Load Cell Output Signal Amplifier Design

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Czexican1329

Joined Apr 23, 2018
57
E,
I have not. Would it be okay to assume that as long as I did not require a negative voltage, I could use a single voltage supply?

I have not. Would it be okay to use a single supply as long as I do not need a negative voltage?

I just found an IA that operates similarly using a single supply, the INA122 (http://www.ti.com/lit/ds/symlink/ina122.pdf).
 
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ericgibbs

Joined Jan 29, 2010
9,574
hi,
I have been searching for INA116 instances where user have only a single supply, so far no luck.
Perhaps Ron may have better news.
E
@ reloadron
 
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ericgibbs

Joined Jan 29, 2010
9,574
hi,
For most general purpose OPA's and IA, their internal output stage [ that drives the Vout] cannot sink the Vout signal to 0v.
Check the Vout plots I have posted you can see as the Vout approaches zero the output becomes non linear and limits at around +50mV to say +100mV

E
 

bertus

Joined Apr 5, 2008
20,331
Hello,

At the site of Texas Instruments you will find 49 instrumentation amplifiers.
There are only 3 that are stated as single supply types.
They are the ina122, ina155 and ina156.

Bertus
 

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Thread Starter

Czexican1329

Joined Apr 23, 2018
57
hi,
For most general purpose OPA's and IA, their internal output stage [ that drives the Vout] cannot sink the Vout signal to 0v.
Check the Vout plots I have posted you can see as the Vout approaches zero the output becomes non linear and limits at around +50mV to say +100mV

E
So is there a way to amplify the lower end of the load cell's output signal range to meet the 100mV minimum, but when the load cell reaches max output signal, it doesn't go over 1.25V? I don't want to exceed my 1.25V max (a limitation due to the power rating of some of my components).
 

ericgibbs

Joined Jan 29, 2010
9,574
So is there a way to amplify the lower end of the load cell's output signal range to meet the 100mV minimum,

This does not make sense, it is the IA thats limit the minimum Vout.

I would suggest you post a diagram of what the Vout is connected too and what are the sensitive components, ref .1.25V max
 

Thread Starter

Czexican1329

Joined Apr 23, 2018
57
The analog input module I'm using is a Siemen's SM1231 16-bit analog input module. It can operate in the following voltage ranges: +/-1.25V; +/-2.5V; +/-5V; and +/-10V. I designed my circuit based on the range of 0V to 1.25V. Vout will be connected to the positive analog input.
 

Reloadron

Joined Jan 15, 2015
5,140
What are your thoughts on my latest idea?
First I agree with Eric's above comment but I also think you are looking in the right direction. If this were my project I would be looking towards an INA125 which if we look at the data sheet even affords a 10 volt reference out. Additionally Single Supply: 2.7V to 36V may be used. This is a chip designed and built around what you want to do. That is how I would likely go about this if I were going to design the amplifier(s) for my load cells.

Ron
 

Thread Starter

Czexican1329

Joined Apr 23, 2018
57
First I agree with Eric's above comment but I also think you are looking in the right direction. If this were my project I would be looking towards an INA125 which if we look at the data sheet even affords a 10 volt reference out. Additionally Single Supply: 2.7V to 36V may be used. This is a chip designed and built around what you want to do. That is how I would likely go about this if I were going to design the amplifier(s) for my load cells.

Ron
Ron,
Thank you. I'm glad I'm finally starting to understand what you all have been trying to get across to me.

E,
Because of the approximate 100mV minimum for Vout, I determined that I should have my analog input voltage range of my AI module be +/-5V (but in reality, it will be 0V to 5V due to using a single supply). Because I will be using the 10V reference voltage as my excitation voltage, I would expect my max output signal voltage from my load cell to be 30mV (3mV/V * 10V). Using the equation 1.1mV/30mV = x/5V, I found that I should expect to have an amplified signal of about 183.3mV using my test load (given a 1.1mV output signal from the load cell under the strain of my test load). This value for Vout should be high enough for my Vout minimum. Using the equation G = Vout/Vin, I determined my desired gain would be about 166.666. Substituting fthis value of G into the gain equation of the INA125 (G = 4 + (60kohm/Rg)), I determined that Rg would be approximately 368.85 ohms. As such, I could either choose a 360 ohm or a 390 ohm resistor for Rg; I chose the 390 ohm resistor for Rg, as (Vout)max (when Rg = 390 ohms) would be 4.73V and would not excede 5V.
Does my procedure seem to be on the right track?
 

ericgibbs

Joined Jan 29, 2010
9,574
hi,
If you have decided to finalise the choice of IA amps to the INA125 with a Vout range of 180mV thru +5v for a bridge signal of 1.1mV thru 30mV,[using a Vex of 10V] it sounds feasible.

Do want a LTSpice sim check run for your values.?
E
 

ericgibbs

Joined Jan 29, 2010
9,574
hi,
This is the LTS sim, note the reduced Rgain resistor value.
Its is common usage to have Rg as fixed and a multi turn variable pot in series, so that you can adjust the gain.
E
Don't forget to decouple the power rails, say a 100n and 47uF
 

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