x100 'scope probe

You need a 90 meg if coming off an X 10 probe, 99 to the BNC.I think.

Off the top of my head I think you'll be down to 1/10th the upper limit ban dwidth, and on AC coupling it will take much longer to settle to the center is there's DC on the input. Also not that in most scope front ends that wi ll NOT give you 10 X the max DC input.

The sell X 100 probes, a bit more money per everything else. I used one for certain things when faced with a scope that lacked 20V/div. Yeah, they gav e you more at the low end but took away from the high end. No more looking at the collector of an HOT/LOPT with a X 10 probe.

Well if the intent is to measure high voltages you'd want a resistor or series of resistors to avoid "flash-over" of the outside of the resistor with dirt or moisture in the air. If you are measuring waveforms, the coaxial cable's capacitance will distort the wave form because the resistor and cable form a R-C integrator.

Normally there's a variable cap in the probe that can neutralize the cable capacitance.

Don't forget the point David Hess frequently makes...

If you are relying on the scope's 10Mohm input resistance to form the lower leg of the potential divider, then consider what happens if you turn the scope to AC input coupling. The

*entire* input voltage appears across the scope's AC coupling capacitor.

That can and should be avoided by having a probe with the potential divider made from two resistors, one for the upper leg and the other is (in parallel with the scope/s input) the lower leg. Then if the scope is in AC mode the voltage "seen" by the scope will be slightly wrong, but the voltage will be limited by the potential divider.

On Tuesday, September 25, 2018 at 10:45:57 AM UTC-4, snipped-for-privacy@gmail.com wrot e:

andwidth, and on AC coupling it will take much longer to settle to the cent er is there's DC on the input. Also not that in most scope front ends that will NOT give you 10 X the max DC input.

or certain things when faced with a scope that lacked 20V/div. Yeah, they g ave you more at the low end but took away from the high end. No more lookin g at the collector of an HOT/LOPT with a X 10 probe.

Right I was going to stick it one the end of a x10 probe... I don't really care if it's a x110, probe... mostly want to look at what might be a high voltage without blowing up the 'scope. (maybe it's a bad idea.)

George H.

I was going to ignore the capacitance... a 1 ms time constant would be OK... (100 meg ohm * ~16 pF ~ 1.6ms)

George H.

Huh, I'm not seeing that at all. Don't I still have 1 meg to ground inside the scope? (putting my DMM across my 'scope input I measure 1 meg ohm for both DC and AC coupling.)

George H.

ground inside the scope?"

Not the ones I've seen. Just as a typical 50-100 MHz scope (plugin) example , in

on PDF page 59 is the input circuit. On the J 10/ R 10 side of C 10 there i s nothing to ground, therefore the resistance is infinity. The voltage at J 10 will go to 100 % of the input, as it would with a 10 X probe anyway, bu t a 100 X is likely to see higher voltages.

I have never seen a scope front end that was not similar.

If DC is not needed a cap of 1/10th the value of C 10 or its equivalent wou ld form a capacitive divider and with even miniscule leakage of C 10 or equ ivalent, the voltage will stay down. Its leakage is like to be much higher than that of the higher voltage cap you add externally to protect the scope 's input cap.

I built an attenuator to measure some high voltage spikes 25 or 30 years ago. I still have it, just checked the values to put on the picture in the link below. I don't recall the attenuation, It might make a 10x be a 20x or a 40x. The values make me think 20x but the cap has me not sure. I was able to calibrate it using the scope calibrator for a good square wave, but the shield was important to allow that. I was always a little concerned for the scope probe coax arcing, but at 800 volts, it never did.

Note: I put a T on the scope, then the probe off one side and the attenuator off the other side. If I get some time I'll set it up and check the attenuation.

Mikek

Not on the scopes I'm familiar with.

The BNC is connected to the capacitor, and the other side of that is connected to the 1Mohm (not 10Mohm, doh!) vertical sensitivity attenuators.

In DC mode the capacitor is shorted out so the BNC is connected to the attenuators. In AC mode the BNC is "floating".

On Tuesday, September 25, 2018 at 3:03:26 PM UTC-4, snipped-for-privacy@gmail.com wrote :

le, in

is nothing to ground, therefore the resistance is infinity. The voltage at J 10 will go to 100 % of the input, as it would with a 10 X probe anyway, but a 100 X is likely to see higher voltages.

Huh. OK... I didn't look at the schematic, but my TEK1001 floats the input at AC, (as you say.) The new Keysight reads 1.0 meg ohm at DC 1.0 meg ohm a t AC. I put in a DC voltage through a 1 meg resistor, and measured the voltage at the 'scope input (AC and DC). It was 1/2 the applied voltage. The same in both cases.

So there is at least one 'scope that is different. (I never knew... thanks)

ould form a capacitive divider and with even miniscule leakage of C 10 or e quivalent, the voltage will stay down. Its leakage is like to be much highe r than that of the higher voltage cap you add externally to protect the sco pe's input cap. Right, I've done that... I wanted DC (well I don't need it any more.. voltage was not as high as I thought.)

George H.

Thanks Tom, I need to correct what I said about the keysight scope (DSOX 1102G) On AC it's 1 meg ohm for gains of 500 mV/ div and less and 1.2 Meg at 200 mV and higher.

George H.

Amazon has decent 100 MHz 100x scope probes for around $35.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

My Rigol measures 1M input on a DVM (to 5 decimal places) on either AC or DC coupling.

Set to 50 ohms, it measures 50.37 whether AC or DC coupled.

Good to know, I guess.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

rote:

t bandwidth, and on AC coupling it will take much longer to settle to the c enter is there's DC on the input. Also not that in most scope front ends th at will NOT give you 10 X the max DC input.

e for certain things when faced with a scope that lacked 20V/div. Yeah, the y gave you more at the low end but took away from the high end. No more loo king at the collector of an HOT/LOPT with a X 10 probe.

e

Right, I was looking at cheap and now. I should buy a x100 probe...

George H.

ote:

ample, in

ere is nothing to ground, therefore the resistance is infinity. The voltage at J 10 will go to 100 % of the input, as it would with a 10 X probe anywa y, but a 100 X is likely to see higher voltages.

ut

m at

.

t would form a capacitive divider and with even miniscule leakage of C 10 o r equivalent, the voltage will stay down. Its leakage is like to be much hi gher than that of the higher voltage cap you add externally to protect the scope's input cap.

I didn't know my 'scope could be floating at AC coupling.... That's even better to know. (shame on me.)

George H.

I set the attenuator up on my bench scope, 300MHz scope with matching probe, makes the 10X probe a 20X probe. I can also calibrate the probe using the built in calibrator.

If making this is to much, how about an on board voltage divider.

10meg from your source to a 1.39meg to ground. Place your 10meg scope probe at the junction. This is will display as a 10 to 1 ratio of the initial HV. This assumes your HV source will drive an 11.22 Meg load without appreciable sag and that your probe is 10meg at the frequency of interest. Mikek

I described this in a later post, but I adjusted the resistor divider to make up for the loading of the probe, so it will be 100X with the the probe attached.

Mikek

Some scopes, such as my TDS 784As, warn you that if you select AC coupling with 50-ohm Zin, your lower cutoff frequency will be much higher than you expect. Thus it seems that the AC coupling happens before the input load, at least on the 50-ohm scale.

I have a couple of Pomona boxes with series capacitors plus protection diodes on the output.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
https://hobbs-eo.com