Hello, I am wondering is there any difference in accuracy between APS, spectre and SpectreX? I am currently simulating a Verilog-A model of a 18 bit ADC, where I encountered a wierd problem: using Spectre X with preset = CX and all other settings default, I get a SNDR of 83dB and a strong 3rd order tone (which is wierd because all components are ideal and linear, and there's no saturation); however, as I switch to APS and Spectre with errorpreset = conservative and all other settings default, I get a SNDR > 100dB with low distortion. The schematic is exactly the same and there's no parasitic since it's a Verilog-A model. As I referred to SpectreX manual, I noticed that SpectreX should have the same accuracy with Spectre. Why there's a such a difference? Which result should I trust? I'm using virtuoso ic618 with spectre 211. PS: some other information which might help: I found that as I set a smaller reltol (i.e., 1e-4 (default) -> 1e-5) and vabstol (i.e., 1e-6 (default) -> 5e-7) in APS and Spectre, I see a growth in SNDR.
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