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Interfacing a 4wire load cell to PIC microcontroller built in ADC

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Hi all,

I am going to Interface 4 wire load cell to PIC 16F877A microcontroller.
range of load cell is 0-40kg,22mV/V,
planing to use 5V excitation voltage, as same source for microcontroller.
planing to measure 0-1.5kg with least 1mg accuracy.
planing to use built in ADC with 10bit.
instrumentation amplifier planing to use AD623,
Pls help me whether it will work as my requirements or not.

If it will not work pls suggest me new.

Thank you
 
hi,
A 10 bit has only 0 to 1023 counts output for a input range of 5V with a +5Vref.

So 1500gm/1024 will give approx 1.46gm per Bit.

You say resolution to +/-1milli-gram,,, do you mean 1milli-gram or 1 gram.???

Even with over sampling I do not think you will get down to 1mg.
 
Hi eric,

I need 1mili gram , if it possible. So should I change the Load cell or, with existing load cell can I change the instrumentation Amplifier and reading method?
 
Hi eric,

I need 1mili gram , if it possible. So should I change the Load cell or, with existing load cell can I change the instrumentation Amplifier and reading method?

hi,
Its not the load cell or the AD623 thats the problem, its the low resolution a 10bit ADC.
 
I thought that too, but I thought to maximize resolution by using VreH and VrefL of built in ADC so my resolution will very high If I set vrefH-vrefL to 500mV.


So eric if I am ready to change the ADC unit to external,
what is your suggestion, should I use differential input ADC or single positive input ADC.
 
I thought that too, but I thought to maximize resolution by using VreH and VrefL of built in ADC so my resolution will very high If I set vrefH-vrefL to 500mV.


So eric if I am ready to change the ADC unit to external,
what is your suggestion, should I use differential input ADC or single positive input ADC.

hi,
You can get 16bit external ADC's that will 65536 counts for a 0 to 5V input.

That will only give 1500/65536, 0.0228Kg/bit ie: 22m-gram

Ideally you would have to consider a 24bit ADC.

As you know the AD623 is a differential input and a single ended output, if you power the AD623 from +5V you cannot get a higher Vout of approx 3 to5 V. A second rail to rail OPA will be required to raise it close to +5V.

What you are trying achieve ie: 1milli-gram with 1500gram Span is not realistic with the components you are using.

1500.000gm +/-0.001
 
Hi,

I thanks I understood that now. In this case I would be change the resolution of measurement up to 50mg. Do you think If I set those analog reference voltages of the pic built in ADC to maximize the resolution, will work?

I can set the references to concern voltage range and devide it to 1023 parts.

Pls advice me
 
Eric,

I was helping him in chat today. I recommended a 100gm 2mV/V load cell (cheapest one from Omega) into a LTC2054 zero drift amp feeding a ADS1100 full differential analog front end (16bit ratiometrc, self calibrating).

That way 0.1gm would be 10uV into a 3uV max IosV.

Dan
 
Hi Eric/Ubergeek,

Thanks for help of both of you. I tried the circuit with two instrumentation amplifiers.

Eric: you are correct about 10bit resolution, I don't know how did I missed such a simple thing at that time.

Ubergeek: You are quite correct about AD623 but it worked with less accuracy, so I found another IC INA128P which with 10uV offset, and I could measure 4grams resolution, for 1000grams span.

Still I didn't use any filter all the circuit if affecting ordinary items so in that scale output is varying by - or + 1 value from ADC units.
I draw the graph with some weights (vegetables from super market with written weight), and it is linear.
I used the reference voltages of PIC to bring values to effective range.
Now I change the plan to AD7730, which is a built in solution for this project.
And mean time I found 12bit ADC, SPI interface MPC3201 and I'm going to give a try and also same series they have 22bit ADC also.
I will attached pictures of the project at the moment, And next week I will come back to you with AD7730 results.

Thanks again for both of you guys and I think my result would helpful for others too.
 
Eric,

I was helping him in chat today. I recommended a 100gm 2mV/V load cell (cheapest one from Omega) into a LTC2054 zero drift amp feeding a ADS1100 full differential analog front end (16bit ratiometrc, self calibrating).

That way 0.1gm would be 10uV into a 3uV max IosV.

Dan

hi Dan,
I use the ADS1100 for 16bit work, the only limitation I find is the low update at 16bits.

hi Swasthka,

Thanks for the update, do you mean the MCP3201.?? 12bit.

I use the MCP3202 [dual] with high over sampling to get +/-10mGram from a 30kG load cell.

E.
 
there are newer versions that cost about the same.... the latest being the ADS1115. Since TI took over they have not been posting the resolution shift with increased sample rates.... the ADS1100 is limited to 12bits at 128 samples per second and the ADS111Xs go to 850 samples per second.

Dan
 
Hi all,

Sorry for the late reply regarding the AD7730. It took me few weeks because I was busy with job also. However I successfully interface the AD7730 with PIC16F877, compiler used is CCSinfo PIC C.
Anyone need help on that pls let me know regarding the code or circuit.
I will comeback to you with more images and codes.

Thanks for the helps you all given to me
 
for mine
im using load cell 0-10kg,2mV/V
actually
im not quite understand the function of bit

and i want to use lm741
is that possible?
 
sir i use pic18f and use adc 1255 for load cell interface and 4-wire loadcell with 0-100kg and gives 10mV ON excitation voltage 5V on 100 kg weight
as i am a beginner plz send sample code to help plz sir
 
sir i use pic18f and use adc 1255 for load cell interface and 4-wire loadcell with 0-100kg and gives 10mV ON excitation voltage 5V on 100 kg weight
as i am a beginner plz send sample code to help plz sir

Dear Manish,

While I am checking the datasheet for the ADC1255 please let me know how much accuracy you are willing to have. Like some commercial weighing machinese are 5mg accuracy.
 
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