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 falls
BTW, 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!