Low Noise Measurement Preamplifier
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.
Features and Specifications
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)
