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Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Use a combination of resistors and a pot such that the pot just exceeds the maximum range of adjustment you need. That way the effect of its variation is minimised. It's also considered a good idea to connect the unused end of the pot to the wiper. It won't hurt and it might reduce noise.
If you use the shielding system explained by Fabio earlier in this thread, I think the division ratio should remain fairly constant with changing parasitic capacitance. You'll have to test it and see and perhaps increase the height of the shield if the division ratio is too susceptible to parasitic capacitance. But bear in mind that a parasitic capacitance might change the actual voltage you are measuring and don't confuse this with a change in division ratio!
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
IntraWinding wrote ...
Us a combination of resistors and a pot such that the pot just exceeds the maximum range of adjustment you need. That way the effect of its variation is minimised. It's also considered a good idea to connect the unused end of the pot to the wiper. It won't hurt and it might reduce noise.
yeah in the pic you see that leg left unconnected, thats becuase the drawing wouldnt let me put a juntion there. But in the prototype i make this week end i will cetianly tie that pin down, just as you have said.
IntraWinding wrote ...
But bear in mind that a parasitic capacitance might change the actual voltage you are measuring and don't confuse this with a change in division ratio!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Patrick wrote ...
ok , well i noticed that you used 4 fixed reistors for the lower limb of your divider but you have to be careful that the wiper or one of the lower resistor dont open, thus becoming a 75kv hazard at the low end. i dont have that as a possiblity so i will try that trim pot then. we will see.
That's why I have four stout resistors in parallel in a mechanically robust arrangement, and a Littlefuse CG2350L 350V gas discharge suppressor in parallel with them, but located inside the main tube, and going directly to earth. Not quite as unsinkable as the Titanic, I'm sure, but getting there...
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
i will commence construction this weekend as i am finnally out of school for the summer now. i still have issues with the tlo82cp texas instruments op-amp limits though, as intrawinding said.
Proud Mary wrote ...
Not quite as unsinkable as the Titanic, I'm sure, but getting there...
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Patrick wrote ...
i will commence construction this weekend as I am finally out of school for the summer now. I still have issues with the TL082CP Texas instruments op-amp limits though, as IntraWinding said.
Great! This is a fascinating project - the very low input capacitance and shielding issues are outside my experience and I'm keen to see how you get on!
I think the improvement you got in performance by changing R6 from 10M to 1M is either due to a peculiarity of the simulation model that doesn't exist in the real product, or, if the model is super accurate, it's due to a poorly defined aspect of the way the op amp functions internally. The input devices are JFETs (Junction FETS) whose peculiarities I'm not familiar with, and additionally there may be protection circuitry not shown even in the detailed circuit diagram on page 9 of the data sheet
If you put R6 back to 10M, change the capacitive division ratio to, say, about 1500 by increasing C2 and increase the op gain to bring the overall division ratio of the system back to 1000 I think the problem is completely solved. I don't think there is any significant price to pay for this approach although I guess it must raise the level of random noise a little, but I don't think that will affect you. I'm not sure if it will have a significant effect on bandwidth though as I don't understand what limits the upper frequency response. Hope this is of some help
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
those are super-fast but are they frequency specific or truly wideband? i have no experience with them.
NOTE: this is a good idea ProudMary has, but so far i cant find any that have more than 12 v output and most are lass than 8 volts. i would like +-15v output if possible.
i wonder i a single jfet could be configured for accurate signal representation?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I bought some AD8056 video amps a while back - 300 MHz, –3 dB Bandwidth (G = +1)1400 V/us Slew Rate, and inexpensive - because I wanted to preserve the geometry of some proportional counter signals, but I haven't gotten around to using them yet. But as you point out, they're low voltage devices.
There's no reason why should feel you have to use an IC, if one or more discrete transistors could serve just as well, or even better.
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Capacitive Voltage divider, for O-scopes (10,000:1)
