I am trying to design a ADA4084-2 into my latest design, but I am receiving strange results when I do noise modeling using the Spice circuit. The voltage noise at the output is extremely large, even with relatively small feedback resistor values. It almost seems like an improper value for the current noise has been input in the spice model.

Basically, the problem can be produced by just putting a 5k feedback resistor (from output to inverting input), while holding the non-inverting input at a constant voltage (-2 V in my case). Power supply rails are +/- 5V.

In this configuration, the resulting predicted voltage noise at the output is 600 nV/sqrt(Hz) ! (~10 Hz to 1 MHz)

If I reduce the feedback resistor to 1 Ohm, I get the much more reasonable value of 4 nV/sqrt(Hz)

I have also performed the same simulation with other parts (AD4528, AD4004) and those behave as expected for all values of the feedback resistor.

I really want to use this part, and the noise values in the datasheet are completely acceptable. I just want to make sure that I am not missing something dramatic.

Thanks for any help you can provide.

Dustin

Dustin,

A couple of things.

-- Try running on +/-15V, with Vcm =0V.

Should be about 550 fA/rt-Hz at 1 kHz.

If that doesn't work, are you running LTSpice? LTSpice sometimesw doesn't like the way we do the noise

modeling.

Harry