This is a circuit for boosting small AC signals for measurement with an oscilloscope, voltmeter, spectrum analyzer, etc. It was specifically designed for testing linear regulator noise, but it’s a fairly generic circuit, so you can probably find other uses for it.
I no longer offer manufactured LNMP PCBs. However, the EAGLE and Gerber files are linked below, so you can get some professionally made yourself. The LNMP is a low enough density design that you should also be able to hand-etch it yourself, though beware that it is a 2-layer design.
Switchable between 40 dB and 60 dB gain to allow for low noise when full gain is not needed or when the signal voltage is unusually high.
Noise -72 dBFS on the 60 dB setting. (AD797 in IC1 and IC2)
Full scale output signal: 1 Vrms at minimum battery voltage.
Minimum input signal: 1 µV (~6 dB above 60 dB setting noise floor)
Maximum input signal: 10 mV (just below minimum clipping point on 40 dB setting)
Powered by eight AAA cells to avoid ground loops and power supply noise.
On-board NiMH fast-charge circuit with -ΔV charge termination.
Idle time > 24 hours on a charge (750 mAh battery pack)
Run time ~20 hours on a charge with a peak signal (no additional output stage load)
High-current output stage for driving long cables.
Gain adjustment circuit to trim out worst-case gain errors due to component tolerances.
4-pole high-pass filter strips DC components. (fc ~5 Hz)
Optional 1-pole low-pass filter to limit test bandwidth. (Useful for noise testing.)
Completed project fits the popular Hammond 1455L12 case. (Board size 100 mm × 120 mm, case height 1.2 inches)
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