Noise Simulation in LTSpice of opamp with resistor

Hi,

I think the problem may be that the voltage source V4, if it has zero series resistance (default), acts as an ideal perfect AC wideband short, and thus shunts away all of the noise current from the input. If you add any parasitic series resistance at all to that source, the current noise will start to be nonzero as the shunting effect diminishes.

For the most accurate result, if you put the resistor R5 in series with the ideal no-Rs voltage source instead, it still models a "grounded" resistor at +IN since the voltage source is still an AC short directly to GND, but all of the input noise current from LT1723 will be forced to go through the resistor. You can see the output noise contribution of only the LT1723 input noise current combined with R5 if you left-click on R5 in LTspice.

Please let me know if you have any further questions.

Best regards,

Catherine

Hi,

I think the problem may be that the voltage source V4, if it has zero series resistance (default), acts as an ideal perfect AC wideband short, and thus shunts away all of the noise current from the input. If you add any parasitic series resistance at all to that source, the current noise will start to be nonzero as the shunting effect diminishes.

For the most accurate result, if you put the resistor R5 in series with the ideal no-Rs voltage source instead, it still models a "grounded" resistor at +IN since the voltage source is still an AC short directly to GND, but all of the input noise current from LT1723 will be forced to go through the resistor. You can see the output noise contribution of only the LT1723 input noise current combined with R5 if you left-click on R5 in LTspice.

Please let me know if you have any further questions.

Best regards,

Catherine