to the input common mode voltage? Since I have a 220V AC voltage on the negative input as attachment.1.docx

  • James,

    I prefer having resistors between input filter (22pF capacitors) and the op amp inputs. 10k should be plenty.

    The voltage divider lowerers voltage provided that the final resistor has a path to low voltage (with power on or off) . The non inverting side is OK if 1.65V supply has a path with power on or off. Same for output with power off. If that is not there, then the diodes on input and output to both supply rails will provide a path.

    In summary, voltage is fine.