I received my AD8648 op amps from Digikey. For small orders weighing 13 oz or less (including packaging), you can't beat their price of their first class USPS shipping. I get my orders on the east cost within two or three days for only a few dollars. Replacement chips in hand, I removed and replaced the op amp, but the EC circuit still doesn't quite work. Because of the higher gain of the AD8648, there are now occasional oscillatory bursts at the peak circuit op amp output. These only occur when the op amp amplitude is relatively low, which happens at low EC values. The net result is that the peak circuit can not measure EC peaks that are lower than the noise burst, because it instead captures the noise burst. But, I still have other chips to fry.

As a general rule, when debugging a board, until I am very sure that it is operating correctly, I power it with a current limited supply that has an output current and voltage display. This helps get an idea of the normal current draw for the board and protects from accidental short circuits. In this case, on one particular power up to continue looking at the oscillation problem, the power supply started current limiting at about 1.5V / 1.5A. Previously, it had hovered around 5V / 150ma, even with the Raspberry Pi running. Feeling the chips, the VCOM regulator (REG2V5 in the schematic) was hot. I measured the resistance between VCOM and GND and it was about 1 ohm. I lifted the VCOM pin on REG2V5 and VCOM was no longer shorted to 2.5V. So, I replaced REG2V5, powered up again, and the supply STILL current limited and the chip STILL got hot. Very depressing. After desoldering and lifting various pins on the +12V_IN net, I determined that the 12V input to REGA5 was shorted to the 5V output. Chips rarely die unless something actively kills them. I could not think of anything that I had done to cause this poor chip to short itself from input to output. It's only providing a few tens of milliamps of current to run the analog portions of the circuit.

Through the years, I have found that keeping a good debug log is often helpful. Just the act of writing down your thoughts about what you know, what you think might be happening, what you think you changed, etc. can sometimes help you find a solution. In this case, the answer did not actually hit me until I was writing up this project log entry. As I typed the sentence above about checking the 12V net, it occurred to me take another look at the TPS73250 (REGD5) voltage regulator datasheet. Low and behold, I blew it. Literally. I blew up REGA5 by driving a 6V absolute max input with 12V. The valiant TPS73250 ran for weeks of testing, providing a nice regulated 5V output, until finally giving up the ghost and shorting from input to output. That of course put 12V on the input to the VCOM regulator (REG2V5), blowing it up as well. So, I need to figure out a way to limit the input to REGA5 to around 6V and to replace REGA5 and REG2V5 (again). Another debugging rule, always order a few spares. Ultimately, I will probably have to replace REGA5 with an LDO that supports 12V input ... and remember to read maximum input voltage specifications more closely. Once I get the power supply issues fixed, it will be back to solving the oscillation issue on the AD8648 peak detect circuit.

An old Linear Systems professor of mine once said "Life is a series of transients waiting to die out". That was certainly the case for the poor LDO that gave its full measure in support of the Spiroboard debugging effort.