ADC122S655 (2 channel simultaneous A/D converter) gets hot IMMEDIATELY! - Page 1
Forum section: electronics / projects

author: Skimask
author_rank: Super Contributor
timestamp: May 27, 2014, 04:03:50 am
content: I've been over and over the datasheet, can't find anything of any use. Maybe somebody else has dealt with this chip before...(schematic will be coming after I get it cleaned up a bit and readable by the masses, 'cause I hate few things worse than a crap schematic)I built a circuit to sample two inputs simultaneously using an ADC122S655 (http://www.ti.com/lit/ds/symlink/adc122s655.pdf) in a single ended configuration. Both + inputs are going to a 1K/10K voltage divider coming out of a pair TL084's (if one or the other was ground, maybe 3ma of draw there), V5 to 5v rail, Vref to the 3.3v rail, - inputs to a 1.65v 'rail' comprised of a simple resistive voltage divider, chip select/clock/data out open circuited, ground to ground, etc.Immediately after applying the V3.3 reference voltage, the chip gets hot...IMMEDIATELY! I can watch it go "white" on my FLIR E4 when I plug in the USB (which feeds power from Vbus thru a 3.3v regulator). The chip case itself gets up to about ~170F within a couple of seconds, and the upstream 3.3v regulator gets hot really fast as well.I've plugged in two chips, checked for shorts, etc. re-checked, re-soldered, etc, cut everything else out of the loop, cut traces to isolate the chip, etc.Question...I'm NOT applying 5v on the Va (pin 7) at the moment, which according to the datasheet, is the analog power supply.The datasheet specifies that Vref should go from 1v to Va.What happens if Vref (eg. 3.3v in my case) is greater than Va, 0v, effectively grounded thru a 25R resistor (using an ATX PSU for power so had to have a bit of a load resistor there)?Is that 3.3v reference voltage running thru the chip, out the Va (which is supposed to be an analog input power supply rail), thru the 25R to ground or what?Or maybe I messed up the single ended configuration...I dunno.Likely I've blown up 2 fairly spendy chips ($6.55USD each). Obviously would like to avoid toasting any more...When I pull the chip out, nothing gets hot, voltages stay where they should, etc...(again, schematic is forthcoming...)

author: TVman
author_rank: Frequent Contributor
timestamp: May 27, 2014, 04:11:03 am
content: Maybe you have A short on your chip or on your PCB.

author: Bud
author_rank: Super Contributor
timestamp: May 27, 2014, 04:20:00 am
content: Quote from: Skimask on May 27, 2014, 04:03:50 amThe datasheet says (page 2) Va "must be applied", not sure why you do what you do.

author: Skimask
author_rank: Super Contributor
timestamp: May 27, 2014, 04:58:19 am
content: While it's been said many times, in many places, and came back to bit many people...I'm going to say it here as well...I don't believe there are any shorts any where.Not saying I'm infallible, but I've been professionally trained in the art. While my soldering abilities may be greater than the masses, I'm sure I'm no better than the bulk of my learned colleagues that frequent these forums. I would, however, venture to guess that my inspection abilities, eye for detail, etc., are likely second only to a select handful in this particular field.So...Soldered/De-Soldered the chip a number of times, tried 3 different chips (the first two of which I now consider garbage/toast until proven otherwise), board doesn't draw extra current without the chip installed, voltage rails are steady and nothing heats up at all, have a compound inspection magnifier eye loupe, meter doesn't show any dead shorts (set on ohms not on diode-check), meter shows minimum of about ~40K between any two pins that aren't supposed to be otherwise connected to something else, without the chip installed, ohm readings rise towards infinity (and beyond)......which again, in the absence of an in-depth datasheet, something tells me there's a diode feeding from Vref to Va inside the ADC122S655, and when I apply everything EXCEPT the Va pin (which runs thru a 25R resistor to ground), the chip gets hot straight away.and if it turns out I've got a dead short that I missed, well, I'm going back to underwater basket weaving

author: Skimask
author_rank: Super Contributor
timestamp: May 27, 2014, 05:03:01 am
content: Quote from: Bud on May 27, 2014, 04:20:00 amI did miss that "must be applied".Nice call...Now I'm wondering if that really is a "must be applied" or "this is where the 5v goes".As for the why... Right now I'm powering the digital parts of the PCB off USB for testing/prototyping/etc...just trying to get the programming working. When I put the thing into a box of some sort, it'll be powered off an ATX connector hooked up to an ATX PSU.Guess I could run a jumper from Vbus over to the 5v rail...temporarily anyways... The datasheet does say 4.5v-5.5v. As long as I'm not pulling too much off the USB for the 3.3v side of things, I should be in there...

author: Skimask
author_rank: Super Contributor
timestamp: May 27, 2014, 05:28:44 am
content: I think that did it! Putting 5v on the Va (pin #7).The PCB is still a bit warm from re-soldering a new chip on there, but I let it go for a couple minutes and the chip only got up to 85F (77F here in the shop now)...in contrast to the 'white-hot' it was before.One little word in the datasheet..."must"...is the key...I think...so far anyways.Getting a bit late here. Will do more testing later this week.If this takes care of the problem, you just saved me a total redesign of my ADC section.