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Is it possible to compensate the VOS by adjusting the common mode outout voltage VCM?

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Junus2012

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Dear friends,

Is it possible to compensate the VOS by adjusting the common mode outout voltage VCM, suppose the op-amp with single supply operation 3.3 V and VCM = 1.65 V, but due to the offset I would connect 1.65+-dv, where dv is opposite to the offset voltage polarity.

The question in my mind that if we fix the VCM from output we still have it at the input, right ?

because VOS referred to the input = VOSI+VOSO/Gain
 

I assume you mean instrumentation amp rather than opamp but yes, you can just as easily correct offset at the input as the output.

At the end of the day the output is all that matters. The offset correction circuit can do anything that nulls your offset at the output.
 
I assume you mean instrumentation amp rather than opamp but yes, you can just as easily correct offset at the input as the output.

At the end of the day the output is all that matters. The offset correction circuit can do anything that nulls your offset at the output.

Thank you asd for your reply

Yes right I am dealing with an Instrumentation amplifier but I think it is also applied to the op-amp.

Any way as you said the fianl target or what matter is what matters, However, the input offset voltage defines the minimum input voltage that can be treated amplifier (in addition to the noise level), if we adjust the VOS to zero by adjusting VCM, it will still keep the VOSI as given in the formula

VOS_referred to the input = VOSI+ VOSO/gain => VOS_referred to the input=VOSI+0/gain = VOSI

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by the way I see this procedure is more easy than adjusting the VOS by applying a DC offset with opposite polarity to the one of the OP-amp or In-amp inputs, at least I am talking for single supply operation
 

Hi,

depending on your circuit: Maybe just a suitable filter in the feedback path is a solution.
Show your circuit and your requirements ... if you need a more detailed answer.

Klaus
 
"However, the input offset voltage defines the minimum input voltage that can be treated amplifier"

I don't agree. It's common to look at signals that are smaller than the offset and it shouldn't matter where you cancel that offset.

In gain of 1000 example you might have 1mV of offset giving you a 1V signal with 0 input. Now cancel it at the output -> 0V signal. Now an input signal of 0.1mV comes in and the amplifier output will increase by 0.1mV*1000=0.1V to 0.1V out. That's perfect.


If your gain is variable and input offset dominates then there might be an advantage to canceling at the input. Now if you change gain the offset will remain mostly cancelled. But otherwise I'd still suggest its an implementation choice: where is it easiest to inject your own offset.
 
If it's possible to "adjust" Vio by pushing Vcm, then the
amplifier is not a very good amplifier. CMRR is desired to
be high.

If you're going to adjust with an external source / trim-pot
/ select resistors, why not sum in via a resistor, the small
current required? If (as in many inst amps) the resistors are
buried inside, adding small external matched series resistors
to both terminals and putting an offset current to one side,
the other, or both might be what you want.

Look at how some old-timey bipolar op amps used to supply
ADJ pin-pairs to allow a trim-pot, series-resistors balance
network externally to influence offsets by injecting two
currents from VCC into the gain stage.
 
The OP is talking about an inst amp and I assumed because of what they said they meant the "ref" pin when referring to "VCM from output". Possibly a mistake?
 
Dear Friends,

Thank you all for your helpful contribution,

And nice to see you again Klaus :)

Below is the bloack diagram and the schematic of my instrumentation amplifier, it is based on indirect current feedback topology

off_3.PNG

off_1.PNG

off_2.PNG

So it is fully differential amplifier.

I came to know about the offset voltage correction by adjusting VCM from the data sheet of these Instrumentation amplifier, However, beside these adjustmet they also using Auto zeo techniques

https://www.ti.com/lit/ds/symlink/ina333-ht.pdf

https://www.analog.com/media/en/technical-documentation/data-sheets/AD8557.pdf

Thank you once again
I am looking forward to your discussion again

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In the last data sheet, still a question for me why should they need to include such an adjustment when they already have an advcance auto zero offset cancalation,
 

Again, it's clear at this point the OP means the output reference more commonly named "ref". Both linked datsheets have offset correction tied there.
 
