Low Noise x1000 Amplifier

Here’s the specs: Voltage noise 2.5nV/√Hz, Gain 1000, bandwidth 0.3Hz to 500kHz. I measure 600nVpp from DC to 1kHz at the output.

Here’s the story: Designing and debugging a high precision A/D stage, you will want to know how quiet the voltage reference really is. When I worked on a 20 bit A/D board, I found this amp a great way to prove that a simple voltage regulator was not good enough as a reference voltage (measured data trumps an any preconceived notions). It was a big help getting the shielding and grounding debugged too!

Here's the amp on the bench, next to a 9V battery. Click on image to enlarge it.

Since most ‘scopes have about 5mV/Div at their highest gain, this AC-coupled amp allows you to ‘see’ to 5uV/Division. For a 5V full scale A/D that’s 1ppm per division on your ‘scope. Trust me, you’ll see things there.  (I read that Pasteur freaked out his dinner hosts, using his microscope to look for germs on his food. He was ‘debugging’ in his day too, with his new favorite toy).

A view inside the prototype x1000 low noise amp. Click on image to enlarge it.

Here’s a pdf of the schematic – it’s based upon the design in the book Low-Noise Electronic System Design, by Motchenbacher and Connelly (a highly recommended text). Since the text came without the amp, I had to make one myself.

SCHEMATIC-low-noise-x1000-amp-MODEL-101

I also have a SPICE model for it, using Linear Tech’s LT-SPICE and I’ll send it to you, if ask.