Hello,
So when I first learned about probing, I learned that you should use the 10x setting of the probe whenever possible so that you don't load down your circuit very much.
Recently, I came upon a video by Mr. Ganssle.
https://youtu.be/aJsJibDNg9M
He shows that a 10pF capacitor goes from over 300ohms of capacitive reactance to just 159ohms of capacitive reactance.
What happened to the 9Mohm resistor in the probe?
Like, it should still be a 10 to 1 input impedance, right?
Formula: sqrt((1/(PI×2×100 000 000×0.000 000 000 01))^2+(9 000 000^2))
He goes on to discuss using just a piece of coax and 1K resistor.
Wouldn't that need some sort of capacitive compensation?
And if this really is *the answer* to high speed probing, why are we using 10x probes at all? Why not just a resistor hanging off of a piece of coax with maybe an attached pogo-pin and alligator clip on the ground terminal?
Like, it should still be a 10 to 1 input impedance, right?
He goes on to discuss using just a piece of coax and 1K resistor.
Wouldn't that need some sort of capacitive compensation?
And if this really is *the answer* to high speed probing, why are we using 10x probes at all? Why not just a resistor hanging off of a piece of coax with maybe an attached pogo-pin and alligator clip on the ground terminal?
Such probes are really only useful for higher frequency signals e.g. >100Mhz and the lower impedance circuits typically found with high speed signals. There are commercial probes available, known as Z0 probes, that have a series 450 resistor and have a x10 attenuation when coupled to a scope with a 50 input. Since the cable is terminated at the scope with the cable characteristic impedance it still looks like 50 at the probe end of the cable and doesn't have the excess cable capacitance that is seen with cable terminated by the 1M input of a scope as for a 10M probe. Without the excess capacitance there is no need for compensation.
However such probe present an excessive loading, especially DC loading, on lower frequency and higher impedance circuits so are not generally suitable for most circuit probing needs.
Hello,
...
He goes on to discuss using just a piece of coax and 1K resistor.
Wouldn't that need some sort of capacitive compensation?
And if this really is *the answer* to high speed probing, why are we using 10x probes at all? Why not just a resistor hanging off of a piece of coax with maybe an attached pogo-pin and alligator clip on the ground terminal?
Thanks!
When I think of high speed, I am thinking digital.
When I think of high speed, I am thinking digital.
Strictly speaking it doesn't matter. If you are looking at a waveform with a scope you are looking at an analogue waveform that something will interpret as a digital signal.
Exceptions: photon counting and femtoamp circuits
Relevance: all the probing considerations apply unchanged to both "digital" circuits and "analogue" circuits.
When I think of high speed, I am thinking digital.
Strictly speaking it doesn't matter. If you are looking at a waveform with a scope you are looking at an analogue waveform that something will interpret as a digital signal.
Exceptions: photon counting and femtoamp circuits
Relevance: all the probing considerations apply unchanged to both "digital" circuits and "analogue" circuits.
In the context of loading and drive, how much it can tolerate, it most certainly matters.
What do you think "most certainly matters"?
What do you think "most certainly matters"?
Anything that effects the SI.
Relevance: all the probing considerations apply unchanged to both "digital" circuits and "analogue" circuits.
Relevance: all the probing considerations apply unchanged to both "digital" circuits and "analogue" circuits.Do you really think rise time and amplitude are equally important in analog and (binary) digital signals? Is amplitude a critical parameter for an accurate eye diagram? Is rise time important in what's generally a more narrow band analog signal like RF?
What do you think "most certainly matters"?
Anything that effects the SI.
Precisely. SI is an analogue phenomenon not digital, and the same issues also occur in analogue circuits.
What do you think "most certainly matters"?
Anything that effects the SI.
Precisely. SI is an analogue phenomenon not digital, and the same issues also occur in analogue circuits.
As I stated, it just provides context which I thought would have been obvious. Sure we can become pendant but it doesn't add anything to the discussion.
What do you think "most certainly matters"?
Anything that effects the SI.
Precisely. SI is an analogue phenomenon not digital, and the same issues also occur in analogue circuits.
As I stated, it just provides context which I thought would have been obvious. Sure we can become pendant but it doesn't add anything to the discussion.
The key distinction between digital signals (0,1,etc) and analogue signals (volts, amps, frequency, etc) is not pedantry.
False distinctions between analogue waveforms and RF/microwave etc is not pedantry.
Failing to grok those is the source of much bafflement, many incorrect statements (especially w.r.t. sampling rate), and many problems seen all too often in circuits (especially SI).
Hence it is important, not mere pedantry.
Strictly speaking it doesn't matter. If you are looking at a waveform with a scope you are looking at an analogue waveform that something will interpret as a digital signal.
What do you think "most certainly matters"?
Anything that effects the SI.
Precisely. SI is an analogue phenomenon not digital, and the same issues also occur in analogue circuits.
As I stated, it just provides context which I thought would have been obvious. Sure we can become pendant but it doesn't add anything to the discussion.
The key distinction between digital signals (0,1,etc) and analogue signals (volts, amps, frequency, etc) is not pedantry.
False distinctions between analogue waveforms and RF/microwave etc is not pedantry.
Failing to grok those is the source of much bafflement, many incorrect statements (especially w.r.t. sampling rate), and many problems seen all too often in circuits (especially SI).
Hence it is important, not mere pedantry.When you consider details, digital is analog. I use the term digital to provide context about what we are probing. Again not 0's and 1's but loading, drive strengths, transition levels, overshooot.... As you previously posted:QuoteStrictly speaking it doesn't matter. If you are looking at a waveform with a scope you are looking at an analogue waveform that something will interpret as a digital signal.Much like my own post so far, I suspect these comments are too high level and obvious. They add little to no value to the discussion.
Hello,
So when I first learned about probing, I learned that you should use the 10x setting of the probe whenever possible so that you don't load down your circuit very much.
Recently, I came upon a video by Mr. Ganssle.
https://youtu.be/aJsJibDNg9M
He shows that a 10pF capacitor goes from over 300ohms of capacitive reactance to just 159ohms of capacitive reactance.
What happened to the 9Mohm resistor in the probe?
Like, it should still be a 10 to 1 input impedance, right?
Formula: sqrt((1/(PI×2×100 000 000×0.000 000 000 01))^2+(9 000 000^2))
He goes on to discuss using just a piece of coax and 1K resistor.
Wouldn't that need some sort of capacitive compensation?
When ballsystemlord titled Probing at high speeds, they never defined what they were attempting to probe. I provided what I considered the topic to be about based on their mention of resistive probes, thinking the OP would provide more details. I really have no idea what that were actually after and assumed after reading some of the posts, lost interest. A shame really.
Note that up at RF frequencies, the loading effect of a typical 10Meg x10 scope probe isn't as simple as 10Meg in parallel with about 12-15pF.
...
I couldn't tell you how many times I have seen someone using that long ground clip lead and hooked tip.