Load cell signal chain

Hello,

I design a signal chain for a weight system.

I have up to 8 load cells on my system (0.55 mV/V at 5000Kg)

I want to be able to read each cell independently.

I have some questions concerning that:

1 - Sensor supply:

I know lower is the suppllower is the thermic disturbance of the load cell. So I want to supply my load cell with 5V.

Other benefit is that 5V is a common value for the ADC reference.

But it give me a poor signal sensitivity: 5V=> 1kg -> 0.55µV

Do you know a good documentation about the thermic disturbance? may be I can choose a bigger supply without problem and it will increase the sensitivity.

2 - 8 Channels:

Lot af ADC have integrated MUX, but not more than 3 (sometime 4) channels and for sur never 8 (differential).

That leads to the use of severals ADC. I prefere to have a reduced ADC and amplifier number.

That's why I want to use a multiplexer like ADG5409 (4:1) coupled with a converter like AD7731 or Ad7190.

( for example: 1 ADG5409 manage the  4 first differential inputs to the channel 1 of AD7190 and a second ADG5409 leads the 4 last differential signals to the channel 2 of the same AD7190.)

My question is: Is it a good idee to use a multiplexer for this function? (in therme of noise, signal disturbance,...)

I see the multiplexer add a Ron resitor (~100Ohm or less) but that is not a problem for me because I have a High impedane input after that (Amplificator AD621 or AD7190 directly). So for me the multiplexer will not perturb my signal. Do I think right this way?

What are the points I should take into consderations?

Thanks in advance for your help

Parents
  • Thanks for your help.

    First I would like to add an answer to my first question:How to choose the load cell supply voltage.

    I found a documentation but in french. So I will not give a link here. I can give the main result I found: The power dissipated into the load cell should be < 0.30 W / cm². For example my load cell are 9mm² and 350 Ohm. That lead to a PloadCell = 0.018W => 0.15 W/cm² at 5V exitation.

    Next point is the multiplexer that I want to use before the ADC. As  I read your answer, It sound not so stupide to use multiplexer on differential load cell signal lines. That a good point for me. What I need is more something like ADG709. Because of the differential lines I need (4:1)*2 multiplexer. The leakage current is very small (< nV for a load cell current of ~10mA) So the disturbance should be insignificant.

    I still have a question concerning the multiplexer: How is the schottky noise in the ADG709?

    Because this time I will have a junction in the line and this noise will be amplified. I need to estimate it influence.

    But I had not found any info concerning that in the spec af ADG709.

    And could you help me to find a good differential amplifier for this application?

    A gainof 100 is OK more than 1000 is unusfull.

    The most important (for me) is that the amplifier has a low noise and that it works with single supply at 5V.

    Concerning Bandwidth: lower is better. My signal is DC.

    Could you give me exemple of parts that correspond?

Reply
  • Thanks for your help.

    First I would like to add an answer to my first question:How to choose the load cell supply voltage.

    I found a documentation but in french. So I will not give a link here. I can give the main result I found: The power dissipated into the load cell should be < 0.30 W / cm². For example my load cell are 9mm² and 350 Ohm. That lead to a PloadCell = 0.018W => 0.15 W/cm² at 5V exitation.

    Next point is the multiplexer that I want to use before the ADC. As  I read your answer, It sound not so stupide to use multiplexer on differential load cell signal lines. That a good point for me. What I need is more something like ADG709. Because of the differential lines I need (4:1)*2 multiplexer. The leakage current is very small (< nV for a load cell current of ~10mA) So the disturbance should be insignificant.

    I still have a question concerning the multiplexer: How is the schottky noise in the ADG709?

    Because this time I will have a junction in the line and this noise will be amplified. I need to estimate it influence.

    But I had not found any info concerning that in the spec af ADG709.

    And could you help me to find a good differential amplifier for this application?

    A gainof 100 is OK more than 1000 is unusfull.

    The most important (for me) is that the amplifier has a low noise and that it works with single supply at 5V.

    Concerning Bandwidth: lower is better. My signal is DC.

    Could you give me exemple of parts that correspond?

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