In my continuous search for noise reduction, I have discovered that the audio developers have placed a great deal of effort into the design of low noise, LINEAR, DC power supplies And for a very good reason - no hum at high gain.
To test my system noise, I wrote some code in "Processing.org" that made a very precise noise chart and measurement on my PC of my 24 bit volt meter. I was using a "switching" power supply at the time. I measured the noise of the output signal at an effective gain of 1,000,000. I then built and used a linear supply and tried it. The linear supply cut noise in half or better. It is my opinion that the switcher introduced high frequency noise to my entire volt meter circuit.
The sketch to the right is the
basic model from which my 24 bit meter circuit is powered but not
showing a center tapped transformer for dual voltages and later
regulation in my circuit shown below. But this does show the use of a
modest inductor which I am going to try.LINEAR VS. SWITCHER
For those of you who could use s quick description of the difference between switcher and linear power supplies, here is a well written WIKI article:
http://en.wikipedia.org/wiki/Switched-mode_power_supply
I have become an strong advocate of linear supplies in high gain ADC circuits and I generally accept the audio designs, as built for high performance, high quality audio preamps, as my low noise standard. The only difference is that I need a tiny amount of power so my supply design works at a milliamp capacity. I tested the idea of a modest low ohm, 100mH inductor and it completely eliminated any high frequency noise the circuit had picked up from outside EMI sources including my hands or oscilloscope probe near the circuit. From this model, I added a second stage of regulation after the full wave rectifier as you will see below.This second stage of regulation is what is called a "reference source" in the industry.
With my power supply, I am using six 2200 uF filter caps and an LM317 and LM337 as preliminary regulators for the plus and minus 9 VDC supply. As an extra and maybe extreme step, I added a Maxim 5 volt reference voltage source as the final voltage source for my op amp and my ADC.
The schematic below is in testing now - it is very early to show this as a design. But it does give you and idea of how much I have borrowed from the high performance audio people (with their very demanding need for clean sound) who know power supplies nose better than most.
