# offset and span adjustment in opamp

#### NAGARAJ_JP

Joined Aug 21, 2020
30
Hi i am designing a circuit that will amplify the differential input from a weigh stone bridge,i am using mcp6004, the gain of the opamp is 10, the circuit i have attached below in which i can get the gain exactly as per the calculation along with the offset, I need a circuit which i could adjust the offset and the span.
Thankyou

#### ericgibbs

Joined Jan 29, 2010
19,091
hi N,
A simple solution for the Span adjustment would be to use one of the Quad MCP6004 other internal amplifiers with the Gain/Span pot in its feedback loop.
E
Update:
Example, R6 can be a variable.

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#### NAGARAJ_JP

Joined Aug 21, 2020
30
hi N,
A simple solution for the Span adjustment would be to use one of the Quad MCP6004 other internal amplifiers with the Gain/Span pot in its feedback loop.
E
thankyou ericgibbs the circuit which you gave is having tracking power supply but i have only postive 5v supply i need to design the circuit with that power only is there any ways that offset and span can be adjuster using only positive 5v power supply.

#### MisterBill2

Joined Jan 23, 2018
19,409
A better choice will be a true instrument amplifier circuit. The gain is adjustable as well as the offset, and the common mode rejection should be much better,.

#### ericgibbs

Joined Jan 29, 2010
19,091
hi N,
Check this image form the d/s, 0V out is not possible.
A way of adjusting the Span is to use a dual pot for R3 & R4.
Say a fixed 8k2 resistor and a 5k dual linear pot, for each.
E

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#### MisterBill2

Joined Jan 23, 2018
19,409
What will become clear in using the posted circuit is that it will not be possible to match the two input gains or impedance. That is why I suggested using an instrument amplifier circuit, since setting the gain will require only adjusting one resistance value. In addition, the common mode gain will stay very small, which is important since a Wheatstone bridge has two outputs that are both at a voltage close to half of the bridge supply voltage.
So the TS does need to look at instrument amplifier circuits to see the benefits.

#### ericgibbs

Joined Jan 29, 2010
19,091
Hi,
I would be interested to hear why you are saying it will not be possible to match the two gains, if the TS uses precision components.?
It is a method I have used with good results.

Also we do not know the budget limitations of the TS, so it maybe a case of using what he has been told to use.?

E

#### MisterBill2

Joined Jan 23, 2018
19,409
Hi,
I would be interested to hear why you are saying it will not be possible to match the two gains, if the TS uses precision components.?
It is a method I have used with good results.

Also we do not know the budget limitations of the TS, so it maybe a case of using what he has been told to use.?

E
A reasonably good quad op-amp costs only a bit more than a single. And the instrument amp uses only three more resistors, so the cost difference is not much. And the number of precise resistors is the same. The advantage of being able to set the gain with only one resistor without having to be concerned about matching two resistor values makes it less expensive.
And in many op-amp ICs the input currents are not identical, and certainly the input impedance is not identical in this circuit, as others have proven over the years.

#### ericgibbs

Joined Jan 29, 2010
19,091
hi Bill,
I agree that an IA would be a better solution, it would be my choice in preference over a Differential single OPA amplifier.

I have seen other TS's, IA builds fail, when using 3, OPA's without considering component tolerance balancing.

Perhaps more detail from the TS will help us.?

E

#### MisterBill2

Joined Jan 23, 2018
19,409
hi Bill,
I agree that an IA would be a better solution, it would be my choice in preference over a Differential single OPA amplifier.

I have seen other TS's, IA builds fail, when using 3, OPA's without considering component tolerance balancing.

Perhaps more detail from the TS will help us.?

E
Almost ALWAYS, additional details from the TS will allow more helpful comments. Is this a one-off instrument creation, or a design for a production run of a million units? Or something in between?
The TS has mentioned that only a 5 volt supply is available, and that does make it a bit more challenging to provide an offset adjustment. But still, we have no clue about the application or the bridge circuit.