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Differential Probe - Reverese Engineered
Differential Probe - Reverese Engineered
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Old 2nd January 2014, 05:44 PM   #1
underwurlde is offline underwurlde  United Kingdom
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Location: S.England
Cool Differential Probe - Reverese Engineered

Hello,

I use a number of Differential Probes at work (like the TesTec SI9000-1 found in RS). The need to calibrate & repair them necessitated a reverse engineering exercise. Not at all easy, all ICs (and some of the 'transistors') have had their markings sanded off. However, some numbers could be made out.

I have to hand 4 of these probes, the earlier version uses through-hole components and the later models use surface mount components. Both types are revealed here:

Through-hole version of the probe:
Click the image to open in full size.

Notes:
Guessed op-amp used for U3
Q3=Q5 (markings sanded off). Guessed this device as TL431
Q4 (markings sanded off). Guessed at BF451

Surface-mount version of the probe:
Click the image to open in full size.

Notes:
U3 (markings sanded off) - can just about make out '74' so guessed at 741
Missing: SMPS chip. Could not make a guess at that!

------

Anyway, comments, discussion, errors, improvements etc. PLEASE!

Andy
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Old 2nd January 2014, 09:43 PM   #2
ScreamingLordByron is offline ScreamingLordByron  United States
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This is really hard to read...can you post pdf versions?
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Old 3rd January 2014, 10:40 AM   #3
underwurlde is offline underwurlde  United Kingdom
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Cool pdf's of above images...

^ OK, as requested... (apologies for low quality images)

pdf of THP version of the differential probes:
Diff Probe.PDF

pdf of SMT version of the differential probes:
Diff Probe SMT.PDF

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

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

Output:
<= 7V Max into 2K

The supply is 6V (via 4 AA batteries).
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Old 3rd January 2014, 12:37 PM   #4
underwurlde is offline underwurlde  United Kingdom
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LM3046M/N, NOT LM3406! Apologies for typo there

Andy
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Old 5th January 2014, 08:13 AM   #5
1audio is offline 1audio  United States
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Differential Probe - Reverese Engineered
I appreciate your work. I think most of these (even the Tek and Agilent ones) are made by a vendor in Taiwan. I met them at a trade show in Taipei years ago and they seemed pretty serious and responsible. I would caution against making a copy China style. Make sure you understand the function of each element first (I'm sure this goes without saying) since the high voltages involved are really dangerous. Even the spacing between traces can be an issue, especially with fast rise HV pulses.

The basic FET buffer into an IC makes a lot of sense. It would not work at 20 MHz without a much more complex divider. I have a couple of Pomona branded versions that I look inside of. The potential of upgrading the opamp is interesting.
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Old 5th January 2014, 09:22 AM   #6
underwurlde is offline underwurlde  United Kingdom
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No intent here to copy 'China style' this design. As mentioned, the need to repair one drove this exercise. Having revealed the design, the basic simplicity intrigued me, and the potential to upgrade / improve the design led me to post this thread and to get some insights into it's design.

Thank you,

Andy
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Old 5th January 2014, 10:46 AM   #7
richie00boy is offline richie00boy  United Kingdom
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Thanks for posting this. I designed a basic diff probe for basic/slow jobs in the lab so that I wouldn't endanger the expensive ones. It's good to know what's inside and learn that they are not that complex.
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Old 5th January 2014, 04:37 PM   #8
underwurlde is offline underwurlde  United Kingdom
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Remember: don't copy this design... I am not sure what they used for the op-amps. Everything else though IS known (the JFETs, the LM3046, the transistors and value of capacitors / resistors)... unless I've dropped a clanger of course

Andy

Last edited by underwurlde; 5th January 2014 at 04:39 PM.
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Old 5th January 2014, 05:49 PM   #9
1audio is offline 1audio  United States
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Differential Probe - Reverese Engineered
The FET bipolar part makes sense, high current gain and low capacitance at the input is critical to getting a high impedance input. good matching of the two buffers helps and the CMRR trim is important. Most of the CMRR comes from the opamp.

The resistors have 2200 pF in parallel? The ratio is roughly 200:1 I think and that would suggest an input cap of around 1 uF. Perhaps those are 2.2 pF HV caps? That would make more sense.

Opamp selection for this would entail understanding the frequency range its supposed to operate over. A 741 would not have enough bandwidth to be very useful. There are a number of new opamps that could give as much as 100 MHz from it, however the rest of the circuit would not be up to the task.

There is a lot to be learned from studying the Tek plugins and probes. Many of the manuals can be found on the web for free.
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Old 5th January 2014, 06:37 PM   #10
JensH is offline JensH  Denmark
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Differential Probe - Reverese Engineered
If the through hole version is using the AD797, perhaps the SMD version is using that one as well? As 1audio points out the 741 would not be useful in a 25 MHz probe.

Could the 9 have been turned into something looking like 4 when sanded?
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