Electronics > Metrology

DIY 0.1 to 10Hz Noise Amplifier

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Svgeesus:

--- Quote from: trtr6842 on May 03, 2024, 05:06:46 am ---The red LED on the left lights up if the LNA is powered on and the DC input voltage goes below -10mV. 
The input protection limits the current to ±2mA, which buys me some time when I inevitably hook up my input backwards!

In past revisions I definitely stressed input protection, since having such a large input capacitor paired with low input impedance is a recipie for trouble.  However, I neglected to protect the signal outputs from any accidental connection to DC sources of up to +30V.  With past revisions that would surely damage or completely destroy the LNA.  On these boards I reworked some series resistors and TVS diodes on the bottom side of the board.  These limit and clamp any injected current, and they do not significantly change any output characteristics when used with 1MΩ scope inputs.

--- End quote ---

Just checking and sorry if this is obvious. So, measuring noise on a +12V source is fine, but to measure noise on a -12V source I need to have a floating DUT ground and connect -12V to LNA ground and 0V to LNA input, right?

trtr6842:
@Svgeesus correct, the LNA ground is tied to the scope gnd, and the LNA cannot handle negative inputs without damaging the electrolytic input capacitor.  However it would be simple to swap the polarity of the input capacitor and that would make the LNA capable of only measuring negative input voltages.

Either flipping the input cap, or floating the whole LNA, swapping the input polarity to make it positive with respect to the LNA input, and using a differential probe/amp on the LNA output would allow for measuring negative inputs.

Alex Nikitin:
If your negative voltage source is not an extremely low noise one, a simple passive filter and a good voltmeter might do the job for both polarities without any additional switching and with a reasonable settling time.

Cheers

Alex

Kleinstein:
Not many voltmeters are really low enough in noise to be useful fora direct noise analysis. This would be manily nV meters and a few more special ones. Also the autozero mode complicated the effective band width. One could get close / comparable, but not really the standard 0.1 - 10 Hz band.  E.g. 40 ms integration would give a 12.5 Hz effective noise bandwidth and the length of the interval to look at would set the lower frequency limit.  So 20 PLC, non AZ mode an looking at some 10 seconds would be relatively close.

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