OP-AMP Saturation Issue

[umesh49]

Hi,
I was using a very simple circuit (amplifier with offset) in one of our board and got an strange issue with one of our product. My op-amp (PN: LMV321IRY) was getting saturated at 0.2V and 3V.
Actually it was suppose to go till 50mV to 3.25V with 3.3V power supply.
It a simple circuit with + was connection to 1.65V and - was connected to a DC input with 3k input and feedback capacitor.

I did multiple measurement and op-amp was not going above 3V (or below 0.2V). This was happening with one board and other boards were performing good.
While I was doing some probing on this faulty board, issue goes away and now this board is also performing well with my saturation limits as 50mV to 3.25V.

Does any one is having any idea about this type of failure in op-amp?

Any input will be appreciated.

Regards,
Umesh

 

[Audioguru]

Its saturation voltage depends on its load current. What was its load resistance and what voltage was its load connected to?
If it has a negative feedback resistor then it is also its load.

 

[D.A.(Tony)Stewart]

Read specs carefully.
Unless otherwise specified, all limits guaranteed for TJ = 25°C, V+ = 2.7V, V− = 0V, VCM = 1.0V, VO = V+/2 and RL > 1 MΩ.
VOL = {60mV~180mV} typ.~max, @V+=2.7, RL = 10 kΩ to 1.35V
VOH={-10~-100}+V+ typ.~max

Thus load spec current=
IOL=1.35V/10k=135µA
IOH=(2.7-1.35)/10k=135µA

Thus effective series resistance(ESR) , from above using VO/IO=ESR output drop is ...

ESR(low)=60mV(typ.)/135µA
=444Ω typ 1333Ω max.

ESR(high)=74Ω typ. to 740Ω max.

Although output impedance drops with negative feedback. Once it saturates, you lose the gain.

 

[umesh49]

Thanks for replying Sunny;
Actually we did enough validation to prove the op-amp operating limits. Base on this we used to write a new spec. Only thing surprising me is, why one board out of thousand boards shows this issue and it disappeared and now my board works good.
I am just trying to understand it it could happen due to some dry solder on board, component ***** or is it op-amp itself which can show such a behavior.

 

[D.A.(Tony)Stewart]

Thanks for replying Sunny;
Actually we did enough validation to prove the op-amp operating limits. Base on this we used to write a new spec. Only thing surprising me is, why one board out of thousand boards shows this issue and it disappeared and now my board works good.
I am just trying to understand it it could happen due to some dry solder on board, component ***** or is it op-amp itself which can show such a behavior.

,

here your ESR results, may be gone 1k average spec to ? 3k ? Back to 1k? What is your load?

That is not solder effect...., maybe flux spurious oscillation, maybe ESD wounded? Do you measure quiescent current? The internal gate voltage and drain-source channel controls ESR.

 

[umesh49]

My output load was a low pass filter with 1k resistor and 1nF capacitor. It goes then to a ADC input which may draw 1uA.
I did not measure the quiescent current. Issue is not there so I am first trying to replicate it again.

 

[Audioguru]

Was there a strong radio transmitter nearby when the opamp saturation was poor?
A cell phone transmits a strong RF burst every couple of seconds so that its signal is picked up by the nearest tower. It causes beeps in many nearby electronic circuits.
Maybe there was a police car or fire truck transmitting nearby?
Maybe the kid next door made a radio transmitter and was testing it?
A lightning storm?

 

[umesh49]

Was there a strong radio transmitter nearby when the opamp saturation was poor?
A cell phone transmits a strong RF burst every couple of seconds so that its signal is picked up by the nearest tower. It causes beeps in many nearby electronic circuits.
Maybe there was a police car or fire truck transmitting nearby?
Maybe the kid next door made a radio transmitter and was testing it?
A lightning storm?

We tested in different environment and different facilities and part was behaving same with always. I suspect there is something on board which got corrected itself while I was probing the board.