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| Tek p5205 HV differential probe teardown. BTW, what are the red and brown wires? |
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| Mechatrommer:
--- Quote from: onlooker on September 28, 2014, 02:30:28 am ---I am also thinking to reduce the attenuation from 50x-500x to 10x-100x since I am really just interested in the differential capability, not the HV capability. Any suggestions? --- End quote --- if you are well versed in "dividers matching" and "frequency compensation" then its possible. all you have to do is replacing the dividers in HV left section with "quality" components of the same sizes. otherwise dont do it, messing with attenuation is messing with differential capability/performance. --- Quote from: onlooker on September 28, 2014, 02:30:28 am ---As to the +-15V power supply, is switch mode supply good enough? --- End quote --- if it doesnt screw the noise and CMRR performance. |
| David Hess:
--- Quote from: Mechatrommer on October 06, 2014, 04:17:39 pm --- --- Quote from: onlooker on September 28, 2014, 02:30:28 am ---I am also thinking to reduce the attenuation from 50x-500x to 10x-100x since I am really just interested in the differential capability, not the HV capability. Any suggestions? --- End quote --- if you are well versed in "dividers matching" and "frequency compensation" then its possible. all you have to do is replacing the dividers in HV left section with "quality" components of the same sizes. otherwise dont do it, messing with attenuation is messing with differential capability/performance. --- End quote --- Especially do not do it unless you are prepared to recalibrate the AC and DC common mode rejection. Of course you may have to do this anyway if you change the probe leads. You may be able to adjust the attenuation on the output side which is low impedance but I would sure want a schematic before trying it which seems like a lot of work for a minor convenience. Why not just set the oscilloscope vertical input to a higher sensitivity? |
| MarkL:
--- Quote from: 128er on October 06, 2014, 03:12:11 pm ---One more thing... After I saw your schematic with the voltage divider for the reference voltage. That was my first idea too. But the reference voltage depents on the supply voltage and it could be unstable. With my circuit i try to sense +- ~250 mV. Is the solution with the zener regulator as a reference overkill, seen from a practical point? The 7815 should regulate well enough, that it won't have any effect on the circuit. --- End quote --- Perhaps it might not be obvious as drawn, but it's really just a classic measurement bridge. It's not dependent on supply voltage variation. For example, if the +5V supply goes up 10% and throws off the 2.5V reference voltage by +10% on the - input, the measured node into the + input is also up by +10%. They track each other. The only things critical to this design is the tolerance of the resistors, and the offset voltage of the comparator (TLC3702 = 6.5mV max). You can work out the exact requirements for the tolerances, but we're trying to detect an almost +/- 25% change in resistance on the brown-to-GND leg of the bridge. There's a lot of leeway. One thing not obvious is that this circuit is operating off +5V and not +15V (so, yes, there's regulator not shown). 5V operating voltage is on purpose because from testing we know the resistance on the brown wire is not valid unless the probe is powered up. That tells you instantly there's some active electronics also on that brown wire and not just a switch with a couple of resistors. On a TDS3054, the brown pin was measured as +5V, so that's the only thing I'm comfortable with, without knowing what else is on the brown wire. Your current source has a compliance voltage > +5V on that pin. I don't know if that's safe or what effect it might have on the probe internals. In fact, that contact on the TekProbe interface, along with another, make up the connections for 5V I2C communication with some of the more expensive probes. That's why the connection is called "Data" (and the other "Clock"). The pin is multiplexed by the scope between the two functions of I2C and attenuation sense. And one additional thing, as you point out the TLC3702 is a dual comparator. If you're not using the other half you should connect one input high and the other low. It has extremely high input impedance and it's probably pretty easy for it to oscillate with open inputs. It's generally good practice to tie levels on unused gates on any chip anyway. --- Quote ---Probably I put too many thoughts at such a small thing. ::) --- End quote --- Sometimes the fun is in the designing and building, more than the final outcome! |
| 128er:
--- Quote from: MarkL on October 06, 2014, 06:42:40 pm --- On a TDS3054, the brown pin was measured as +5V, so that's the only thing I'm comfortable with, without knowing what else is on the brown wire. Your current source has a compliance voltage > +5V on that pin. I don't know if that's safe or what effect it might have on the probe internals. --- End quote --- That's a huge no go for +15V. Didn't thought of that, or missed it in the thread. Thanks again. One more power rail required. One thing leads to another . . . |
| NiHaoMike:
Put in a zener to prevent that pin from going above 5V. |
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