Using a TL081CN in an open loop config and tried to find on the PDFs the minimum voltage differential that it can detect when comparing the inverting and noninverting inputs. Can it go as low as uV or pV?

 

The minimum is quite small. There is a offset voltage that might mess you up. Also a small voltage takes more time to resolve.

 

The lowest my bench PSUs go are 10mV and detecting that is no problem. However, I did notice some drift which I assume is due to temperature i.e. warming from the ±15V on the supply pins. I didn't think about nulling it but with that low of an input voltage it should be. Or use a precision op amp instead of a general purpose one that has had its rough edges smoothed a bit.
OK, looking at the input offset voltages pretty much answers my question with it being several mV. Thanks Ron!

 

If want very low offset you can use a chopper-stabilized type op amp, which can have µV of offset (often limited by thermo-voltaic effects of the wires and connections).

 

Received a surprise from the TL081CP. Hooked on up as shown on the PDF for null balancing the output using a 10K pot and didn't seem like anything was happening. So went to 100k pot and the output would swing off of zero but quickly return to zero. Even though it has the external null offset pins, it also has internal zeroing! Still scratching my head a bit over that one and even in the PDF it shows external balancing. On my Fluke bench meter, it swings a bit but mostly in the -10 to -50uV range. For the C suffix the PDF gives 3 mV typical and for the H suffix 1mV typical but it's under 100uV. That is with the input pins 2 & 3 grounded, balance pins 1 & 5 NC, meter on output pin 6, and ±15Vcc.


Then with NC for pins 1 & 5!

 

Received a surprise from the TL081CP. Hooked on up as shown on the PDF for null balancing the output using a 10K pot and didn't seem like anything was happening. So went to 100k pot and the output would swing off of zero but quickly return to zero. Even though it has the external null offset pins, it also has internal zeroing! Still scratching my head a bit over that one and even in the PDF it shows external balancing.

Are you sure your scope was not on AC coupling?

Bob

 

-10 to -50uV range

The Engineers that design the front end of OP-amps are smart and have experience. They worked hard to get +/-3mV and most likely you will never see more than 2mV. (this depends on how you use the amp and what resistors you used and if the impedance on both inputs are the same) They aimed for "0V". It is hard to hit zero because 100uV is not zero and 10uV and 1uV and 0.1uV........
If you use the part correctly there is no reason to think you will ever see 3mV. Also you will see a different number from the next batch of parts.

 

Are you sure your scope was not on AC coupling?

Not using my scope as it's not as accurate as my Fluke 8050a bench meter.