# Amplifier noise app note from TI/periodic random noise

In my battle with transfer function estimation, ed I have been dealing with many noise problems lately and have come across this application note from TI regarding the calculation of noise figures for basic op-amp circuits.  This noise figure deals with the ratio of circuit signal-to-noise ratio (SNR) at the input versus the output. The article goes through the derivation of noise analysis equations due to thermal noise in resistive elements and due to rms noise figures of the active device and goes on to quantify the noise figure as a function of temperature, resistances and op-amp parameters. This can be useful in determining performance properties of circuits given a set of passive components and can be used to define a “best case scenario”.

This all well and good, but then one might ask what noise has to do with system identification (transfer function estimation)? The simple answer is that the frequency-domain transfer function can be determined by passing “noise” through a system and comparing the spectral properties of the output versus the input. The idea is that white noise has a flat spectra (over infinite time) so the transfer function can be accurately determined for all frequencies (again, given infinite time). If infinity is too long a time to wait, one trick is using something called periodic random noise to give a well defined spectral distribution in finite time. A Gaussian random number generator can create white noise, however, an inverse Fourier transform is used to to generate the periodic noise.

Essentially, enough sinusoids are added together to cover the frequency range of interest with equal amplitudes and randomized phases that are distributed over +/- pi. The amplitude will relate to the desired resulting rms value for the noise and the number of summed sinusoids. The frequencies of choice should line up with the sampled frequencies in the following FFT that will be computed to compare the spectra of the input and output signals. The signal will now look like noise and will be “random”, however, all of the frequency domain components will maintain their amplitude and phase through the whole procedure leading to less variance in the FFTs.

# IC Friday: Maxim Semi’s MAX274

This week we are presented with the MAX274 from Maxim (thanks again Neil!). This is an older low-pass filter IC and it should be noted that it requires substantial external components. The on-chip capacitor design is an interesting island pattern, pancreatitis it is possible that the inductances of the traces that link up the squares are also integrated into optimizing the design.

A quick reminder about the file naming scheme: the 10x means that the 10x magnification objective was used in addition to another 10x magnification in the optics. This means that a file name with 10x in the name is actually 100x and 20x is 200x. I know it might be slightly confusing, rx however, I don’t know if I should change at this point given that I used this poorly designed naming convention for all previous posts.

max274-10x-stitch.jpg

# Entry into low-power computing

As some of you may have known, gerontologist
I am a bit of a FreeBSD enthusiast and have been using it regularly since around the time of the Linux fragmentation/teardrop vulnerability. I have always kept a FreeBSD system running at home so that I could perform various network tasks and automate things. In an effort to conserve power and reduce noise, I have been using my trusty Sony z505sx (pictured above). Recently, I switched to a Jetway MINI-ITX board and have been quite happy. The board runs a 1.2GHz Via C7 Eden processor, is fanless and consumes only a few watts of power while operating. It is loaded with 1GB of ram and a 750GB SATA (also low power) drive to provide ample storage. Since the system has RCA/S-Video out, I added a 5″ LCD screen on the top. So far, the machine performs well running as media/web/ftp/ssh server and is barely audible with no fan attached.

I am manufacturing a custom case for this machine and will post a full writeup once I have a cover that I like, for now, you will have to enjoy the single image below. As far as the z505sx is concerned, I decided to put it on eBay in a preemptive spring cleaning effort and to avoid clutter.