Low Noise Preamplifier With 220 PV/sqrt(Hz) And Customizable Frequency Response.pdf

Gerhard W. Hoffmann designed a 220 pV/sqrt(Hz) low-noise preamplifier with the following key features and specifications:

Performance:
- Voltage noise vs. frequency: Green represents input shorting, while red shows noise from a 60 Ohm resistor (1 nV/sqrt(Hz)).
- Gain options: 20, 40, 60, or 80 dB with +6 dB gain boost to compensate for impedance mismatch.
- Lower corner frequency: 0.1 Hz, adjustable to 10 Hz.
- Upper corner frequency: 1 kHz, adjustable to 100 kHz or 1.3 MHz.
- Input impedance: 10 kOhms.
- Output impedance: 50 Ohms.

Noise Level:
- Internal verification: A 60 Ohm internal resistor provides a thermal reference, and an input short after the AC coupling capacitor ensures accurate noise measurements.
- Equivalent input noise density: 0.9 nV/sqrt(Hz) for 20 dB gain, decreasing to 220 pV/sqrt(Hz) for higher gains (40-80 dB).

Controls:
- J1: Signal input (SMA).
- J2: Signal output (SMA).
- SW1: Lower corner frequency adjustment.
- SW2: Upper corner frequency control.
- SW3: Gain selection (20, 40, 60, or 80 dB).
- SW4: Gain boost (+6 dB or 0 dB).
- J3: Battery charger input.

Design Details:
- The preamplifier utilizes 20 ADA4898 op amps in parallel to achieve the desired noise density.
- Input bias resistor (10 kOhms) and coupling capacitors are chosen for a 10 Hz lower corner frequency, with larger values required for lower frequencies.
- The first stage has a voltage gain of 10, followed by an inverting amplifier in the second stage to sum currents from all 20 first stages.
- Final gain control is achieved through a rotary switch selecting appropriate amplifier outputs and providing additional +6 dB gain if needed.
- The design includes DC blocks and low-gain stages to manage offsets and maintain overall gain.