Offset in voltage follower - 2nd

[Denis1024]

Hello,
I'm a bit confused.
The guaranteed offset voltage of the LT1013 is 150µV, but in my application, it is close to 4 mV.
So I made a basic configuration (see attached drawing)
R1=0 ohms, R2 absent. A good 5 1/2 multimeter is connected to TP1 and TP2 : V ~ 4 mV approximately (I expected ~100µV)
OK, now I measure the offset with a gain of 1000
R1=100k R2=1k : V=80mV.
80mV/1000 = 80µV, correct according to the specifications.
Same thing with different LT1013, same thing with other operational amplifiers. Same thing with different multimeters.
If the multimeter is shorted, V=0mV.
Is there something I never understood? like "in a voltage follower, the offset voltage is not guaranteed"?

I found that topic, but this case is not mine
https://www.eevblog.com/forum/beginners/unexpected-offset-in-voltage-follower-circuit/msg4081153/#msg4081153

Thanks
Denis

[StillTrying]

I can't see anything obvious on the data sheet.
Have you got the floating inputs of U1b connected to something, both inputs connected to your center 0V would probably do.

[Denis1024]

The second part is connected as follower, input at GND

[Manul]

Speaking about voltage follower configuration, try measuring voltage directly between inverting and non-inverting input (preferably directly on IC pins), this will exclude possible measurement errors like ground voltage drop for example. By the way, have you checked output with a scope? Is it stable? It might be oscillating at unity gain.

[Andreas]

Is there something I never understood?

Do you use switchmode supplies for the +/- 5V?
What happens if you use battery supplies?

With best regards

Andreas

[oPossum]

Both inputs should have the same source impedance to minimize the offset voltage.

[tggzzz]

Are you using a solderless breadboard?

Are there any decoupling capacitors?

Is the PSU lead inductance causing it to oscillate?

[David Hess]

Same thing with different LT1013, same thing with other operational amplifiers. Same thing with different multimeters.
If the multimeter is shorted, V=0mV.
Is there something I never understood? like "in a voltage follower, the offset voltage is not guaranteed"?

The voltage offset specification applies just as much to the voltage follower configuration.

You might be having a problem with oscillation.  Are the supply rails properly decoupled?

Try measuring the output voltage with a small value of series resistance to isolate the multimeter capacitance from the operational amplifier output.  The voltage follower is especially sensitive to output loading.

[Denis1024]

@Manul : The measurement is made on the pins. The oscilloscope shows only noise.
@Andreas : The supply was linear lab supply, now with battery : Same results
@oPossum : With follower, one input is connected at the output, without resistor.
@tggzzz : The first circuit was on breadboard, the second is PCB with ground plane. The +/- rail are decoupled with 100n
@David Hess : A resistor makes the same results.

I made a setup (attached) with better wiring.
2 battery coins -3V 0 +3V
The first amplifier is a follower with input at ground
The ouput is connected to the second non-inverting amplifier, gain=100
The multimeter is connected via coax + 50 ohms load
Output 1 : 500µV with noise, but without oscillations (without the 50 ohms load : 4mV)
Output 2 : 40 mV, confirmed with oscilloscope, noise not visible

40mV / 100 = 400 µV = (Voffset1 +/- Voffset2 ) * 100
The datasheet shows a span of Voffset between -200 / +200 µV

So I assume that measuring 100µV without noise requires a lot of skill
If the second amplifier says ~200 µV for each amplifier, that is the thruth

The Voffset is in the specs. The follower follows the theory. I will not the next Nobel !
Thanks.
Denis

[David Hess]

So I assume that measuring 100µV without noise requires a lot of skill

I have never had a problem doing it and getting the same results with a follower and non-inverting amplifier unless the operational amplifier was oscillating.  I just used common test leads instead of a shielded cable.

[KT88]

@Denis1024: did you try another part? It could be an ESD damage...

Just an fyi: Having the same resistance (impedance at both inputs won't help - this kind of bipolar opamp usually has input bias current compensation with possibly opposite input bias currents...

[Denis1024]

I probably found the bug :
LT1013 has V offset = 60µV typ.
But
LT1013D from linear has 200-800µV, (a little more from Ti )

I use LT1013D.
Thanks to all.