Offset Null for Single Supply Op Amp.

Thread Starter

FrancescoC

Joined Nov 22, 2014
31
Hi,
I was reading the datasheet for an LN1006 Op Amp from Linear Technology.
The device is equipped with offset null connection and the datasheet says to connect the wiper of pot to -V.
Because I want to use a single supply, I was wandering if you can connect the wiper to +V instead.

I am sure I have seen this done before. Or am I mistaken?
Regards

FrancescoC
 

OBW0549

Joined Mar 2, 2015
3,075
Why in the world do you want to connect the offset pot wiper to V+???? As you said, the datasheet says to connect it to V-. So connect it to V-.
 

hp1729

Joined Nov 23, 2015
2,304
Hi,
I was reading the datasheet for an LN1006 Op Amp from Linear Technology.
The device is equipped with offset null connection and the datasheet says to connect the wiper of pot to -V.
Because I want to use a single supply, I was wandering if you can connect the wiper to +V instead.

I am sure I have seen this done before. Or am I mistaken?
Regards

FrancescoC
That "-V" is -V of the op amp whether it is connected to a negative voltage or ground.
 

Thread Starter

FrancescoC

Joined Nov 22, 2014
31
Hi,
Sorry is not LN1006 is LT1006.
The label V- (not -V) is on Pin4. It is the negative supply.
My point is, if I use both Negative and Positive supply, the wiper of the pot is connected to the negative supply.
Therefore I can use offset null. Section 4 of the datasheet explains this.

So what can I do if I wish to Offset-null the op-amp when using a single supply?
Can it be done at all?

Regards

FrancescoC
 

wayneh

Joined Sep 9, 2010
16,124
There's no such thing as a dual supply, there is only a supply of two voltages, one higher than the other. If you must, call your lowest voltage -5V and your highest voltage V+ minus 5. There, now you have a negative supply!
 

#12

Joined Nov 30, 2010
18,167
"The LT1006 is the first precision single supply operational
amplifier. Its design has been optimized for single supply
operation with a full set of specifications at 5V."

"Note 4:
Optional offset nulling is accomplished with a potentiometer
connected between the trim terminals and the wiper to V–."

I don't understand what is so difficult about connecting the wiper of the pot to the most negative supply voltage which is connected to the chip.
Why is this difficult for you?
 

OBW0549

Joined Mar 2, 2015
3,075
My point is, if I use both Negative and Positive supply, the wiper of the pot is connected to the negative supply.
Therefore I can use offset null. Section 4 of the datasheet explains this.

So what can I do if I wish to Offset-null the op-amp when using a single supply?
You're making this a lot more complicated than it really is: for single-supply operation, the V- pin of the opamp is connected to circuit ground rather than to a negative supply voltage; therefore, simply connect the wiper of the offset nulling pot to ground since that is where the V- pin is connected.
 

Thread Starter

FrancescoC

Joined Nov 22, 2014
31
Hi,
I think I may have misunderstood something.
I thought that to offset-null an op-amp a negative voltage supply is required.

Are you telling me that this is not the case?
I have not been able to see an example of offset-null with a single supply op-amp.

Regards

Regards

FrancescoC
 

OBW0549

Joined Mar 2, 2015
3,075
I think I may have misunderstood something.
Yes, you have.

I thought that to offset-null an op-amp a negative voltage supply is required.
Wrong.

Are you telling me that this is not the case?
That is correct.

I have not been able to see an example of offset-null with a single supply op-amp.
When we operate an opamp from a single supply (i.e., without a negative supply voltage), all we are doing is connecting the opamp's negative supply pin to circuit ground-- that is, circuit ground IS the negative supply for the opamp. And therefore, in this case, that is where you connect the wiper of the offset trimpot.

The reason you haven't seen any examples of this is most likely because it was assumed to be blatantly obvious, and therefore in no need of explanation.
 

Thread Starter

FrancescoC

Joined Nov 22, 2014
31
Yes, you have.


Wrong.


That is correct.


When we operate an opamp from a single supply (i.e., without a negative supply voltage), all we are doing is connecting the opamp's negative supply pin to circuit ground-- that is, circuit ground IS the negative supply for the opamp. And therefore, in this case, that is where you connect the wiper of the offset trimpot.

The reason you haven't seen any examples of this is most likely because it was assumed to be blatantly obvious, and therefore in no need of explanation.
Hi,
Thank you for the straight answer.
It is also blatantly obvious that I know very little about opamps!
Thanks to everybody for the help.

Regards

FrancescoC
 

dannyf

Joined Sep 13, 2015
2,197
I thought that to offset-null an op-amp a negative voltage supply is required.
That's generally true: as you typically don't know the direction of the offset.

To put it another way, in a single supply circuit, you may not be able to fully null the offset.
 

OBW0549

Joined Mar 2, 2015
3,075
That's generally true: as you typically don't know the direction of the offset.

To put it another way, in a single supply circuit, you may not be able to fully null the offset.
That is simply not true. In all my 45+ years of doing analog circuit design, I've NEVER encountered any opamp, or opamp circuit, for which that is so. In all cases, such as this one, in which an opamp provides connections for an offset trim pot, the adjustment is adequate for nulling out even the worst-case offset, positive or negative, regardless of whether the opamp's V- terminal is connected to ground or to some negative supply voltage.

For opamps that DO NOT provide offset trim pot connections, nulling offset externally can be a bit more tricky in single-supply circuits than in dual-supply circuits; but that's not the case here.
 
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