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Trouble with the PCM1804.. shorting?

Heya,

I've soldered a texas instruments PC1804 A/D converter to a SSOP28 adapter board. At the moment, all adjacent pins are measuring about

400-700 thousand ohms of resistance between eachother.

However, when I hook up the digital and analog voltages to the chip, it appears that the analog voltage pins (Vcc, 5 volts) are getting very hot and possibly reflowing the solder and shorting out? The leftover flux will start to smoke. I'll disconnect everything and measure the resistance between the Vcc and Analog Ground pins, and there's anywhere from almost no resistance to 10 thousand ohms.

I'm hooking it up like this:

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Could it be that the pins are just shorted? or am I hooking something up wrong and it's shorting internally? All I've had hooked up to the PCM1804 was just the Digital and Analog voltages and grounds, nothing else. Any ideas?

Regards, Matt

Reply to
mattcarpenter
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The datasheet is pretty clear that pin 14 is Vdd and pin 13 is Digital Ground, so I don't think that's the case... Anyways, the magic smoke is comming either from pin 22 or 23. I'm pretty sure I havn't swapped those either.

Reply to
mattcarpenter

Ah, sorry about that, the schematic I drew up is incorrect, however it's hooked up correctly on my prototyping board. Updated Schematic. (Or at least trying to.. my computer doesn't want to cooperate with me)

Reply to
mattcarpenter

I am finding it somewhat strange, however, that I'll re-work some solder joints, then check the resistance between two adjacent pins, and it'll be somewhere around 400k ohms to 700k ohms, but I'll hook the power up, apparently it'll start shorting somewhere (the chip doesn't get hot but the pins do, as do the voltage regulators, and I can hear the leftover flux sizzling, letting out smoke, etc..) then I'll unplug everything and check the resistance between the pins again and there will be none.

So could a 500 ohm connection between +5v and ground be able to generate enough heat to reflow the solder and create a bridge? If there are any bridges, they're probably under the chip. It's a SSOP package which is pretty small and almost impossible to see between the fluxed-up legs. I'll try to resolder the part again I suppose.

Reply to
mattcarpenter

You have to be smarter than whatcher f*ckin' with ;)

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Best Regards,
Mike
Reply to
Active8

No. That's only 50 mW.

Pull the chip, fire up the regulators and check the voltages. It's easy to get a regulator hooked up wrong when 2 regs have different pinouts. You can thread a piece of wire behind the pins and tie one end off to another part. Then pull the wire under the pins as you heat them. It'll lift the pins off the PCB.

Maybe it would be easier for you to solder the thing by first soldering the PCB pads. You don't need much solder and you can wick the excess off the pads with Solder Wick. Then flux the chip pins and hold it down on the footprint. Just heat the ends of the legs and the solder will reflow. I'd remove the flux afterwards.

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Best Regards,
Mike
Reply to
Active8

I think you have swapped pins 13 and 14 and you have probably let the magic smoke out of the 1804

martin

"An eye for an eye makes the whole world blind" Gandhi

Reply to
martin griffith

pin 13 digi gnd in your pic is connected to your regulator out

martin

"An eye for an eye makes the whole world blind" Gandhi

Reply to
martin griffith

well, I assume you have checked with a magnifying glass for shorts, so all thats left is to replace the chip/ remove and resolder it.

Never work on a project at the weekend, when the shops are shut, and you have no spares at hand

martin

"An eye for an eye makes the whole world blind" Gandhi

Reply to
martin griffith

Do the maths, dude. 5^2/500 = 50mW, so clearly not. Look at your pcb with a decent microscope. Or crank the power supply current limit up to, say, 20A and fry the short. Dont do what one of my techs did once though

- he couldnt fry the short because of its impedance, so cranked the voltage up to about 35V. It fried the short, then f***ed the micro. oops.

Cheers Terry

Reply to
Terry Given

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