OPA4277: Opamp Heats up

Morning Shekhar, 

You might try removing those direct cap loads on the 2 stages that have them and see if the power goes down - if so, you are likely oscillating. 

Hi Shekhar,

Michael's suggestion is a good one. Check the OPA4277 outputs, especially U100A and U100C, with a 10x scope probe and a wideband DSO for oscillation. If high frequency oscillation is present, then compensation could be added to the stages to eliminate it.

Regards, Thomas 

Hello sir ,

I had remove the capacitor and test the ckt but opamp still heating . one more thing i would like to mention if we give only supply no other component connected that time also opamp gets heat .

Hello Sir,

Thank you for your suggestion.My output and input is DC signal. as per your suggestion i had check the signal on DSO but there is no oscillation in that .one more thing i would like to mention if we give only supply no other component connected that time also opamp gets heated .

Hi Shekhar,

The OPA4277 normally runs with a low operating current of about 800 uA per amplifier, or 3.2 mA total. Using +/-12 V supplies the quiescent power is on the order of 77 mW, which doesn't result in much heating; about 5 degrees above ambient. Therefore, for a correctly operating OPA4277 it shouldn't be getting hot and something unexpected has to be occurring with the circuit.

Which resistors are populated in the U100-B stage? It looks like R114 connects the non-inverting input directly to the output. Also, what load is connected to the connector at the output of U100-C?

Did you obtain the OPA4277 directly from TI, or one of TI's authorized distributors? If the op amp was obtained from another source it may be suspect.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Shekhar,

your thread title mentions the "OPA4277", but your schematic shows the "OPA4227". So, what OPAmp are we talking about?

Also, can you please show an (easy to read !) schematic which shows the actual component values? Concretely spoken, R114, is it 0R or is it removed?

Kai 

Hello Shekar,

Kai caught something I missed. If the op amp is an OPA4227, and not an OPA4277, then the quiescent current moves up from 800 uA per op amp section to 3.7 mA per op amp section. The total quiescent current is then approximately 14.8 mA. With +/-12 V supplies the quiescent power would be 0.355 Watts. The package would have to dissipate that power through conduction and radiation, and I expect would feel noticeably warm compared to room temperature.

Please do verify which op amp type is being used.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hello,

 I am sorry its my mistake . I am using OPA4227UA For my application .R114 is not assemble on my board its optional( it bypass the U100-B -R112 & R113 disconnected ) . OPA4227 is assemble separately on different board and give supply only no input , no resistance and still its heat . i bought this IC from TI and TI's distributors both so i dont think it will make any problem Because i had used both ic's .   

Hi kai,

 I am sorry its my mistake . I am using OPA4227UA For my application . Please Find the CKT below. R103 is Not connected

Hi Shekhar,

Since you are indeed using the OPA4227 there is the issue of the much higher power dissipation that I previously mentioned. Certainly, that device will get hotter than the OPA4277.

Otherwise, I don't really see anything about your circuit that would explain why the op amp would be getting very hot. Is it possible that you have a PC board error and something is not connected as you expect?

Aside from the heating one thing that I did notice is you are using the OPA4227 in the U100-D gas sensor potentiostat circuit. Gas sensors of this type usually produce very small output current, and the input bias current of the op amp needs to be much smaller than it. The OPA4227 has a typical input current of ±2.5 nA, and ±10 nA maximum. Usually, the gas sensor op amp is one specified as an ultra low input current op amp having tens, or hundreds of femptoamperes. This is something else you need to check.

Gas sensor circuits are not fast and you may find that the applications can be better accommodated by a low quiescent current, ultra-low input current op amp. You may find with a low current op amp that the heating issue goes away.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Thomas,

 Thank you for the solution .Can you suggest me which Opamp i should use for gas sensor ckt.

Thank you in advance.

Hi Shekar,

can you measure the temperature of your "hot" OPAmp, please? Take a tiny thermocouple for this and apply a drop of heat-conductiong paste on top of the OPAmp package to ease the heat flow to the thermocouple.

You might also want to have a look at these product sites:

See also the "technical documents" and the "design & evelopment tools" there.

This is another very interesting site discussing a gas sensing reference design:

Kai