SPICE model for OpAmp Noise

Although the following was concerning the ADI SPICE model for the AD8676 opamp, this could be relevant to many other opamps:

Using PSpice, the noise from the AD8676 opamp model from ADI was much higher than the expected data sheet value of 2.8nV/rt(Hz).

It seems the explicit noise sources are contributing less than the thermal noise from each resistor.  As shown below, by replacing every resistor (except the one used to generate the 2.8nV) with a resistor model fixed to absolute zero temperature, the broadband noise was reduced to around 3.5nV.  This value was the sum of 1)thermal, 2) flicker and 3) shot noise of the input transistors, as shown in the attachment.

1)  Is this a feature in many SPICE models?

2)  Is there a method to remove the input transistor shot noise?

...

.SUBCKT AD8676  1 2 99 50 45

...

Q1   15  7 501 NIX

Q2   6  2 502 NIX

RE1 501 5 RZERONOISE 1.731E+03

RE2 502 5 RZERONOISE 1.731E+03

IOS  1  2 500E-12

...

...

* VOLTAGE NOISE REFERENCE OF 2.8nV/rt(Hz)
*
VN1 80 98 0
RN1 80 98 16.45E-3
HN  81 98 VN1 2.80
RN2 81 98 RZERONOISE 1

*flicker noise
*
D5 69 98 DNOISE
VSN 69 98 DC 0.6551
H1 70 98 VSN 12
RN 70 98 RZERONOISE 1

...

...
*

.MODEL RZERONOISE RES (R=1,T_ABS=-273.15)

...

Parents
  • Hi Kapati,

    Sorry for the delayed response. Please see attached file for the revised model of AD8676.

    Please let me know if you have any concerns regarding this.

    Thanks,

    Roel

  • Hi

    Has this issue been resolved? In ADISimPE, the noise is still wrong (not matching datasheet) for AD8676.

    In LTSpice I used AD8675 for simulation because there is no model for AD8676 but the results do not match the datasheet as well.

    - Input Voltage Range: Error starts to increase @11.5V instead of the 12.5V as according to datasheet)

    - Offset ist >110uV compared to the max 75uV (datasheet)

    The AD8672 performs better in simulation than AD8675, though according to the datasheets, it should be the other way around.

    Is this a problem with the model, my simulation or the actual part? Especially the reduced IVR would be a problem, as we plan to use +-13V supply voltage for a +-10V signal.

    Best regards

    Haje

Reply
  • Hi

    Has this issue been resolved? In ADISimPE, the noise is still wrong (not matching datasheet) for AD8676.

    In LTSpice I used AD8675 for simulation because there is no model for AD8676 but the results do not match the datasheet as well.

    - Input Voltage Range: Error starts to increase @11.5V instead of the 12.5V as according to datasheet)

    - Offset ist >110uV compared to the max 75uV (datasheet)

    The AD8672 performs better in simulation than AD8675, though according to the datasheets, it should be the other way around.

    Is this a problem with the model, my simulation or the actual part? Especially the reduced IVR would be a problem, as we plan to use +-13V supply voltage for a +-10V signal.

    Best regards

    Haje

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