Differential Probe - Reverese Engineered


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:

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:

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! :)

pdf's of above images...

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

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

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

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).


Paid Member
2004-03-24 5:16 am
SF Bay Area
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.


Paid Member
2004-03-24 5:16 am
SF Bay Area
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.


Paid Member
2009-07-30 9:54 pm
You are right about the bandwidth. The AD797 has got a relatively high GBW product, but there isn't much output in the MHz region, and certainly not at 25 MHz. I guess I was mislead by the AD797 shown in the schematic of the through-hole version. Probably just a placeholder.