PDA

View Full Version : ADC interface with load cell



ssbhide
04-26-2010, 11:35 AM
Hi All,
I am facing a common problem, but I don't know how to solve it. I am interfacing a load cell to a pic 18f452 ADC. I am using a opamp with a gain of 470K. Now the problem is. If I reduce the gain then the minimum weight I wish to sense is not sensed by the circuit due to the limitations of the ADC. And at this high gain the noise distortion is so much that I am not able to get any stable readings. What is the solution? Will a low pass filter (RC filter) solve the problem. What should be the cutoff frequency for which I need to design this filter?
Thanks in Advance waiting for your replies
kind Regards
suchitra

Ulhas
04-26-2010, 01:38 PM
gain of 470K in single stage seems to be horrible. In fact which scheme you have to adopt can be cleared only when you post some details of load cell.
However why don't you select the load cell of 4-20mA output signal. This will reduce you problems to great extend.

Also if you want to do it your self then you need precise instumentation amplifire and OP amps along with high grade shielded cables.

ssbhide
04-26-2010, 10:15 PM
Hi
My gain is 470 and not 470K sorry for the typo. The load cell I am using is a micro load cell whose rated output is 2mv/v. And the excitation voltage is 5 V.
I am using TI's opamp OPA333 in non inverting mode. with Rf = 470K and R1 = 1 K. What do you exactly mean by a load cell of 4 to 20 MA? Is it possible to give further details about it especially the price.
Kind Regards
suchitra

Ulhas
04-27-2010, 08:30 AM
Hi
See this "http://news.thomasnet.com/fullstory/11844"
In price it may be costly but reliable. Let us know about your application details amd the datasheet of your finalised load cell so that it can be explored further.

However if it is strain gauge based then go for National Instrumentation amplifier ics.
Look here too "http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=1612&uiTemplateId=NODE_STRY_PGE_T"
Order free samples.

see this and get the cost http://www.ajinkya.in/measuring-instrument.html