Using AD623 or AD8226 LTSpice Circuit Problem

Thread Starter

ridvanmelih

Joined Jun 24, 2021
10
Hello Guys,

I have a question and Please Help Me!

I interested in whatstone bridge amplifier and I try some circuit design. I will show that the this circuit. I try some different combiniation and I selected more different value but the result is so different. WheatStone Bridge output is normally 0 Volt (You can see the figure). but I didnt solve this problem. Yes I know, If I change or add a some component, circuit working normal but ın this time my maxcimum voltage is the 4.35V, 4.4 or 4. etc. But I want ycalculate a 5Volt.

Ify you want, please reead this post and write some command.

ThntAllAnoutCircuit.JPG
 

Thread Starter

ridvanmelih

Joined Jun 24, 2021
10
Hi eric,

Thank you for your reply.

I shared it.

If I want to If Wheatstone Bridge Resistors (R1=120 R2=120 R3=120 R4=120), I want to see a 0 Volt on Amplifier Output.
If I want to If Wheatstone Bridge Resistors (R1=120.2479 R2=119.7521 R3=120.2479 R4=119.7521), I want to see a 5 Volt on Amplifier Output.
 

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Thread Starter

ridvanmelih

Joined Jun 24, 2021
10
hi r,
This is what I see with a Gain of 100 times, for a 10mV input test voltage.

What is the full mV range output of your Bridge.?

E
My Bridge output range is 0mV..10.33mV and I want to see a if Bridge output is 0mV, amplifier output equals to 0v and if Bridge output is 10.33mV, amplifier output equals to 5V.

But I see the your study, this scenario is not possible. Did I get right?
 

Thread Starter

ridvanmelih

Joined Jun 24, 2021
10
hi r,
This is what I see with a Gain of 100 times, for a 10mV input test voltage.

What is the full mV range output of your Bridge.?

E
Why is the amplifier output is not equal to 0 V, normally your bridge outpu is 0mV, and you supply to signal, I'm sorry, your knowledge may be higher, but it didn't seem quite right to me.
 

Irving

Joined Jan 30, 2016
2,308
This is why I drum into my students the importance of reading and understanding the data sheet for a component... The headlines don't tell the whole story... (the classic one is Rds(on) for a MOSFET).

@ridvanmelih you could increase your supply voltage.if you're planning to feed this to an ADC you probably need it below the rail anyway.
 

Irving

Joined Jan 30, 2016
2,308
Why is the amplifier output is not equal to 0 V, normally your bridge outpu is 0mV, and you supply to signal, I'm sorry, your knowledge may be higher, but it didn't seem quite right to me.
Eric is introducing an offset voltage into the amplifier input rather than modify the bridge resistors. This is less likely to introduce user error!
 

Thread Starter

ridvanmelih

Joined Jun 24, 2021
10
This is why I drum into my students the importance of reading and understanding the data sheet for a component... The headlines don't tell the whole story... (the classic one is Rds(on) for a MOSFET).

@ridvanmelih you could increase your supply voltage.if you're planning to feed this to an ADC you probably need it below the rail anyway.

Hello Sir,

I try this and I increasethe supply voltage but I think the system is not very stable.I try to more different configuration
 

Irving

Joined Jan 30, 2016
2,308
This is a common configuration. My students do an investigation using a strain-gauge and an AD623. They use 0.5 to 4.5v output, avoiding the rail issue, and use REF to set bridge zero as 2.5v so the output swings either way +/- 2v. The ADC is 0 - 5v but only 0.5 - 4.5 is used actively. Even rail-to-rail output often can't get closer than 0.01 - 0.05v to the rail, so if you are aiming for sub-10mV resolution, i.e 10bit ADC or higher you're not going to get useful accuracy at the extremes. Much better to use a smaller non-rail upper and lower level, more bits and scale appropriately (subtract 0.5, multiply by 1.2, in postprocessing.)
 

Alec_t

Joined Sep 17, 2013
12,309
Why is the amplifier output is not equal to 0 V
Because the IC is not an ideal device. Internal currents involved in its operation make theory and practice somewhat different.
I have my doubts, too, as to the accuracy of the LTspice model for the IC. For example, if R1=R3=R4=120 but R2=121, the IC output goes to -98mV although there is no negative supply rail.
 

Irving

Joined Jan 30, 2016
2,308
LTSpice is only as good as the underlying models, which is why there are multiple models for MOSFETs depending on what you're trying to simulate. As long as you stay away from extremities it's usually pretty good. My students do an exercise comparing simulation with reality to clarify this.
 
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