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Design >> Analog Design >> A question about device noise simulation of sigma-delta modulators
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Message started by alixluohao on Aug 10^th, 2009, 8:10pm

Title: A question about device noise simulation of sigma-delta modulators
Post by alixluohao on Aug 10^th, 2009, 8:10pm

Today I simulated the device noise of my swiched-capacitor sigma-delta modulator with the method using a ideal S/H to close the loop, as the designer's guide "Device Noise Simulation of sigma-delta modulators" said. The ideal S/H is implemented by Verilog-A as follows:

module sah_ideal(vin, vout, vclk);
input vin, vclk;
electrical vin, vout, vclk;
parameter real vtrans_clk = 1;
real vout_val;
analog begin
@(cross(V(clk) - vtrans_clk, 1.0))
vout_val = V(vin);
V(vout) <+ vout_val;
end
endmodule

The tran simulation may be OK, but when implementing a PSS simulation, the "vout_val" is refered as a hidden state and the PSS simulation is terminated. How to eliminate the hidden state so as to apply the S/H to the device noise simulation? :'(

Thanks!
LUO

Title: Re: A question about device noise simulation of sigma-delta modulators
Post by currant on Aug 10^th, 2009, 10:57pm

Look this article about hidden-state free SH
http://www.designers-guide.org/Analysis/hidden-state.pdf

As I understand, it is impossible to simulate sigma-delta modulator by periodic analysis, because the output of modulator is not periodic, but chaotic. I've read about this in Spectre RF docs.

Best regards.

Title: Re: A question about device noise simulation of sigma-delta modulators
Post by thechopper on Aug 11^th, 2009, 5:37pm

Hi LUO,

Usually the noise performance of a sigma delta modulator is dominated by the first integrator. Try then to analyze the first integrator noise with PSS. In this case PSS will work and you will get a good idea of what your main input noise source will look like.

Regards
Tosei

Title: Re: A question about device noise simulation of sigma-delta modulators
Post by alixluohao on Aug 11^th, 2009, 10:59pm

Thank you for your suggestions. I will read the pdf "Hidden state in specreRF" and pay attention to the noise of the first intergrator. :)