Simple loadcell amplifier schematic please

[Morcego]

Hi,
I need a simple loadcell amplifier schematic for a project.
Loadcell is a 350R 4 wire bridge 3mv/V.
I need a fullscale output of around 5V.

I had tested the INA125P but I was very disappointed. I used the recommended circuit from BB in the application note, but the linearity was very poor (I dont know why) I had it compared with a commercial amplifier and the result was a huge diference. As I increase the load, the diference was bigger. Can anyone point me to a simple, and reliable schematic please. Someone told me that the AD627 was a good solution.

Thanks in advance

Joaquim

 

[Skogsgurra]

The BB instrument amplifier should work quite well. How bad is the linearity?

A common problem is power supply decoupling missing and the + input picking up noise so that the amplifier oscillates at a high frequency. What does the output look like on a scope?

Gunnar Englund

http://www.gke.org

 

[Morcego]

Hi Gunnar,
Thanks for your awnser. I had put 5kg in the load cell. The commercial amplifier shows 5kg. I had mine adjusted to get also 5kg (gain adjust). Then I put another 5Kg on top. The commercial unit shows 10Kg, mine shows about 14.

Gets worse as load increases.

I'm using linear power supply(not switching because of high frequency noise). I'm using the circuit in BB datasheet. Should I have to take extra care? The ina125 is powered a 12V (single supply) as as a 100nF decoupling capacitor to ground.

I will checkit with a scope.

Thanks for any help

 

[jimkirk]

What's the input voltage? Each input must be at least 1 volt above ground for linear operation.

 

[richs]

Hiya-

There's a nice little circuit in the app notes on the
Microchip site (No, I don't work for Microchip). They
have a pretty good write up of interfacing op amps in an
instrumentation interface to a typical A/D converter. They
address such things as amp band width and filtering in it.
I do like the section on the hardware types making the
software types do the digital filtering........

I'm sorry I don't have the link with me directly. Oh wait,
here it is:

http://ww1.microchip.com/downloads/en/AppNotes/00695a.pdf

You might want to wander through this app note and
(pic, sorry couldn't resist) up some tips.

Even though you are interfacing to a load cell rather
than a pressure transducer, the signal requirements are
quite similar.....

Hope it helps. I found it useful.

Cheers,

Rich S.

 

[Warpspeed]

There are all sorts of possible causes of non linearity, but my nose tells me to be that bad, you may have a severe noise pickup problem.

If the input signal has a fairly high ac noise voltage superimposed, the output signal may not be the nice clean amplified dc voltage you expect. The output may actually be swinging wildly from rail to rail with so much gain.

Look at the output on an oscilloscope, and see if it is truly clean. The recommended circuits of B.Brown or A.Devices are always of a very high standard, and your circuit layot may follow the schematic exactly. But the physical layout, particularly grounding, screening, and supply bypassing may be suspect.