This project contains design files for a low-frequency (0.1 - 10 Hz), AC-coupled low noise amplifier intended for measuring noise of voltage references, low noise linear supplies, etc. A number of other designs utilize input high pass filters with large (1 - 2.2 mF) electrolytic capacitors and low (ca. 1 k) resistors, this approach suffers from several drawbacks. Electrolytic capacitors exhibit high levels of dielectric absorption, leading to long settling times. Leakage can also be problematic and requires selection of parts to ensure that the amplifier does not saturate when DC bias is applied. Moreover, measurements of sources like unbuffered DAC outputs suffers from considerable input signal attenuation due to low input impedance in the passband. This design instead utilizes a much smaller (4.7 uF) polypropylene AC-coupling capacitor and large (10M) resistor with a hybrid JFET input amplifier to remove the DC bias from the signal of interest. Dielectric absorption and leakage for polypropylene capacitors is much lower than that of Al electrolytics, allowing faster settling and obviating part selection.
While this amplifier is yet to be built and tested, the design concept borrows heavily from another project (https://github.com/curtisseizert/LowFreqLNA). That project was aimed at lowest possible noise and greatest bandwidth, with different gain and bandwidth options accessible via front panel switches, but the resulting complexity, part count, and cost were relatively high for a DIY project. This project is a simpler and less expensive version with fixed gain and bandwidth. The board is a 4-layer 120x78 mm PCB designed to fit in a Hammond 1455K1201 enclosure. The amplifier can be powered by two 9V batteries or an external power source. The "settle" switch, which discharges the interstage HP filter capacitors and saturation indicator are reasonably simple quality of life features that were maintained from the more complex version. The expected noise floor is about 60-75 nV p-p depending upon some parts choices noted in the schematic. BOM cost is ca. $75 from Digikey.
I will update this repository with testing results once I have built a prototype.