# I'm working on an auto balance wheatstone bridge sub system

#### soufianeelbir1998

Joined Oct 14, 2020
9

I'm an aerospace engineer and I don't have a lot of expertise in electronics. So I was working on the design with a freelancer, and this is the design that he had provided, the problem is that I have not understood the 3 TL084ACN amplifiers configuration. This circuit is based on the article attached.
Please can anyone help me understand the amplification part? I can share the design file if you want to check it.

#### Attachments

• 1.9 MB Views: 6

#### ericgibbs

Joined Jan 29, 2010
11,651
hi 1998,
Welcome to AAC.
the problem is that I have not understood the 3 TL084ACN amplifiers configuration.
The 3 OPA's are wired as an Instrumentation amplifier.

E

#### soufianeelbir1998

Joined Oct 14, 2020
9
Thanks, ericgibbs. I really want someone who could help me. can I find a consultation service here ?

#### ericgibbs

Joined Jan 29, 2010
11,651

Do you have a specific question/s I am sure we can help.

#### soufianeelbir1998

Joined Oct 14, 2020
9
Yes, you did. Thank you.

I asked just to check if someone can analyze the design with me, so I can know why exactly the values of resistances were used
and why the connections are like that...

#### ericgibbs

Joined Jan 29, 2010
11,651
hi,
Referring this clip from your PDF.

RA,RB Rsens, Rvcr form a Wheatstone bridge circuit

When the bridge is balanced, ie: RA=RB and Rvcr=Rsens there is no differential voltage between Va and Vb.
So Vout is zero volts, also the OUT of 'OA' is zero, so there is no Vcntrl voltage and Rvcr does not change.

If Rvcr value changes, the Va and Vb apply a voltage to amplifier 'A', then thru the 'OA' amplifier.
This change of amplified voltage changes the value Rvcr so that the bridge is brought back into balance.

This covers the amp section.
E

#### crutschow

Joined Mar 14, 2008
26,064
I would use an instrumentation amplifier IC rather than making one with op amps.
The resistors in such an amp are more precisely matched to give better common-mode and temperature performance.

#### soufianeelbir1998

Joined Oct 14, 2020
9
hi,
Referring this clip from your PDF.

RA,RB Rsens, Rvcr form a Wheatstone bridge circuit

When the bridge is balanced, ie: RA=RB and Rvcr=Rsens there is no differential voltage between Va and Vb.
So Vout is zero volts, also the OUT of 'OA' is zero, so there is no Vcntrl voltage and Rvcr does not change.

If Rvcr value changes, the Va and Vb apply a voltage to amplifier 'A', then thru the 'OA' amplifier.
This change of amplified voltage changes the value Rvcr so that the bridge is brought back into balance.

This covers the amp section.
E
View attachment 219600

#### soufianeelbir1998

Joined Oct 14, 2020
9
I would use an instrumentation amplifier IC rather than making one with op amps.
The resistors in such an amp are more precisely matched to give better common-mode and temperature performance.
can you suggest the one that can be used here? thank you.

crutschow

can you provide me a paid analysis? ( do you provide paid quick consultations? )
I want someone to explain to me very well why the values were chosen and why each component was used? I'm not a specialist in this kind of design, and I have hired a freelancer to do the design for me but he didn't explain it to me. It is very important for me to be able to discuss with my supervisor the details.

#### crutschow

Joined Mar 14, 2008
26,064
can you suggest the one that can be used here?
There are many available.
It depends upon the requirements of the circuit (signal stability, temperature range, etc.)
Does it need to be military approved?
can you provide me a paid analysis? ( do you provide paid quick consultations? )
I want someone to explain to me very well why the values were chosen and why each component was used?
Sorry, I don't do consultations.
That's the normal sort of analysis the consultant who built the circuit should have done as part of the design.
Can you pay him to do it?

It can be difficult for someone else to definitely determine the reason for the values chosen or why that particular configuration was used.

#### soufianeelbir1998

Joined Oct 14, 2020
9
There are many available.
It depends upon the requirements of the circuit (signal stability, temperature range, etc.)
Does it need to be military approved?
Sorry, I don't do consultations.
That's the normal sort of analysis the consultant who built the circuit should have done as part of the design.
Can you pay him to do it?

It can be difficult for someone else to definitely determine the reason for the values chosen or why that particular configuration was used.
Thank you for your time. He said he doesn't have time .

#### DNA Robotics

Joined Jun 13, 2014
598
I would find the TL084ACN amplifiers data sheet. They may tell you why. Search for TL084ACN Instrumentation amplifier. Results may explain it. Those may be standard values, used by everyone.

#### sparky 1

Joined Nov 3, 2018
408
Yes the word "instrumentational op amp" is the correct application type because they are intended for a jump to instrumentation.
also we see that simulator is being used. A keep it simple ltspice approach that he could convert to multisim.
To first break down the 3 op amp configuration to show an example why and how the resistors are chosen in a single op amp circuit.

I think what is being explored involves the scope and background that starts with the question: 1. How to make an instrumentation op amp.
Then if the title is relevant next logical question and learning curve jump is: 2. How to make an auto zero bridge amplifier.

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