very low noise Active Probe - Page 1
Forum section: electronics / projects

author: xzswq21
author_rank: Frequent Contributor
timestamp: November 15, 2017, 08:02:07 pm
content: HelloI have a signal within 2Vpp and up to 10MHz. I need to buffer it with a high impedance probe and then measure the SNR.but the probe should be ultra low noise to not affect the SNR of the signal.Do you know any reliable circuits or ideas or low price Active probe?(I have some ultra low noise LDOs and some very low noise and ultra low noise op amps)(Actually I have looked at many active probe circuits! but I don't have any information about their noise!)update:Actually I have a current source. I want to apply the output current to a resistor and then measure the output voltage.200 uApp current---> 10k resistor ----> measure the output voltage (2Vpp)I used 10K resistor instead of using 1kohm+10V/V gain like the below:200 uApp current---> 1k resistor ---->+10V/V of a gain----> measure the output voltage (2Vpp)Now I can measure the THD and SNR and ... of the system ..... .The current is a stepped sine signal (I mean I increase the frequency step by step)Best Regards

author: awallin
author_rank: Frequent Contributor
timestamp: November 16, 2017, 05:51:08 am
content: a few distribution-amp designs listed over here:https://www.ohwr.org/projects/pda-8ch-fda-8ch/wiki/similar-projectsthey are 50-ohm in, 50-ohm out designs but you might be able to modify them for high input impedance.this one is worth a look also:https://www.tapr.org/kits_tns-buf.htmlif you have a decent 10MHz signal you will need an instrument with a very high dynamic range to measure the true SNR (on not just the instrument noise-floor)...

author: ejeffrey
author_rank: Super Contributor
timestamp: November 16, 2017, 06:11:19 am
content: You can buffer that signal with a low noise opamp. Just go to analog devices website and use their parametric search to find something suitable.But the measurement is tricky even after you buffer the signal. 2 Vpp is a big signal, so you can't just use a low-noise opamp with a gain for 100 to bring the noise up above the rest of the measurement apparatus. What are you planning to use as a digitizer?What is the noise floor you expect? Is this a few nV/rtHz johnson noise? Or do you expect a higher noise floor?What kind of noise do you care about? Broadband noise, or noise close to the signal frequency? Or do you actually want to measure something like harmonic distortion / waveform distortion that isn't random noise?How much control do you have over the signal? Do you know or control the exact frequency, or do you only know the range? Is it a sine wave or something else?Pending the answers to these, my inclination is to try to filter out the "signal" passively to get a better look at the noise. If you don't have to measure noise super close to the signal tone, you might be able to use a notch filter to kill the signal, which would allow you to amplify the noise up to a more convenient amplitude.

author: xzswq21
author_rank: Frequent Contributor
timestamp: November 16, 2017, 06:41:00 am
content: Quote from: ejeffrey on November 16, 2017, 06:11:19 amActually I have a current source. I want to apply the output current to a resistor and then measure the output voltage.200 uApp current---> 10k resistor ----> measure the output voltage (2Vpp)I used 10K resistor instead of using 1kohm+10V/V gain like the below:200 uApp current---> 1k resistor ---->+10V/V of a gain----> measure the output voltage (2Vpp)Now I can measure the THD and SNR and ... of the system ..... .The current is a stepped sine signal (I mean I increase the frequency step by step)