This project started as a simple amplifier for measuring noise in linear-regulated power supplies. This is necessary because even a cheap linear regulator only puts out a few mV of noise, and most hobbyist test equipment runs out of resolution in the mV range. A good linear regulator has noise way below that.
The previous version was quite simple: two fixed 10× gain stages preceded by a single RC filter. The gain stages had DC offset nulling, so there was no need for an output cap. It was a wideband amplifier: no bandwidth limiting.
That version was simple enough that I was able to make a hand-etched single-sided PCB for it with only two jumpers. It worked well enough, but there were a few problems. First, some of the lower signals ended up 40 dB or more below my spectrum analyzer’s lowest range, so the measurement system’s noise floor obscured more of the signal than I liked. Second, I occasionally had instability with the circuit. Third, it being a quick-and-dirty design, I knew I could improve on it.
Then, by chance, I came upon Linear Technology’s Application Note 83, Performance Verification of Low Noise, Low Dropout Regulators by Jim Williams and Todd Owen. In it, they give a circuit for a low-noise preamp for testing linear regulators, and another for a thermally-responding voltmeter. (Thermal voltmeters are nice for noise tests because they’re RMS by definition. Sadly, the circuit can’t be made, since the key IC is obsolete now.) I took some ideas from both of those circuits, kept some from the original preamp (dropping some dross in the process), and added the charge controller from the PPA battery board; that was LNMP v0.5.
Due to a miscalculation in the board size, I made some small changes to the board and called that v1.1. Those boards became available in June 2006.
The phase lead cap idea was inspired by aos’s comments in response to my Head-Fi post. He also suggested limiting the supply current to the output op-amp to lower its bandwidth.
The high-pass filters and the adjustable gain stage ideas came from Williams and Owen’s AN83 article. The idea to use the LT1206 also came from there, though in the app note it was used for a different purpose.
The fast-charge circuit is pretty much identical to Figure 14 in the MC3334x datasheet.
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