Measuring noise on DC rails with active probe

Question

I have a Tektronix P6245 active probe (allows only DC input) and wish to use it to see noise on various power supply rails. I've done this already on a +3.3V rail, and can get sufficient DC offset on the scope (Tek TDS744a) to see millivolts.

However, it gets tricky with higher voltages (within the probe limits). For example, a +12V DC rail is tough. Bringing the vertical resolution to a level that will provide meaningful data is simply not possible.

I'm considering a dc blocking capacitor between the probe and the rail under test. Is this a reasonable option? Any suggestions for best way to do this?

Answer 1

Yes, go ahead and use a DC blocking cap. Active probes do have high input impedance, so peculiarities of capacitor actual R-L-C do not matter much. If you need more accuracy along the spectrum, you can always calibrate your tip.

I usually use the standard Tektronix tip (plug), shave the sharp edge flat (with Dremel), and solder a 0603 ceramic capacitor as a new tip. A 0.1uF 25V MLCC cap should give you a bandwidth from about 1.6Hz and up.

CAUTION: However, be aware that the single-ended probe must have good grounding, so be careful not to pick up noise on the ground lead over the air, or from improper grounding point. Instead of long lead, use a pogo-pin (spring-loaded) plug into ground receptacle of the P6245 probe, and have an open ground surface near your point of measurement. You will need to properly align both tips to make good connection.

Answer 2

Options:

1. Money no object? R&S has a new probe specifically for this application, with separate AC and DC signal paths so that large offsets can be accomodated while measuring small AC signals. The cost of a new R&S scope to use it with may be prohibitive, though. I'd be surprised if Tek, Keysight, LeCroy, etc., don't come up with similar probes in the near future.

2. Use a different probe that allows for ac-coupling.

3. Forget the fancy probe and just contact the power rail through a 50-ohm resistor with the center conductor of a chopped off piece of coax (and contact the o.c. to ground nearby), and ac-couple the other end to your 'scope.

Answer 3

Your probe seems to have a gain of x1, therefore it provides only a dubious benefit in this case.

Since a power supply has low impedance (hopefully it has decoupling capacitors!) the easiest (and also best) way to measure its noise is to solder a 50 ohms resistor to your test point, and connect this to the scope using a piece of 50 ohms coax with a BNC at the end.

Set scope to AC coupling. You don't need to enable internal 50R termination, since you have 50R source termination.

Soldering the coax frees up your hand to perform other tests, if you want to measure your supply noise with your microcontroller/board/DSP/whatever doing various things, for example.

I like MELF resistors for this purpose, because they are small, have good HF behavior, and unlike SMD chip resistors, won't break when the cable pulls slightly.

This will give you the best measurement possible.

The place you solder the coax ground to is also important.

Answer 4

So the best way would be to just use a normal probe lower bandwidth probe (if you don't have one maybe you should get one) and run the probe in AC mode at a higher gain than 1x and take out the hassle of adding capacitors.

I also find it hard to believe that you can't set the scope to AC coupling.