INA163 getting hot

[AudioWizard]

Hi all,

I'm using an INA163 chip in one of my prototype preamp design, and I noticed it gets pretty warm. Power draw is between 10 and 15 mA... (which seems normal). I power it with +/- 12 V.

Is this supposed to get that warm while in use? And if so, should I get it some kind of heat sink (I wouldn't know how, it's rather small). I haven't measured the temperature, but while we can still touch it, it gets noticeably warm (so I would say probably above 50°C).

Just wanted to make sure the heat was normal... Anyone has used this chip and can confirm? Oh, and since I'm going to put this small preamp inside a magnetic pickup, there won't be much room for dissipating the heat. Is it going to be a problem?

Thanks!

[Brion55]

Make sure it isn't oscillating RF.

[AudioWizard]

Thanks for the pointers; actually, that's what I was suspecting.

My oscilloscope isn't quite good enough for me to see the RF oscillation, but it was probably there. I did something that probably shouldn't be done: when first testing my preamp, I left the inputs floating: probably what caused the oscillation.

Hopefully this hasn't killed the INA163.

[AudioWizard]

I think I'm beginning to understand...

Turns out my power supply has a problem. I made it with a 7812/7912 pair; it was actually made just for testing purposes. Anyway, the problem is that there is some high-frequency noise on the negative output (-12 V) with a significant amplitude. No such thing on the positive output (+12 V). I have no idea why. Is the 7912 more prone to oscillation than the 7812?

Is this oscillation in the negative rail the possible cause of the INA163 heating - and malfunctioning? I did put decoupling capacitors on its power rails (100 nF). Doesn't seem to be enough here.

Anyway, I'm going to improve my power supply unit; but I was wondering if you guys can explain a little bit about the problem I'm running into (power supply oscillation leading to oscillation in the differential amplifier....) and ways to cure it (obviously, I'm guessing better PS decoupling)?

I admit the power supply was made in a quick and dirty way for testing purposes, but I've never seen any problem like this. I guess the INA163 is a bit tricky to use and sensitive to power supply quality?

Thanks a lot for any pointers/comments!

[AudioWizard]

Still replying to myself (but that might interest someone).

So, I improved my power supply (adding a couple decoupling capacitors did the trick). But that was not all.

I think I got confused with the whole "floating inputs" issue. I had not put two resistors between each input and ground. I had directly connected my magnetic pickup to the inputs of the INA163... it didn't like it much. With two 2k2 resistors to the ground, everything is fine... (for the record, my magnetic pickup has a 50 ohms DC resistance...)

So anyway, the "floating" inputs did make the INA163 overheat. (I thought they were not quite floating, since a coil was permanently connected between them, but I was clearly wrong.) With resistors providing a path to ground, the INA163 doesn't get as hot as it did without. It still gets warm, but I guess that's normal (about 10 mA quiescent current, that's gotta make some heat ;-) ).