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| Looking for a Probe to Use for 5V DC Power Supply Ripple Measurement |
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| bendras:
--- Quote from: precaud on May 12, 2018, 02:11:57 pm --- --- Quote from: bendras on May 12, 2018, 01:46:46 pm --- --- Quote from: Cerebus on May 12, 2018, 12:44:26 pm ---You need a differential probe or all the common mode noise that's generally slopping about will completely swamp the noise that's actually being generated by the power supply and that's all you'll see. --- End quote --- This is exactly the reason why I am looking for a way to do a differential measurement. --- End quote --- OK. How about a Tek AM502 diff preamp? PAR made a similar standalone unit, the 113. Signal Recovery has the 5113, that will be in the same price range used as the SR560, though. --- End quote --- All of these preamplifiers seem to have maximum bandwidth of 1MHz. As far as I know the standard for measuring power supplies is 20MHz. Do you know of any models with 20MHz or higher bandwidth? |
| bendras:
The HP Hewlett Packard 1141A seems like a good candidate, but I am a bit confused about the specification. The datasheet https://www.testequipmentdepot.com/agilent/pdf/1141a.pdf on page 54 says that without attenuators (i.e. in the 1:1 mode) it can handle up to +/-20V in "Common-mode Operating Range DC" and up to +/-300mV peak in "Differential Input Range". Does this mean that it is suitable for measuring 20V DC rails with 300mV peak-to-peak ripple on them? |
| nctnico:
--- Quote from: bendras on May 12, 2018, 11:34:13 am --- --- Quote from: nctnico on May 11, 2018, 10:14:25 pm ---The easiest way is to solder a piece of coax to the board and use a DC blocker and 50 Ohm termination at the oscilloscope. --- End quote --- I have investigated this technique but at 5V it seems to be loading the circuit with a few tens of mA of current draw. That is the reason why I am looking for a way to do differential measurements. --- End quote --- No it doesn't. The DC block (just a capacitor in series) is placed before the 50 Ohm termination so no DC current flows into the oscilloscope. |
| David Hess:
--- Quote from: bendras on May 12, 2018, 04:14:52 pm --- --- Quote from: precaud on May 12, 2018, 02:11:57 pm ---OK. How about a Tek AM502 diff preamp? PAR made a similar standalone unit, the 113. Signal Recovery has the 5113, that will be in the same price range used as the SR560, though. --- End quote --- All of these preamplifiers seem to have maximum bandwidth of 1MHz. As far as I know the standard for measuring power supplies is 20MHz. Do you know of any models with 20MHz or higher bandwidth? --- End quote --- The Tektronix 1MHz AM502 was the standalone implementation of the Tektronix 5A22/7A22 for the 5000/7000 series mainframe oscilloscopes. It is great for these kinds of measurements but of course only up to 1MHz. For higher bandwidth in the 7000 series, Tektronix had the 100MHz 7A13 which provides 1mV/div sensitivity over a common mode range of +/-10 volts and can be used as a differential comparator similar to Keysight's power rail probes or as a differential input amplifier. The problem with the 7A13 (besides being old) is that its switchable bandwidth limit is 5 MHz (1) although this could be modified by changing a capacitor. My solution is to connect my DSO to the vertical output from the 7000 mainframe and implement the 20MHz bandwidth limit there; so effectively the 7A13 and 7000 mainframe become a 100MHz 1mV/div differential front end for my DSO. Preamble made a standalone version of the 7A13. They were bought be LeCroy who still produces their products but that would be an expensive way to go although cheap if you design power supplies for a living. You might be able to find a used Preamble instrument on Ebay. Some external differential probes might be suitable also. The Pintek DP-60HS could be used with external AC coupling capacitors but it only has a 15 MHz bandwidth. The easiest way and what I would try first, which also happens to be recommended for ATX power supply testing, is using a pair of x1 probes and a 2 channel oscilloscope configured for differential (invert and add) input operation. The problem is gain matching between the input channels which some DSOs do not handle well and increased quantization noise. Analog oscilloscopes and some very rare old DSOs can use their analog "variable" vertical controls to match the gain of the channels for pretty good performance when used like this while avoiding an increase in quantization noise. (1) I think the reason for a 5MHz bandwidth limit instead of 20MHz on the Tektronix 7A13 was that 20MHz had not be standardized for power supply noise measurement when it was designed and the 7A13 is pretty noisy due to its complexity so 5MHz was a good choice for operation at its maximum sensitivity. The specified noise of the 7A13 is less than 200uVrms and I measured mine at about 100uVrms which is about 10 times what a simple but good single ended 1mV/div input with a common mode range of +/-10mV has. The difference here is that the 7A13 is differential which doubles the input noise and has a cascode to support operation over +/-10V or 1000 times what a normal oscilloscope input can handle. When used as a differential comparator, it is like having an oscilloscope position control which operates over 20,000 divisions. |
| precaud:
--- Quote from: bendras on May 12, 2018, 04:14:52 pm ---All of these preamplifiers seem to have maximum bandwidth of 1MHz. As far as I know the standard for measuring power supplies is 20MHz. Do you know of any models with 20MHz or higher bandwidth? --- End quote --- Not offhand. I think you're in diff probe territory to get useful CMRR at that bandwidth. |
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