The other week I got a few samples of some MAX187 ADC chips, as I was in part curious what was so special about them that warranted the high price tag. Looking into ADC chips all started when I wanted to find a solution to the ESP8266's fairly poor internal ADC. Somewhat off-topic notes:  If anyone wants the board files (and code)for the MAX187 PCB I can post them, although it's a bit cost prohibitive if your not getting MAX187 samples.  I've done a more practical design around a MCP3304 ADC chip that I will be posting in the future.

During the process of testing the code that I wrote to use it with an ESP8266 on a Wemos D1 board, I mistakenly connected the VIN pin instead of the GND pin to the ADC input pin.  This by itself wouldn't have been a huge deal by itself, except I was powering the board via a wall wart, which sent around 12V to the poor ADC input pin.  I discovered the issue as I started to smell something and then found the ADC chip to be streaming hot.  After correcting the issue, the ADC chip appeared to become unscathed.

Or so I thought.  Last night, I was writing code to use the MAX187 with a ATmega328P (I did this to do a comparison with the ESP8266 in terms of noise the ADC picks up and found that the ADC picks up a bit more noise from the ESP8266.)  Upon testing the code, I noticed when using a 50K pot, the voltage was staying at full scale during most of the rotation and only dropping off close to 0v when hitting one end. I thought there was possibly an issue with my code and busted out a multimeter to confirm the strange readings.  Strangely enough, the readings were correct.  I then thought perhaps there was something wrong with the pot.  I tested it with the wiper pin disconnected from the MAX187 and it behaved normally.

I then checked on the PCB for the MAX187 to make sure there wasn't any short between the ADC input pin and VCC. There was no short, but I did discover that the socket that I used was very slightly conductive, in the 6-18 or so megaohm range.  The plastic itself was ever so slightly conductive.  Good thing to note, as that can be problematic with dealing with high impedance sources.

I didn't think that the very slightly conductive socket was the problem so I turned to the MAX187 chip itself.  I measured the ADC input pin voltage left floating, at it was around 5V. There was my problem.  A low impedance or dead short must have formed between the ADC input pin and VCC pin. That would explain why the input kept reading full scale until the pot was in the position that the resistance was low enough between the wiper and ground pin that it was able to shunt the supply voltage that was going into the ADC input pin. 

Further investigation with a multimeter showed  sure enough,  there was a low impedance connection between the ADC input pin and VCC pin with a reading of 117 ohms.  Making an educated guess there's and ESD protection diode between the ADC input pin and VCC pin. That diode protected the input when it got 12v but sometime or after that diode failed in the way of forming a low impedance connection.  The diode that protected the MAX187 chip ended up rendering the chip useless in the end.

That was quite a journey. I believe in the years that I've gotten into electronics, this is my first chip that I've effectively killed.