I can't read any sense in the idea to vary VCM.

Dear FvM,

I mean by the VCM is the reference voltage (VREF), if you see the data sheet please of the two instrumentation amplifier I posted, Ideally it should be in the middle of the supply voltage for single-supply operation, but as you see they are perturbing this value to adjust the value of the offset voltage to zero.

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The OP is talking about an inst amp and I assumed because of what they said they meant the "ref" pin when referring to "VCM from output". Possibly a mistake?

Dear asdf44

Yes I mean the VCM at the output which is the VREF in the datsheet

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"However, the input offset voltage defines the minimum input voltage that can be treated amplifier"

I don't agree. It's common to look at signals that are smaller than the offset and it shouldn't matter where you cancel that offset.

In gain of 1000 example you might have 1mV of offset giving you a 1V signal with 0 input. Now cancel it at the output -> 0V signal. Now an input signal of 0.1mV comes in and the amplifier output will increase by 0.1mV*1000=0.1V to 0.1V out. That's perfect.


If your gain is variable and input offset dominates then there might be an advantage to canceling at the input. Now if you change gain the offset will remain mostly cancelled. But otherwise I'd still suggest its an implementation choice: where is it easiest to inject your own offset.

If according to your kind explanation that offset voltage can be compensated from the output side, which I convinced by your formula, then applying offset voltage correction is only another additional option.
Second thing how can I simulate the output offset voltage ?
I usually simulate the total input referred offset voltage

Thank you
 

Yes it's optional...in digital systems with an ADC its common to measure and store the offset (at say powerup) and subtract it later. Though very large offsets could eat into dynamic range or saturate intermediate stages. Those problems are only solved with actual analog cancellation.


Simulate by finding an inst amp model that includes both (test with different gains and you should see whether it does) or just build the 3-opamp inst amp circuit up yourself and inject offsets where you see fit.

LTSpice would be good for either and already has many analog devices inst amps.
 
The term VCM is just confusing related to the circuits. They don't have (or at least don't show) VCM control feature. And no CM reference voltage to set the output VCM. In so far it's unclear what you mean with "adjusting common mode output voltage".

The offset can be manipulated by injecting a differential voltage or current between Vref+ and Vref- in your circuits, not by shifting both in the same direction.
 
The term VCM is just confusing related to the circuits. They don't have (or at least don't show) VCM control feature. And no CM reference voltage to set the output VCM. In so far it's unclear what you mean with "adjusting common mode output voltage".

The offset can be manipulated by injecting a differential voltage or current between Vref+ and Vref- in your circuits, not by shifting both in the same direction.

Dear FvM,

You are right, sorry for this mistake of not showing the VCM in my circuit,
The VCM was hold at one of the CMFB amplifier input to compare it with the VOC, as shown in the figure below.. So I am using the VCM term while in the data sheet of the instrumentation amp are referring it as REF (althaugh the presented IC are single ended output), so I wanted to adapt that idea to adjust the offset voltage by adjusting the VCM in my design.

cmfb.png
 

O.k., I see the CMFB input node in the fully differential amplifier. I agree with dick_freebird's analysis in post #6. VCM has ideally no and practically maximal small effect on input offset. It's no useful way to adjust Vos.
 
Dear Friends,

Thank you for your help, and sorry if my helping rate is over for this day but I do appreciate all the replies

I am sure after your kind explanation that adjusting the offset voltage of my fully differential Ins Amp is not the right procedure, because changing the level of VCM which I suppose to call it VREF basically has direct affect on the output common mode voltage level (ideally VOC = (VO1+VO2)/=VCM) and has no relationship on adjusting VOS.

So changing VCM will change VOC which is the y-value of the intersection point between VO1 and VO2 while the VOS is the x-axsis of this intersection point and it ideally should not change with chaning VCM level.

In contrst to the signle ended InsAmo or the non inverting amplifier, this voltage can be used to adjust the offset voltage or even introducing if needed a required DC voltage shift.


For my case there I must use different approach for offset voltage correction, some of you thankfully has already gave a suggestion, but I would like to post this issue in a new post
 

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