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| Looking for a Probe to Use for 5V DC Power Supply Ripple Measurement |
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| David Hess:
--- Quote from: Kosmic on May 14, 2018, 05:29:07 pm --- --- Quote from: icpart on May 13, 2018, 10:16:14 pm ---Also I find another great improved version of that amplifier int that AN http://www.analog.com/media/en/technical-documentation/application-notes/an159fa.pdf --- End quote --- The amplifier described in an159 look really interesting. I will definitely try to build one of those! --- End quote --- Do you mean the one shown in figure 15? That can work but the specific example was intended for measuring the noise at the output of a linear regulator and has a lot more gain than required. This sort of things works well if it is just built into the original board layout and only populated for testing. |
| Kosmic:
--- Quote from: David Hess on May 14, 2018, 06:14:52 pm ---Do you mean the one shown in figure 15? That can work but the specific example was intended for measuring the noise at the output of a linear regulator and has a lot more gain than required. This sort of things works well if it is just built into the original board layout and only populated for testing. --- End quote --- Yes page 4 or 15 (page 15 is the differential version). On my side I'm interested in linear regulator noise and/or residual switcher noise after a linear regulator. So I guess if the amplifier is not close enough to the measurement point it will pickup a lot of external noise ? |
| David Hess:
--- Quote from: Kosmic on May 14, 2018, 11:39:35 pm ---So I guess if the amplifier is not close enough to the measurement point it will pickup a lot of external noise? --- End quote --- All of the usual probing rules apply which is why building the test circuit into the layout is advantageous. Maintaining a good common mode rejection up to 20MHz and beyond is not trivial either and unless calibration is performed, external dividers become a problem. I have not tried them but I was thinking parts like the LT1187, LT1189, or LT1193 video difference amplifiers would work well in these applications but like all video amplifiers, they are pretty noisy and too noisy to measure linear regulators although they could be used with a low noise preamplifier. They have the advantage of getting rid of the external feedback networks back to the inputs so common mode rejection should be easier to maintain without calibration and their datasheets show about 40dB CMRR at 20MHz. |
| Kosmic:
--- Quote from: David Hess on May 15, 2018, 02:47:18 am --- --- Quote from: Kosmic on May 14, 2018, 11:39:35 pm ---So I guess if the amplifier is not close enough to the measurement point it will pickup a lot of external noise? --- End quote --- All of the usual probing rules apply which is why building the test circuit into the layout is advantageous. Maintaining a good common mode rejection up to 20MHz and beyond is not trivial either and unless calibration is performed, external dividers become a problem. I have not tried them but I was thinking parts like the LT1187, LT1189, or LT1193 video difference amplifiers would work well in these applications but like all video amplifiers, they are pretty noisy and too noisy to measure linear regulators although they could be used with a low noise preamplifier. They have the advantage of getting rid of the external feedback networks back to the inputs so common mode rejection should be easier to maintain without calibration and their datasheets show about 40dB CMRR at 20MHz. --- End quote --- OK interesting, thanks for the clarification. |
| JDW:
I found this thread while Googling "power rail probe" today, having a similar need as the opening poster. In my case, I want to measure ripple and noise for 5v & 12v switch mode supplies. (Scope: Rigol DHO804) A $4,000 power rail probe from Keysight, Tektronix, etc. is laughably out of the question for a hobbyist or casual user, and yet, these specialty probes are superior to other probing solutions out there, including general purpose active probes. The upside is that I came across a low-cost solution, albeit one that I have not personally purchased and tried yet. I first discovered the following power rail probe design by Andrew Levido, which is quite interesting: https://circuitcellar.com/research-design-hub/projects/building-a-power-rail-probe/ Googling further led me to Patrick Coleman, who has forked Levido's design and posted all the details on his Github page here: https://github.com/blinken/power-rail-probe/tree/main Coleman even sells a fully assembled device, complete with a matte black enclosure professionally overlaid with white text: https://paradar.co.uk/products/low-noise-oscilloscope-power-rail-probe £249 (US$317) fully assembled £45 (US$57) for blank PCB & enclosure (must buy components on BOM separately yourself) |
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