Chip is incredibly hot and not operating - no PSU short circuit, seeking advice

Hello TueJBEC,

May I ask how were you able to measure the temperature of the chip? Where are the +IN and -IN inputs connected to? What is your load? 

In addition, how is the amplifier configured? It would be difficult for us to know the root cause of the problem without sharing the schematic. Though, would it be possible to share not the whole schematic but only the part/stage where it shows how the amplifier is configured?

Thanks!

Regards,
Paul 

Hi Paul,

Firstly, thanks for the response. 

I have measured the chip temperature firstly with a calibrated finger, and then with a FLIR camera. Whilst I appreciate the FLIR is not the most accurate measure, I trust the result as the heat coming from the chip was immediately obvious. The chip itself is configured using common circuits including a unity gain buffer stage, and loading circuits as detailed within the data sheet.

I am, as mentioned, unable to share schematics publicly. I am happy to take this discussion private with a member of Analog's staff, as they have explicitly stated is possible, and I await this opportunity.

Regards,

Berin

Hello  TueJBEC ,

Can you share and send your schematic through my email? You can find my analog email by navigating through my EngineerZone profile.

In addition, can you please answer these questions?
1. How many AD826 units does the issue occur? Is it repeatable?
2. What is your load? Capacitor? Resistor?
3. What is connected to the +IN and -IN inputs of AD826? When powering up the unit, is there a signal at the input of the part?

Thanks!

Regards,
Paul

PaulCA , thanks for you response.

I have tested two units so far, and the problem has been immediate on both units. After observing the first unit, I removed the chip, cleaned the PCB, and tested for any problems on the board - nothing shows up as shorted or otherwise different than expected from the schematic. The replacement chip was then placed and measured with still no signs of shorts or production problems. Upon power-up the device immediately heats up as before.

In terms of the circuit, the initial stage is a unity-gain buffer; +IN is connected to an empty SMA connector. The load is into the second circuit of the chip (+IN again). Obviously more will become apparent when I am able to send you schematics.

In short, to my mind I am not doing anything particularly exotic with the amplifier but it seems to just fail immediately.

Thanks in advance,

Berin

Hello Berin,

TueJBEC said:

In short, to my mind I am not doing anything particularly exotic with the amplifier but it seems to just fail immediately.

Well, if I understand you correctly, you may operate the amplifier chain with a high impedant open input.

TueJBEC said:

he initial stage is a unity-gain buffer; +IN is connected to an empty SMA connector.

Am I interpreting your description correctly? Or is there some termination resistor to GND besides the empty SMA connector at the input?

If the input is really open, it can bring your amplifier chain in an undefined operating point beyond the limits of specified operating range. It can pick up and amplify all kinds of interference. And some parasitic feedback from the output to the open input can cause oscillations, which may significantly heat up the chip.

I would suggest setting the input of the first stage to a defined (and sensible) potential and checking whether this alone solves the problem of heating.  

best regards

Achim

Hello TueJBEC,

After reading your reply, the same thought occurred to me. Is the +IN pin really floating? In addition to what Achim said, if an input is left unconnected or floating, there is a risk that stray electrostatic fields will cause that input to go outside the supply rails. This could potentially lead to latch-up conditions and possibly destroy the whole chip.

If you are not planning to input any signal at the +IN of the first stage, I suggest connecting it to GND. If you will input a signal eventually, I suggest connecting +IN on a voltage/signal source equipment whose channel is turned OFF initially before powering ON the AD826. When, the channel of the voltage/signal source equipment is OFF, the +IN pin is essentially at GND potential. Once the AD826 is ON, you can now turn ON also the channel of the voltage/signal source equipment and apply any voltage/signal level that you want on +IN.

Note: To avoid power sequencing issues, the voltage at the supply pins should be established with or before any input signal is applied on the inputs of the op amp.

Hi Paul,

I have tried a few things since my last message, primarily being to apply a resistor between input and ground thus removing the floating input. Unfortunately this has not been the silver bullet I had hoped for.

I am hopeful however that, following further disclosures from my customer, I am on the way to having the problem resolved. For certain I shall be keeping the load resistance on the input, so I would like to thank you (and Achim) for your input.

Regards,

Berin