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Original question:

OPA1678: opa1678 OUTPUT （ Automotive Audio）

OPA1678: Noise Simulation

Antenna_Head
Genius 12900 points
Part Number: OPA1678


The OPA1678 noise is spec'ed for 4.5 nV/sqrt(Hz) at 1 kHz.

Using the following circuit in TINA, where the VG1 source = 0V,

and performing Analysis->Noise Analysis->Total Noise returns this result:

I expected this value to be closer to 4.5 nV * sqrt(1000 Hz) = 126 nV.  

Using the Analysis->Noise Analysis->Input Noise returns,

Which also returns a value higher than expected.

Can you please let me know what I'm doing wrong here?  Is it related to the Flicker Noise of the device?

These noise voltages are RMS, as confirmed by simulating the noise of plain resistors.

Thanks, Best, Steve

  • 0
    TI__Genius 10850 points

    Hi Steve,

    Thank you for your post. When you are calculating op amp total noise, you have to take into account both broadband and 1/f total noise. The formula you have will only apply if the broadband noise extended across the entire frequency range. But since the OPA1678 does have a 1/f component, this needs to be included. The Analog Engineer's Pocket Reference guide goes over these formulas / calculations on pages 61-62 (https://www.ti.com/seclit/ml/slyw038c/slyw038c.pdf).

    In regards to the second curve for input referred noise, this is a slight error in the model but expected. The model mimics the datasheet parameters closely but it isn't necessarily exact. 

    -Tamara