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The load cell I bought looks like this

I have bought a 200kg load cell and an AD620 in-amp. I only want to use 100kg as the maximum weight capacity. I want to power the load cell and the in-amp from the 5v pin of the arduino. The seller can't give me the specifications of the load cell so i'm actually having some difficulty computing for the gain.

I want an output swing of about 0 - 5V from the in-amp so i can utilize the 10-bit ADC resolution of the arduino with the default 5v reference.

Do I have to use a dual +-5V supply to have an output swing of 0-5V? What could I connect to the reference pin of the in-amp if i only have a single supply as I mentioned earlier.

I would really appreciate your help, this is for my thesis.

-----------------------------------UPDATE--------------------------------------

After I have read the datasheet and some books about amplifiers, I have tried to make a circuit so I can check if I could get the -Vs+1.1 to +Vs-1.2 output swing of the Ad620. Below is the circuit: enter image description here

I have referenced the REF pin of the AD620 to half the supply by connecting it to the output of the LM741 op amp with two 10k resistors on the non-inverting pin. After testing some load and testing some gain resistor(as i have said earlier, the seller didn't give me the datasheet of the load cell so i dont know exactly what the exact sensitivity is), I have read an output swing of approximately half of my supply to 3.6V. That would be like 2.2V(because when i test the voltage supply from the arduino its like 4.4V) to 3.6V. Am I getting the right output from the AD620?

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    \$\begingroup\$ The AD620 is not a good choice for 5V single supply usage. It requires over 1V of headroom at each rail. That means best case, your usable ADC span is 1-4V. \$\endgroup\$
    – Matt Young
    Dec 12, 2014 at 3:23
  • \$\begingroup\$ Is this a quarter-bridge, half-bridge or full-bridge load cell? \$\endgroup\$
    – WhatRoughBeast
    Dec 12, 2014 at 3:25
  • \$\begingroup\$ @MattYoung, if I will use dual supply for my in-amp, would i get the 0-5v swing? \$\endgroup\$
    – Rhonald Rei Pahayac
    Dec 12, 2014 at 3:42
  • \$\begingroup\$ @WhatRoughBeast I have added the image of the load cell on my post. i would think this is a full-bridge \$\endgroup\$
    – Rhonald Rei Pahayac
    Dec 12, 2014 at 3:42
  • \$\begingroup\$ If you use a dual supply, you can get 0-~4V. \$\endgroup\$
    – Matt Young
    Dec 12, 2014 at 3:46

1 Answer 1

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You don't need a dual supply, but you'll need more design work than you seem aware of. You can use the AD620 with a single 5 volt supply since the load cell outputs ought to be about 2.5 volts and are within input limits. Set the reference to 1.1 volts. The ouput will then be restricted (according to the data sheet - and you DO have a copy of the data sheet and you HAVE tried to understand it, right?) to the range 1.1 to 3.7 volts. If you're willing to deal with 200 gram resolution you can stop right there.

If not, you need a second amplifier / offset generator. But first you would set the AD620 reference to something useful like 1.5 volts using a pair of resistors, and the gain for a 2 volt swing. Then, using a rail-to-rail op amp you would make a gain of 2.5 difference amplifier, and add in a 1.5 volt offset. The result would be (nominally) a 0 to 5 volt output. And before you ask - no, I won't give you a design. Do some research first. Google "differential amplifier" and actually try to understand what you read. Make a preliminary design and mess around with it. If you can't get it to work, describe your problem in detail on another question. Folks, including me, are glad to help - but I'm not going to do your work for you.

There is another consideration. First, you want to set your zero point at very slightly above zero volts. You should be able to check your data at zero load and see a little bit of noise. If the output of the amplifier is dead zero you know that your zero load level is less than 0 volts, but you don't know how much less. You can always subtract out a known zero offset, but there is no way to recover a completely unknown quantity. Losing a little bit of dynamic range or resolution is much less of a problem than not knowing what your zero point is.

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  • \$\begingroup\$ By looking at the question, OP will not get (understand) your answer by itself. OP may need a more cooked solution. \$\endgroup\$
    – Chetan Bhargava
    Dec 12, 2014 at 5:33
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    \$\begingroup\$ Yeah. But he has to try. \$\endgroup\$
    – WhatRoughBeast
    Dec 12, 2014 at 5:43
  • \$\begingroup\$ @WhatRoughBeast thanks for this, I have read the datasheet and some books about instrumentation amplifiers and how they function. I think I understand some of them a little and come up with a circuit above with my original post updated. As you have said, I set the ref pin to half of my supply(not 1.1 because im testing it first) and my output is restricted from 2.2V to 3.6volts. I think I need a second amplifier like you have said by using a AD822(dual rail to rail op amp). \$\endgroup\$
    – Rhonald Rei Pahayac
    Dec 13, 2014 at 4:37
  • \$\begingroup\$ Glad to help. The AD822 is marginal on speed. Go with something a bit faster. \$\endgroup\$
    – WhatRoughBeast
    Dec 13, 2014 at 5:23
  • \$\begingroup\$ ok maybe I could use AD8051 or AD8052 for dual. what voltage reference could I use to have a stable 5v for the excitation voltage? i think my source is the reason why my values are unstable. I'm only powering my arduino from my laptop's USB port. \$\endgroup\$
    – Rhonald Rei Pahayac
    Dec 14, 2014 at 15:14

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