General > General Technical Chat
weird ways to damage a ad620 chip
DiTBho:
I don't know if this is the proper section of the forum, but the context is very unusual and informal, like a chat.
Yesterday my colleague was testing four ad620 units bought on eBay because found for cheap. Farnel lists a single chip for about 10 euro (plus postage), on eBay four units were purchased for 25 euro including postage.
The AD620 is a high accuracy instrumentation amplifier that requires only one external resistor to set gains of 1 to 10000.
Datasheet of AD620 says that this resistance (called Rg) must be calculated like this:
Rg = 49.4 kOhm/(G-1)
G = gain
As far as I see from his breadboard, he used 47 Ohm resistor, so gain is about 1000. Reference voltage is 0V, there is a single power supply 5V, and shorted V- V+ to the ground through a couple of 10K resistors.
Results? He said two chip units are damaged by these experiment, but I don't see why :-//
I have here a working circuit that uses a ad620 unit to measure the output of "KMZ10B magnetoresistive" sensor, which has a Wheatstone bridge. In this case the reference voltage is about 2.5V.
I removed my ad620 (which works) and tested the two chips used by my colleague: and booom! It's true, those two tested chips really don't work! Their Vout doesn't move from 3.8V, so probably there is a damaged input section or something.
But remember? He has four chips, and it's more interesting that the other two untested chips do work!
Seriously I don't know if and how his testing could damage a chip.
DiTBho:
I have already tried to simulate his testing scenario like the schematic attached in the previous post, but TINA (it's like PSPICE) doesn't report anything weird.
NiHaoMike:
My guess is something not reflected in the schematic such as ESD.
Ian.M:
The AD630 is *NOT* a single supply instrumentation amp. He's violating the permissible Vref input range, which must not approach either rail closer than 1.6V from it.
However there's a high risk that expensive chips from Ebay are duds to start with - ESD damaged, failed production test, remarked 'pulls' or even outright fakes - so his test jig, although incorrect, may not be responsible for the damage.
DiTBho:
In my circuits, I never used Vref shorted to the ground. The data sheet doesn't warn you avoid doing it, but it tells to avoid high capacitance to the output.
Probably ESD, I don't know if he worn an anti static bracelet, for sure the ad620 has high impedance inputs, not sure if it also has strong protective diodes.
X-FILE :o
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