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:
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:
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
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:
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:
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
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.
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).
^ 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.
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).
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.
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.
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
Thank you,
Andy
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.
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
Andy
Last edited:
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.
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.
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?
Could the 9 have been turned into something looking like 4 when sanded?
I'd say something like a LM6171 or THS4011 would be a more appropriate choice. The main features of the AD797 (DC precision, low distortion, low voltage noise) are of little relevance here, and the higher speed/bandwidth of the above parts probably necessary for 25 MHz operation.
Samuel
Samuel
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.
@JensH : yes, just a place-holder, an OpAmp I chose purely because a) it was in my schematic library and b) the pin-out was the same.
Apologies for any misleading confusion there.
Note use of TrimPot for offset null on the OpAmp (on the THP version), hence maybe not either an LM6171 or THS4011.
Andy
Apologies for any misleading confusion there.
Note use of TrimPot for offset null on the OpAmp (on the THP version), hence maybe not either an LM6171 or THS4011.
Andy
Last edited:
What is the package size of C1? I see the front-end R's carrying the big voltage are 1206. But are the front-end C's also?
TIA
TIA
Ok, something went wrong here...
What is the most important in a diff probe , right layout seems very critical to maintain a good CMRR ?
What is the most important in a diff probe , right layout seems very critical to maintain a good CMRR ?
If these are really this simple, then why are they charging hundreds of GBP for them?
What is increasing the asking price so much?
Testing, calibration?
What is increasing the asking price so much?
Testing, calibration?
One would imagine the calibration of such devices would be the major cost impact. Other than that, I like yourself cannot see why they would be any more than say £30ish if produced in any great quantity. Maybe they are low (sub 10K p.a.) volume and that too would have an impact on pricing. Shipping and retail markup will add a chunk of cost too.
A
A