what type of ADC is suitable???

[ex4]

ina114 load cell amplifier

i want to measure voltage from a load cell with max load 10ton. what type of adc does suits??
can i build it with only discrete parts and a microcontroller?
is dual slope adc will be suitable?? any suggestion about the circuitry???

 

[ravimarcus]

what is ad7705

There are two ways to interface a load cell to a uC.

1. Build a discreet differential amplifier and then connect to the ADC of the uC

2. Use AD7705 and digitially connect to the uC. AD7705 and others of the same family have PGA, filter and ADC built in.

Your choice will depend on the availability of the ADC's and cost.

Cheers

Ravi

 

[ex4]

ina114 loadcell

i have try number 2 and i'm getting pretty good reading, but the stability is quite bad for 10ton load or more. Cannot determine between 1kg and 2 kgs
but for 5kg or more increment reading it is very good.

number 1 is something that i think not very good too . I've try it and the stability also bad. Right now i'm trying to use op amp to make a DUAL SLOPE ADC, but unfortunately the circuit that i got from the net is in dual supply

got any suggestion on how to make it single supply?

 

[ravimarcus]

i have try number 2 and i'm getting pretty good reading, but the stability is quite bad for 10ton load or more. Cannot determine between 1kg and 2 kgs but for 5kg or more increment reading it is very good

Impliment a moving average filter in software. This will improve stability.

number 1 is something that i think not very good too . I've try it and the stability also bad. Right now i'm trying to use op amp to make a DUAL SLOPE ADC, but unfortunately the circuit that i got from the net is in dual supply

Type 1 will cause drift due to the tolerance of the components used and you will need trimming often.

got any suggestion on how to make it single supply?

You will have to lift the ground connection to 1/2 supply voltage.

Cheers

Ravi

 

[bobsanjose]

The question is what is the resolution of your ADC? If it is 10-bit I am surprised you get 5kg because 10 bit is equivalent to 1024 steps. Dividing 10 tons into 1024 steps would result in 10 kg resolution. Assuming you have 12 bit resolution AND the accuracy is 1 LSB, you can get to approx 2.5kg but to have a 1kg resolution you need at least 14-bit.

You can use a slow ADC, so a delta sigma could be an option, high resolution but relatively slow.

Hope this helps, Bob

 

[Tohu]

Hi,
there are not difference betwwen 10 kg and 10000 kg load cell.
First if your target is 1 kg accuracy, this is a 0.01 %. You must use a very precision component in your schem. I am not using the AD7705 or similar directly, but may be for this accuracy this not very good idea. What type of opamp you use in discrete decison. You must use a very precision opamp-s, the best way is to use a chopres, like an ICL 7650. What is you Vref source? Hw you made sample, if you give only one sample, this is more that normal. You must get many sample, and after then get the averrage, the some simple is to get 32 or 64 or 128 samples, add to one variable, and after that shift this variable to right with 5,6 or 7 bits. The sigma delta ADC has a some stranges, i cag suggest toread carrefully the data sheat. To build similar application i use the ICL7650 and TC500. If this is a truck scale, then you must has a algorithm to reject the error from movement and vibration. How you supply the cell, you hav to diferent way, one is to supply directly from Vcc, and use to same methode to get the Vref, second is more compliced.

 

[user11]

Hi


There various methods of doing that ...

1. Use Instt. Amp. to Amplify LoadCell.
2. Check the Power Supply is very steady should drift due to temp,humidity,over time.
3. Use Successive Approx. ADC.
4. Dual Slope may be consider depends on the response time.
5. OP-Amp sould not drift due to temp.
6. Offset should be corrected periodically.

Thx+

 

[ex4]

thank you for the warm reply
now i'm using sigma delta 24 bit adc from Linear LTC2440 using on board PSU from my ATMEGA16 and instrumentation amplifier INA114.
I can get approximately usable 20bit data and 1kg fidelity using 20ton load cell.
for the filter i'm implementing Moving Average Filter with 11 point, great stability i can get
but now i'm going to test it on the field

 

[artem]

ex4, have you done whatever described by you above in ONE day ?
Would you apply for Guinness ?)

 

[ex4]

ha..ha..ha..
not really
actually i design it almost a month. starting with selecting the microcontroller, the adc architecture and manufacturer, istrumentation amplifier, nd the power supply unit.
but i get very random, with noise pattern that linear, so first i only implement averaging filter. so i ask for another way, and ravimarcus suggested to use moving average filter. and the resut is satisfying
now, i can determine 1 kg in change with 20ton load cell
i just need to add some other features

 

[pardeepgaur]

hi iam pradeep
can you send me your shematic and code as i am also working on similar project
for my class

thanks
pardeep

quote="ex4"]ha..ha..ha..
not really
actually i design it almost a month. starting with selecting the microcontroller, the adc architecture and manufacturer, istrumentation amplifier, nd the power supply unit.
but i get very random, with noise pattern that linear, so first i only implement averaging filter. so i ask for another way, and ravimarcus suggested to use moving average filter. and the resut is satisfying
now, i can determine 1 kg in change with 20ton load cell
i just need to add some other features[/quote]