Author Topic: Differential probes...why so !@$%@ expensive?  (Read 10152 times)

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Offline tweek

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Differential probes...why so !@$%@ expensive?
« on: March 13, 2011, 01:30:16 am »
Even the cheap ones cost more than I paid for my DS1052E.  What's in these things?  Is the circuit board hand-crafted by nude virgins or something?
 

Offline Psi

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Re: Differential probes...why so !@$%@ expensive?
« Reply #1 on: March 13, 2011, 01:49:29 am »
If you dont need it to be that sensitive or accurate and just want to probe a differential signal you can set the scope math function to "A - B" and connect your signal between the tips of two probes, one on each channel.

Poor mans software based differential probe.
Just make sure both channels are set to the same settings (probe x1/x10, volts/div, not inverted etc.)

Of course you can only look at one signal, since it uses both channels. 
« Last Edit: March 13, 2011, 01:54:32 am by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline tweek

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Re: Differential probes...why so !@$%@ expensive?
« Reply #2 on: March 13, 2011, 02:26:35 am »
I considered this, but I was admittedly ignorant as to whether the particular math function in question would simply be implemented in software, requiring me to fully connect the probes, or if it would actually switch in a differential amp to do the measurement.  I get the impression that some scopes do in fact do the latter, but wasn't sure if the cheap Rigol does this as well.
 

Offline Psi

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Re: Differential probes...why so !@$%@ expensive?
« Reply #3 on: March 13, 2011, 02:44:31 am »
i just assumed the rigol did it in software, i cant say i know for sure.

My old 50mhz analog scope does seem to do this A-B pseudo differential much better than the rigol, which may be for that very reason, being analog the CRO done it in hardware.
With my CRO atleast, it involves setting one channel to inverted as there is only A+B and no A-B
« Last Edit: March 13, 2011, 02:50:12 am by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline tweek

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Re: Differential probes...why so !@$%@ expensive?
« Reply #4 on: March 13, 2011, 02:50:27 am »
I'm guessing it's software.  I just tried it on a 9V battery so I could be sure there was no common ground reference, and I actually got V/2 on the math output.
 

Offline tweek

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Re: Differential probes...why so !@$%@ expensive?
« Reply #5 on: March 13, 2011, 03:03:08 am »
I'm guessing it's software.  I just tried it on a 9V battery so I could be sure there was no common ground reference, and I actually got V/2 on the math output.

Oh boy.  I just did it again and now I get the correct voltage out of the math function.

I think it's time to call it a night.
 

Offline NiHaoMike

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Re: Differential probes...why so !@$%@ expensive?
« Reply #6 on: March 13, 2011, 04:36:21 am »
High speed isolation just isn't cheap. However, I think it's possible to make an isolator circuit that is a lot cheaper than the ones out there.
Cryptocurrency has taught me to love math and at the same time be baffled by it.

Cryptocurrency lesson 0: Altcoins and Bitcoin are not the same thing.
 

Offline grenert

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Re: Differential probes...why so !@$%@ expensive?
« Reply #7 on: March 13, 2011, 05:14:43 am »
If you go the A-B route, you need to make sure your probes or matched, or at least the same model.

If you want to try DIY-ing a differential probe, there have been a couple of recent magazine articles (downloadable from the magazine websites for a fee):
Elektor USA, Sept. 2010: High-voltage probe (home made and differential, too) by Alfred Hesener.
Nuts and Volts, Nov. 2010: Oscilloscope probes and probing by Robert Reed.  Actually, I see that this is a high-speed active probe, but not a differential one.  Still, it includes a nice discussion of problems that can occur when probing circuits.
« Last Edit: March 13, 2011, 05:21:22 am by grenert »
 

alm

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Re: Differential probes...why so !@$%@ expensive?
« Reply #8 on: March 13, 2011, 11:25:12 am »
There is of course the issue of economy of scale: much fewer differential probes are sold than low-end scopes. It is also hard to design a differential probe with good CMRR at high frequencies, both channels needs to be very closely matched over the frequency range (which is why CH1-CH2 sucks). A 60dB CMRR requires matching of both sides of better than 0.1%. This gets progressively harder as the frequency increases. On top of this, the high-voltage models also have to conform to safety requirements.
 

Offline saturation

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Re: Differential probes...why so !@$%@ expensive?
« Reply #9 on: March 13, 2011, 12:40:22 pm »
Try to make one.  To better appreciate good probes see:

http://www.slack.com/TE/TekConcepts/TekProbeCircuits.pdf
Best Wishes,

 Saturation
 


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