Load Cell & 24 bit ADC help

KevinJRClark

I really really really need to measure mass with a load cell, do a bit of math and turn a bit on or off as a result of the math. I have the easy stuff worked out. My issue is the load cell. I have a 2.3kg load cell that puts out 1mV/V. I need about 18 bits to get the 10gram accuracy I require. I found a ADS1210P 24bit delta sigma ADC. It has a lot of functions and is really fast. Neither of which I need. I could stand a sample rate of 1Hz. Sooo.... My question is: has anybody worked with the ADS12XX ADCs before. If so would you share code? and setup experience? If not has anyone worked with other SIMPLE 18-24bit ADCs? At this point help would really be.... well helpful.
K

Post Edited (KevinJRClark) : 3/27/2010 3:53:47 AM GMT

dre

Kevin,

Although I realize most replies to your query will offer help with chosing and coding for 24 bit ADCs, I would like to offer another approach.

My experience has been with using an instrumentation amplifier to produce a near 5 volt full scale output, which can then be fed into a more common and stable 12 bit ADC. Instrumentation amplifiers are relatively stable and inexpensive; and they offer the ability to examine the load cell output [noparse][[/noparse]with a digital voltmeter] before converting the signal to digital. My favorate instrumentation amplifier is the INA125. In addition to a range of amplification of 4 to 10,000 it has· built-in voltage references of 2.5, 5, or 10 volts for bridge supply.

Your figure of 1 mv/V, at full scale I presume, seems low for a strain gage based load cell. Most commerical load cells produce around 3mv/V. But even at 1m/V, if the cell will take a 10 volt supply you will need only a amplifier gain of 500 to feed a 0-5 volt ADC; well within instrumentation amplifier capability.

As for the ADC, I use the TLC2543 as it offers 12 channels and is excellently documented for use by the Stamp :

http://www.emesystems.com/OL2tlc2543.htm



cheers, David

SRLM

What microcontroller are you planning on using? You'll get more specific responses if you let us know...

Anyway, from the datasheet (www2.informatik.hu-berlin.de/~hochmuth/bvp/ads1210.pdf) it appears that the communication is straight forward, if a little bit awkward. On the lowest level it's just a clocked interface, something the propeller or the BS2 can do easily. From the next higher level, you have to do some register writing. You send it a command (which is written to the instruction register), and that determines what sort of data you should send afterwards (such as a command, or a wait for the ADC reading). You'll have to sift through the datasheet, since communication is mostly a series of finicky setup parameters. BTW, I wouldn't label this as a simple ADC.

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Tracy Allen

I'm a fan of the 16 bit ADS1100, which with an internal PGA gain of 8 resolves 19µV when on a 5 volt supply. It is ratiometric, which is good for bridge transducers, and it comes in an SOT23 package than can fit in tight spaces right next to a transducer, and while slow (8SPS), the I2C interface is easy. However, it couldn't match the ADS1210 at 24 bits also with an x16 PGA. What is the load cell output in terms of mV/gram/Volt?

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Tracy Allen
www.emesystems.com

kwinn

Kevin, you may want to take a look at the AD7730 from Analog Devices. It is a complete analog front end for weigh-scale and pressure measurement applications using strain gauge load cells. Everything you need to get a number out from the load cell is there in one chip, the analog front end, auto zero DAC, load cell excitation, ADC, and calibration microcontroller.