TL071 question: 71 has offset adjustments. What are the offsets for?

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,067
I've rarely messed with an op-amp. I'm going through my stockpile of stuff and have several TL071's (8 pin dip). Looking at the data sheet, the 71 has offset adjustment pins. I've never used an offset adjustment. Can you tell me what they're for and how to use them?
 

Papabravo

Joined Feb 24, 2006
16,472
I've rarely messed with an op-amp. I'm going through my stockpile of stuff and have several TL071's (8 pin dip). Looking at the data sheet, the 71 has offset adjustment pins. I've never used an offset adjustment. Can you tell me what they're for and how to use them?
Input Offset Voltage is the differential voltage at the opamp input that produces 0 volts out. Because of the high gain this can be tricky to measure and in modern devices has been reduced to a pretty low value. On some older devices you can use the offset adjust pins to get rid of the Input Offset Voltage.

https://en.wikipedia.org/wiki/Input_offset_voltage
 

MrChips

Joined Oct 2, 2009
23,898
I've rarely messed with an op-amp. I'm going through my stockpile of stuff and have several TL071's (8 pin dip). Looking at the data sheet, the 71 has offset adjustment pins. I've never used an offset adjustment. Can you tell me what they're for and how to use them?
With an op-amp with bi-polar supply voltages, when you input zero differential voltage at the inverting and non-inverting inputs, i.e. (Vin+) - (Vin-) = 0 you would expect the output Vout to be zero.

However input offset voltage in the op-amp itself may cause the output voltage to be non-zero. Offset adjustment allows you trim the output voltage to zero.
 

dl324

Joined Mar 30, 2015
12,917
There are some general purpose methods for nulling offset voltage on opamps that don't have explicit pins.
1606587461100.png
I was checking tracking on a DIY tracking supply and threw this together to measure differences that were in the 3-30mV range (on a 1.25-18V supply output range). On the 20V range, my DVM couldn't measure a difference that small.
 

bertus

Joined Apr 5, 2008
21,447
Hello,

Have a look at the datasheet.
On page 37 is the offset circuit:
TL071_offset.png
On page 38 there is the internal schematic, where the offset for the TL071 is done:
TL07X internal.png
Bertus
 

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Papabravo

Joined Feb 24, 2006
16,472
Awe geez. The hard way. Thanks. I'll give that a search and see if I can spot it.

[edit] If I'm seeing this right, page 15 of the TI Data Sheet says typical ±1mV, max ±4mV. Is that what you read?
p. 37, Fig 7.3 on the version of the datasheet you posted. I didn't give you a reference because my datasheet is a couple of years old, but it is the same figure.
I knew you'd find it eventually, and I couldn't recall the precise details. Sorry you had to work for it.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,067
We learn little when information comes too easily. It's too easily forgotten.

I've always been a poor reader. LAUSD (Los Angeles Unified School District) I graduated with remedial reading skills. After all, ours was the #4 from the bottom school in the USD in 1976.
 

bertus

Joined Apr 5, 2008
21,447
Hello,

Yes, when you look at the internal schematic, it is changing the balance between the emittor resistors.

bertus
 
If you read some of the datasheets, there are some penalties which I forget what they are.

It's not a bad application for digi-pots and auto-zero routines.

I was working on a potential auto-zero function for a multi-range I-V converter, but it got pulled. DC was not a requirement. I didn't add null pots because I had expected it to be done electronically and I goofed.

I managed 40pA of offset which wasn't shabby without any nulling.

The idea was to do, Two ranges with a zero input to measure Vos with it configured as an voltage amplifier.

SURPRISE: When you output 0V from a DAC, you don't get 0.

AC measurements, were the primary purpose and that worked out very well.
 
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Papabravo

Joined Feb 24, 2006
16,472
We learn little when information comes too easily. It's too easily forgotten.

I've always been a poor reader. LAUSD (Los Angeles Unified School District) I graduated with remedial reading skills. After all, ours was the #4 from the bottom school in the USD in 1976.
B5/B6 1958/1959. Got out after 1 year
 

Wolframore

Joined Jan 21, 2019
2,317
Is it better being top of class in bottom school or bottom of class in a top school? I can relate to either... until I started caring... :D
back on topic. I believe that they just had so many pins on a DIP-8 that they broke out a bunch of stuff which seems to be set by uA741 to control Null Offset and sometimes compensation in the single opamps. Notice you don't get these on the dual or quad opamp IC's.
 
Last edited:

Ian0

Joined Aug 7, 2020
2,612
My view is if you want low offset voltage, then buy a low-offset op-amp, like a OP07. Don't bother faffing about with a offset-adjust preset.
Only use a TL071 if you're not bothered by a few millivolts of offset error (which you won't be, if you're amplifying audio signals with it)
 

atferrari

Joined Jan 6, 2004
4,380
There are some general purpose methods for nulling offset voltage on opamps that don't have explicit pins.
View attachment 223592
I was checking tracking on a DIY tracking supply and threw this together to measure differences that were in the 3-30mV range (on a 1.25-18V supply output range). On the 20V range, my DVM couldn't measure a difference that small.
This is my actual implementation with a TL072:



Offset nulling.png
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,067
Notice you don't get these on the dual or quad opamp IC's.
Yeah, I noticed that. I have some TL072's as well. Noticed that they are dual op's. Also see that there are 10 pin op-amps in the TL07x's as well.
buy a low-offset op-amp
This thread isn't about buying something for a project. I'm gathering all my scattered stock into a single place and came across 10 TL071's and 20 TL072's. Not that I have a project for them, the thing is that I HAVE them. Looking up the data sheet I saw the two Null's and know I don't know how to use them. I may eventually use them; and knowing how to is the thing. But I understand the statement. I do the same thing too, offer advice to go a different direction on a project. But often I get scolded for not answering the TS's question directly. Even when their approach is entirely wrong. It's like someone asking how to crack an egg using a 315 Cummins Diesel engine. If I don't answer exactly how to use the engine some here scold me. "THAT'S NOT WHAT THE TS IS ASKING!". But I understand; and am grateful for advice that redirects my thinking.

There IS a project in the back of my mind, been there for decades. I may use one or two of these op's for that. But being that it has sat idle for so many years, it may sit for a few more. Right now I'm in the process of planning an addition above the garage for a wood shop. Meanwhile I'm trying to clean up my clutter.
 
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