The peak in noise spectrum during noise simulation.

Dear lowENC,

lowENC said:

So where should I begin to solve this peak? the stability? or some other way I don't know?

I was studying your post plots and have an idea that may be responsible for your "peak", but without more knowledge of your test bench, circuit, or measurement, I can not be sure. I thought I would pass it by you if it provides some possible insight.

I extracted data points from the noise versus frequency plot you included with your question. I re-plotted the data using both a linear y-axis (to replicate your plot) and a logarithmic y-axis to validate my thought. As shown in Figure 1, the slope of the output noise characteristic prior to the peak frequency of the 30 Hz s about +10 dB/decade. In addition, I would expect to see the low frequency noise be dominated by flicker noise - which your post plot does not show. These two observations suggested that you are observing the noise using a first-order high-pass filter of about 30 Hz bandwidth. The "peaking" you are observing is not a physical peak, but rather the frequency at which the high-pass filter has a loss of about 3 dB. Without the 30 Hz high-pas filter, I suspect the noise would continue to increase at about +10 dB/decade for frequencies less than 30 Hz. However, the presence of the high-pass filter with slope +20 dB/decade below its 3 dB bandwidth, causes the flicker noise with a slope of about -10 dB/dec to appear as -10 dB/decade + 20 dB/decade ≈ +10 dB/decade. This approximate slope and behavior is what I observe in the noise plot with the logarithmic y-axis shown in Figure 1.

Is it possible you are measuring the noise with about a 30 Hz first-order high pass filter?

Shawn

Figure 1: Figure is too large to include with post, hence it is at URL:

www.dropbox.com/.../noise_data_summary_all_plots_091722.png

Dear lowENC,

I updated Figure 1 at URL:

www.dropbox.com/.../noise_data_summary_all_plots_091722v1p1.pdf

to include a test bench and noise simulation of a simple CMOS amplifier with and without a 30 Hz High-pass filter to illustrate its impact on the amplifier output noise. The test bench and simulation results for a noise measurement with and without a 30 Hz High-pass filter are added as page 2 and page 3.

Shawn

In addition to Shawn's observations, I did immediately think that your loop gain suggests that it's barely stable - I'd not be happy with 19º phase margin... (not that this is the cause of the problem). I too would have expected the noise to keep increasing at lower frequencies due to flicker noise, so it's odd that it doesn't - that's a pretty low frequency for a high pass filter too (for an IC).

Andrew