I am using the AD7656-1 for monitoring power supply signals. The input signal level is +/-10V. All 6 inputs are buffered by AD8221BR(Diff opamp) and then connected to the AD7656-1.
AD7656 power supplies: VDD=15V, VSS=-15V, AVcc=DVcc=5V, V-Drive=3.3V, Vref(ext)=2.5V.
AD8221BR VDD and VSS are tied to AD7656.
Designed was done about 2 years ago. So far, built about 200 units. I have 3 or 4 units that worked more than a year without failure. About 10 more were just turned on recently, the rest of them are in storage and never turned-on. Seven of them were “popped” during turn-on within two weeks time frame; they worked for a few days and then popped during power turn-on. The all have the same failure symptom, made a soft pop sound and a puff of smoke came out from the chip near pins VDD, VSS and pin-32(AGND). I removed all the voltages and then connected one voltage back at a time. On the bad chip, I found the DVCC(5V) is tied to all the analog input pins and measured about +2V. There is no DVCC on analog input pins on good chips.
Anyone encountered the chip popping problem or have solution?