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| Pintek DP-60HS differential probe (low-voltage, high-sensitivity, 60 MHz BW) |
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| David Hess:
What I have sometimes done in the past with circuits where it is difficult to attach a probe without affecting the circuit is to build essentially the front end of the probe into the circuit to drive a test point. Something like a pair of bipolar or FET buffers can work into the 100s of MHz or simple operational amplifier followers for DC precision at lower frequencies. For power supplies, the built in test circuit might involve something like a wide bandwidth high side current sense circuit with a transconductance output driving a low side termination resistance to take the place of an AC/DC current probe. |
| IDEngineer:
--- Quote from: David Hess on October 15, 2017, 09:35:40 pm ---What I have sometimes done in the past with circuits where it is difficult to attach a probe without affecting the circuit is to build essentially the front end of the probe into the circuit to drive a test point. --- End quote --- You're a mind reader! I've already spec'd in a dual FET opamp for the prototype PCB's, for a low leakage front end that has a nice low impedance output to drive test equipment. Then we can selectively populate it for development vs. production, or remove the artwork if the real estate is needed for other purposes. I included opamps in an earlier breadboard and the concept worked extremely well. This breadboard is more focused on the isolation aspect of things and adding the opamps would be inconvenient; I'd have to build another breadboard with both isolation and opamps. We're close enough to PCB layout that I'm going to wait for that. In the meantime, I have this breadboard on the bench and it would be nice to use the diff probe with it, so I'm going to proceed with the Rube Goldberg "banana tip" probe idea and see where that, ahem, "leads". I expect improvement, but not perfection. Worst case I gain more experience with the diff probe. |
| IDEngineer:
Just checking in with some observations after a couple of weeks of experience with the DP-60HS. Short version: I love having a diff probe. I have learned so much in such a short time, it's amazing. Longer version: First, the "banana leads" work great. I purchased a few sets of banana plug tips from DigiKey. They have screws on the back that make it easy to add short wires clipped from resistor leads. These short wires can be carefully attached to the circuit in question. While the bulk and weight of the diff probe body makes for some unwieldy positioning, you get nice clean signals that are entirely isolated from ground (as long as you're careful to insure no other connections to a common ground exist, as discussed elsewhere in this thread). And wow, those signals reveal a lot. I know all the theory about scope ground leads, removing them for sensitive measurements, etc. What I didn't realize until now was the effect of having an earth ground reference (from the scope probe ground) applied directly to the circuit being evaluated. It's amazing. Way more effect than I ever realized. It's almost entertaining... I find myself thinking "I wonder what THAT signal *really* looks like" and probing all sorts of stuff I thought we understood. Boy, did some uglies crawl out from under the rocks! As just one example, we have a measurement circuit that has long had a weird sort of variability to it in the real world. Connecting up a traditional (grounded) scope probe shows nothing... all is well, signals as we'd expect them, etc. But with the (isolated) diff probe, suddenly it all made sense because it turns out we're picking up some nasty square waves from elsewhere in the system! Depending upon when the measurement is taken, that square wave introduces variability - and we've never been able to see it before. Of course, the very next thing we did was to leave the diff probe hooked up and connect a traditional scope probe to it too. Voila - the superimposed square wave disappeared, because now this circuit had a ground reference from the traditional scope probe. You could see everything happen on the diff probe's trace. This allowed us to correct for the noise problem, a problem we didn't even understand before. Couldn't even SEE before, because the traditional (grounded) probe was hiding the problem. Keep in mind, this specific diff probe is intended for small signal work - unlike most diff probes which are intended for high voltage isolation. This one has x1/x5/x10 ranges whereas many start at x100 and sometimes go to x1000! Your typical diff probe won't necessarily reveal the stuff we're seeing with this "high sensitivity" (hence its HS suffix) diff probe. You need the correct tool for the job. As I said, it's almost not work anymore to use the diff probe. It's fun to connect it to things and see how they differ from what we thought, expected, or had seen with a traditional grounded probe. I wouldn't be surprised to find another one (or three, one for each of four channels!) around here soon. I'm almost to the point that I don't trust what I see on the scope unless it was measured with the diff probe. Highly, highly recommended. |
| David Hess:
Now you know why I am so attached to my Tektronix 7A13 and 7A22 differential amplifiers. |
| IDEngineer:
Indeed... a Tek mainframe with a good selection of properly functioning modules is a very nice thing to have. Long ago I had one of those TM500(?) series standalone mainframes with a few specialized modules in it, feeding into the standalone scope it sat next to. My favorite module was the current probe amp, which connected to one of their clamp-around probes. It measured low currents with pretty wide bandwidth. I wish I could have taken that mainframe with me when I left the company, I suspect no one else there knew what it was or how to use anything in it. |
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