Unable to identify a component in a NI SCC-A10 attenuator

[flongb]

Hello,
 
I've been given a SCC-A10 attenuator for a class project but it appears it is not working propertly. I identified the component that died but I cannot for the life of me find its reference or nature even.
 
It is a 3 pins SOT component with "NDEN" written on top. The bottom left pin is unused while the top pin and the bottom right pin are linked by a mystery component.
It seems to be some kind of diode(s) with a threshold voltage of 0.6V from bottom to top and 0.55V the other way around.
Could it be parallel diodes? Or maybe some kind of weird zener? I don't think it's a transistor but I might be wrong.

I used a component tester on working component of the same kind (CR 2) but it came up as unknown.

Here's the said component


As you see it's labeled as "CR" which seems to indicate "current regulation", which would make sense considering the whole block is an overvoltage protection

Here's the "schematic" given by the datasheet :


And here is the board itself (I can provide a schematic drawn by myself if needed)



I also posted in the NI.com forum


Thank you for your help

[carl_lab]

"CR" = "crystal rectifier" = diode

[flongb]

Well yes but with a DMM it was detected as a diode in both ways... Parallel diodes? I first thought of this but it doesn't seem right


Even if it were a single diode, the block would be equivalent to 2 serial diodes and it wouldn't make much sense as an overvoltage protection

[carl_lab]

Have you removed the SOT from PCB before measurement?
Is the PCB double sided or multilayer?

If you know somebody with an old HAMEG oscilloscope with "component tester" you can record a V-I-curve to identfy component function:



Example: zener diode
 

[floobydust]

SMD marking code "NDEx" I show as Supertex LND150 or LND250 depletion-mode N-ch DMOS FET. These might be used as the protective clamp if this board has very high input impedance/low leakage?

It's more common to use dual protection diodes for the op-amp, like BAV199 diode. Looks like a clamp to +/- power, one dual diode for each op-amp input. They are very tough and rarely fail, as the input blue resistors should limit current if an overvoltage occurs.

I would compare readings with the other channel. Ohms is better to find a shorted semi.

AD620 Datasheet

[carl_lab]

That's standard input clamping circuitry, but I could not find any visible +V, -V or GND connection to the questionable components CR1-8 (that's the reason why I asked for possibility of multilayer pcb).

Maybe CR1-8 is JFET in current limiter circuitry back to back?

[floobydust]

Ahh yes it is a series current-limiter based on depletion mode mosfet/JFET. Having flipped PCB pic helped draw it out.
The AD620 has built-in protection diodes. No clamping to power or GND this way.

So the part is indeed a depletion mode mosfet/JFET, LND150 has IDSS=1-3mA. Depletion mode semi's are normally on, they conduct until biased off at pinch off.

OP should measure low ohms across each part, and between input (+) and op-amp (+) and between input (-) and op-amp (-).

[carl_lab]

If OP pulls out one of the working JFET/MOSFET, he should check G-S path of the component for diode characteristic (JFET) or open (isolated gate, MOSFET).

[flongb]

The G-S path is isolated, so I guess it would be a MOSFET, everything checks out
It makes sense now and I've shown that to people who, while they would not have thought of it, agree that a MOSFET answers my questions.


Thanks a lot for your time, I can't believe I searched for so long when you guys figured it out in a couple of messages