Offset in voltage follower - 2nd - Page 1
Forum section: electronics / beginners

author: Denis1024
author_rank: Newbie
timestamp: June 24, 2022, 10:38:13 am
content: 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 1000R1=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 minehttps://www.eevblog.com/forum/beginners/unexpected-offset-in-voltage-follower-circuit/msg4081153/#msg4081153ThanksDenis

author: StillTrying
author_rank: Super Contributor
timestamp: June 24, 2022, 11:50:00 am
content: 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.

author: Denis1024
author_rank: Newbie
timestamp: June 24, 2022, 12:37:22 pm
content: The second part is connected as follower, input at GND

author: Manul
author_rank: Super Contributor
timestamp: June 24, 2022, 01:14:44 pm
content: 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.

author: Andreas
author_rank: Super Contributor
timestamp: June 24, 2022, 01:27:59 pm
content: Quote from: Denis1024 on June 24, 2022, 10:38:13 amDo you use switchmode supplies for the +/- 5V?What happens if you use battery supplies?With best regardsAndreas

author: oPossum
author_rank: Super Contributor
timestamp: June 24, 2022, 01:49:07 pm
content: Both inputs should have the same source impedance to minimize the offset voltage.

author: tggzzz
author_rank: Super Contributor
timestamp: June 24, 2022, 06:14:06 pm
content: Are you using a solderless breadboard?Are there any decoupling capacitors?Is the PSU lead inductance causing it to oscillate?

author: David Hess
author_rank: Super Contributor
timestamp: June 25, 2022, 02:30:16 am
content: Quote from: Denis1024 on June 24, 2022, 10:38:13 amThe 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.

author: Denis1024
author_rank: Newbie
timestamp: June 27, 2022, 08:59:07 am
content: @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 +3VThe first amplifier is a follower with input at groundThe ouput is connected to the second non-inverting amplifier, gain=100The multimeter is connected via coax + 50 ohms loadOutput 1 : 500µV with noise, but without oscillations (without the 50 ohms load : 4mV)Output 2 : 40 mV, confirmed with oscilloscope, noise not visible40mV / 100 = 400 µV = (Voffset1 +/- Voffset2 ) * 100The datasheet shows a span of Voffset between -200 / +200 µVSo I assume that measuring 100µV without noise requires a lot of skillIf the second amplifier says ~200 µV for each amplifier, that is the thruthThe Voffset is in the specs. The follower follows the theory. I will not the next Nobel !Thanks.Denis

author: David Hess
author_rank: Super Contributor
timestamp: June 27, 2022, 09:25:06 am
content: Quote from: Denis1024 on June 27, 2022, 08:59:07 amI 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.

author: KT88
author_rank: Frequent Contributor
timestamp: June 27, 2022, 11:05:00 pm
content: @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...

author: Denis1024
author_rank: Newbie
timestamp: June 28, 2022, 07:42:00 am
content: I probably found the bug :LT1013 has V offset = 60µV typ.ButLT1013D from linear has 200-800µV, (a little more from Ti )I use LT1013D.Thanks to all.