What am I?  Mystery IC competition! - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: October 13, 2015, 11:48:34 am
content: I'm reverse engineering / learning about high voltage differential probes, so i got a broken one to (don't turn it on,) take (it) apart!It's a Pico Ta041 1000Vdc, 25Mhz probe:Inside, as you might expect, it uses a classic capacitor compensated resistor divider network, forming a virtual ground and two differential voltages for measurement.How ever, the makers have helpfully () scraped off most of the IC tops so it's a bit of a mystery as to what does what.In that pic, the two inputs cascade in from the LHS of the pic (capacitive divider shown, the resistor strings were mounted on small daughter boards above each capacitor string), VR1 trimmer is the DC offset adjustment and VC1 & VC2 are variable capacitors to trim the AC divider common mode.D1 & D2 are diodes coupled to the -15v power rail (to clamp the min voltage) but then the input paths converge on a SOIC-8 part (U1, removed in that pic)Anyone like to hazard a guess as to what U1 is? I don't think it's a front end amplifier, as it only seems to be fed by the +15v rail and it's pinout isn't immediately familiar to me. Could it be a clamping diode array or similar TVS array? I could be totally wrong, as trimmer VR2 is tracked onto this mystery IC. If it were a low input bias current / high impedance buffer amplifer (JFET etc) then i would have expected it to be fed from the split supply, rather than just the +ve one?U2 is i guess a quad amplifier of some flavour, set up as an instrumentation amp? VR3 and SW1 set the gain (it's selectable for 10:1 and 100:1) i think.VR4 is the low battery warning trimmer (iirc, the power LED on these flashes when the battery voltage fallsBTW, check out all the bodges on this board! Lots of Resistors doubled up (either on top of each other or lent tombstoned together etc. Also, SOT-23 parts cross soldered over two pads etc!

author: Andy Watson
author_rank: Super Contributor
timestamp: October 13, 2015, 12:48:32 pm
content: Quote from: max_torque on October 13, 2015, 11:48:34 amHave you tried measuring the voltage on those pads? Given that there is both +15V and -15V on the board I would be surprised if it's not used for powering an input buffer amplifier - probably a dual-package - forming a complete differential amplifier with that other anonymous 11-pin package.

author: cat87
author_rank: Regular Contributor
timestamp: October 13, 2015, 01:50:41 pm
content: Most likely it's a matched JFET pair, something like the LSK389 or the U440 from Vishay. That makes up the differential input stage and it's only going to be tied to +15V. Ans I'd bet there is a current sink somewhere, in the tail of the differential stage, and that will be tied to -15V.I'm trying to design my own differential probe (1KV input, @20MHz bandwidth) and I am using a similar approach to thisAlso, there should be some BJTs tied to the JFETs....somewhere

author: LukeW
author_rank: Frequent Contributor
timestamp: October 13, 2015, 02:00:33 pm
content: Lots of brands of these seem to look identical... same OEM, nearly identical units, just rebadged.Look at some of these pics.http://www.edaboard.com/thread126987.html#post553732http://bardagjy.com/?p=1664https://www.eevblog.com/forum/testgear/probe-master-differential-probes-anyone-used-these/msg351503/#msg351503SST441 sounds like the one.

author: cat87
author_rank: Regular Contributor
timestamp: October 13, 2015, 03:08:22 pm
content: And another link that might help:http://www.diyaudio.com/forums/equipment-tools/248505-differential-probe-reverese-engineered.htmlThe schematics are kind of dodgy, but it might help you get a general idea of what's going on inside one of these puppies

author: sorin
author_rank: Frequent Contributor
timestamp: May 01, 2017, 07:00:22 pm
content: HelloDo you have made any progress with reverse engineering this?Can you post the schematic here?

author: emocabras
author_rank: Contributor
timestamp: May 24, 2017, 11:02:01 pm
content: Sorry I have to replace sst441 my probe. I can use two low noise j-fef n-channel. sst441nl have a gate diode. What be diodes i can use with 2 j-fet replace sst441? thank you.

author: emocabras
author_rank: Contributor
timestamp: May 28, 2017, 04:45:55 pm
content: Can you help?How to replace sst441 with two j-fet

author: Kleinstein
author_rank: Super Contributor
timestamp: May 28, 2017, 05:44:32 pm
content: The two diodes in the SST441 circuit are diodes to the substrate. The main effect is to give some extra capacitance - so with two separate JFETs a small extra capacitance might be needed -though parasitic capacitance could be enough.The main difficulty might be getting two FETs with close enough matched parameters.

author: emocabras
author_rank: Contributor
timestamp: May 28, 2017, 08:46:12 pm
content: thanks you very much Kleinstein, What j-fet can i use? What external diodes to replicate sst441?