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)
