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Amplifying load cell signal with AD623 problem! Answered

Hello folk! I am having some trouble with this. I have set up the circuit. Thing is, when i put a 1K resistor between pins 1 and 8, i am supposed to get a gain of 100, instead i am getting a gain of 50. If i put 500ohm resistor, instead of getting a gain of 200, i am getting a gain of 100. What is going on here? My load cell is a bridge type. im getting an output in the mVs so i know my load cell is working. Just the amplification part. Note that the load cell and ad623 have the same ground and the reference is grounded too[0V]. Whats happening? any help appreciated. Diagram is attached.

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Joloblair (author)2013-11-18

This reminds me of a similar problem that happened to me. It came to the point where I gave up and asked for professional advice from Force Logic UK. They were very helpful in overcoming my problem. They said that the main reason why it stopped working was that the load cell had gone down.

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gelopes (author)2013-02-25

Chris, I have the same problem. Did you fix it?

Thanks,

EugĂȘnia

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Chris414 (author)gelopes2013-02-26

No i was not able to get the amplification factor as in the datasheet. It was always half of what it should have been. Other than that though it is working fine.

I still know there is something wrong with my chip but I keep working with it since it gets the job done even if the output is not similar to what the datasheet says.

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user

Split your voltage on the reference pin, from the positive of your 10 volt supply to the negative connect two 1 meg resistors in series. Connect the reference pin in the middle. That should cure the problem.

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steveastrouk (author)2012-11-09

Its just dawned on me, You've exceeded the maximum common mode voltage for the gain and supply you have - What's the common mode input voltage ? Measure pin 2 to ground and pin 3 to ground.

Use the diff-amp to remove the common mode voltage and then amplify it after that in another device

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Chris414 (author)steveastrouk2012-11-10

At 2.5 kg load: pin 3-2: 0.8mV

Pin 2 to ground: 4.58V
Pin 3 to ground: 4.58V(i guess a bit more that since pin 2-3 is 0.8mV)

I don't really understand the problem. Can you explain please. Is there any way to fix this without using another IC. Would changing the supply voltage of the AD623 or load cell make a difference? Thanks

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Orngrimm (author)Chris4142012-11-27

800uV are darn low, but should be around an usable level since the AD623A has 200 uV maximum input offset voltage.
Thus your output doesnt start to change for the first 200 uV.

Could it be that the calc is something like
(Input - offset) * Factor = Output
where factor is defined by the resistor, Input is whatever you hav from the load-cell and the output is... welll... The output. :)
With the assumption that the offset is deducted from the input, it may just fit the bill...

my 2 cents ;)

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Chris414 (author)steveastrouk2012-11-10

ya. the output of the load cell when carrying 2.5 kg is 800 microvolt or 0.8 millivolt

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steveastrouk (author)Chris4142012-11-10

What the he?? change the amp and see what happens.

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Chris414 (author)steveastrouk2012-11-10

I really appreciate your help steve. I do not have another AD623 with me but come monday i will get a new one. I will try to change to another IC even. The output of the lamp is just connected to my voltmeter

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steveastrouk (author)Chris4142012-11-10

Also, what load are you putting on the output of the amp

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steveastrouk (author)Chris4142012-11-10

Sorry Chris, I put your output into my inputs.....Let me rethink this.

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steveastrouk (author)2012-11-09

Put a scope on the output, and make sure it isn't oscillating. You aren't showing the bypass caps in your circuit, which are essential for good design.

Make sure the gain you want isn't too much for the rail you are giving it.

Steve

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Chris414 (author)steveastrouk2012-11-09

I added the capacitors as said in the datasheet. Same output.
As an example: i have 2.3kg on the load cell: output voltage of load cell is 0.75mV, i placed a 200ohm resistor between 1 and 8, which according to the datasheet should yield 500x multiplication, i am getting 214.5mV, which is 268x multiplication( i consider as 250 bcz of resistor not being exact and all).

Is there anything wrong with my circuit? what could be the issues? maybe ad263 burnt? but then it shouldnt be able to amplify? or should i just go with what i am getting and fix the resistor in order to get the gain i want, regardless of what the datasheet says?

Thanks again for the help

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steveastrouk (author)Chris4142012-11-09

Several things to look at - like what load are you delivering the output into ? The 623 has a pretty low output drive capability.

What do you get out on zero load ?

Like I said, look for nasties on the output with a scope - I've seen amps like this ringing and that appears to rob you of gain.

Its quite possible RF's being picked up on the inputs too. What's the output impedance of the loadcell ? AD's Appnotes show the approach to take.

You MIGHT try sticking 100pF of capacitance across the gain setting resistor, and see if that lifts the output

Steve

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Chris414 (author)steveastrouk2012-11-09

Here is what comes up: i didnt put a resistor between pins 1 and 8 so Gain=1 (should be)

at 0 load on load cell: 0.2mV output from load cell, 17mV output from AD623 (gain is not equal to 1)
at any other load: i get a decent output from the load cell. but still 17mV output from AD623.

Output impedance of load cell is 350 ohms


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