ADG749: SPICE Noise Analysis


I'm perfoming a noise analysis in LTSpice. My circuit includes the ADG749, therefore I downloded the provided Spice model (.cir), and created an appropriate symbol in LTSpice (which I have already done several times).

By performing my noise analysis in LTSpice, the results are "terrible" large. It seems, the used switch (ADG749) causes the additional noise. 

I attached the simulation result obtained in LTSpice for a single switch. The integrated RMS voltage noise from 1 Hz to 100 MHz is about 540µV. As can be seen, Rin and R1 are noiseless, so the whole noise is added by the switch. Comparing this result with my theoretical resullt of v_n = sqrt(4 x k x R x T x B * pi/2) = 4.26 µV (k = Boltzmann constant; R = 7 Ohm, worst case value according to the datasheet; T = 300 °K; B = 100 MHz), the simulatuion result is way to large.

I had a lock in the .cir file. I wasn't able to identify any noise-values. So I think the switch doesn't include a noise model. But I'm still curiose why the switch has this strange noise behaviour.

Has anyone experience with noise simulations including switch models?


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  • Hi,

    I'm analyzing a measurement structure including the AD8099. The input signal of the AD8099 should be selectable, so I placed the AD749 in front of the opamp.

    Is there an other possebility to model the ADG749 which results in the correct transfere function e.g. by its typical ON resistance and capacitances?


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