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Differential Probe - Reverese Engineered
Differential Probe - Reverese Engineered
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Old 11th June 2019, 02:05 PM   #41
jan.didden is offline jan.didden  Europe
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Yes you can use much larger caps, as long as they are the right ratio. But then your input Z gets much smaller than what you expect based on the resistors. Normally you try to keep all capacitances as small as practical.


Last edited by jan.didden; 11th June 2019 at 02:29 PM.
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Old 11th June 2019, 03:33 PM   #42
Slobodan covek is offline Slobodan covek  Serbia
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Thank you.

Yes, ratio of capacitance is the same as those of the resistors.
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Old 11th June 2019, 03:55 PM   #43
jan.didden is offline jan.didden  Europe
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Yes, impedance ratio is the same which normally means the cap to resistance value is inverse ratio.

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Old 12th June 2019, 02:42 AM   #44
PRR is offline PRR  United States
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Differential Probe - Reverese Engineered
Originally Posted by Slobodan covek View Post
...simulations in TI Tina, and my input divider stage dos not attenuate the signal amplitude at all regardless of the frequency (tried 200 MHz max)....?
Does for me (PSpice). Above 1KHz it is near exact 10:1 attenuation. Below 30Hz it is a small hair more (round-off error in your 444.44... resistors).

However as you and Jan say, at high frequency the input impedance gets very-very low due to the roughly 1,000 pFd equivalent input capacitance. A lot of things which actually work OK will work poorly when 10 Ohms reactance (@ 70MHz) loads it.

My C46 should probably be 50pFd, or even 5pFd, with a few-pFd trimmer. And all other caps in proportion. Then input impedance would be hundreds of times higher.
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Old 23rd October 2019, 04:07 PM   #45
Chupacabras is offline Chupacabras
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Originally Posted by underwurlde View Post
^ OK, as requested... (apologies for low quality images)

pdf of THP version of the differential probes:
Attachment 391348

pdf of SMT version of the differential probes:
Attachment 391347

These differential probes I have reverse engineered are:
Testec type: TT-SI9001
1:10 / 1:100 attenuation
25MHz bandwidth.

±700V @1/100
±70V @1/10
DC & Peak AC
1000Vrms MAX, CAT III

<= 7V Max into 2K

The supply is 6V (via 4 AA batteries).

Thanks for those schematics, good stuff to learn for me.
I have simulated it in LTspice (used much better opamp instead of 741).
Unfortunately the simulations look like these circuits do not work properly.
-3dB bandwidth is about 9MHz.
Am I missing something?
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