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  • TI Thinks Resolved

ADC for 10V load cell

Prodigy 90 points

Replies: 8

Views: 659

Hello, 

I am designing a system to measure for load cells with the following characteristics:

- Sensitivity of 2mV/V

- Accuracy class of 3000 

- Input Voltage of 10V

- Max load 20kg

- Min load 10gr

- Only Force in one direction is measured

The system is power with 24V. I am using two LMR14203: One for the Analog circuit (10V)  and other for the Digital Circuit (5V).

So far I am using INA826 in my simulations to amplify the load cell voltage and everything works nice.

My concern is about the ADC conversion. I was thinking in an ADC with 20bit with ratiometric measurement, but I don't find any ADC with

an Analog Input Ref Voltage of 10V.  What do you think about everything? Is there and ADC with 10V analog ref?

Can I use ratiometric measurement in some way with an ADC with 5V analog ref?

Thank you so much for your help!

  • I don't know if a voltage divider feeded from the load cell input, so as to get an analalog ref of 5V, it's a good idea...
  • In reply to Daniel Casale Linde:

    Hello Daniel 

    What ADC have you been using thus far?

    If you are looking for a sample and hold ADC, commonly known as SAR ADCs, it is not common to use a 20 bit SAR ADC in a ratio metric reference design. This is considered high precision devices, and need the reference input to be clean and stable. 

    It is not necessary for the device  to have a reference of 10V to obtain an input range up to 10V, thus I doubt you will find any ADC with such a high reference input; devices are designed to be able to use a 2.5V or sometimes 5V reference to be able to extend their input range to 10V.  You will need to focus on the driving input circuit design of the ADC, as the reference for your application will not be able to be used for a ratiometric design. 

    There are ADC input designs in the ADC Cookbook that I think will prove helpful. 

    Regards, Cynthia 

  • In reply to Cynthia:

    Hi Daniel,

    I just want to add to a few comments to what Cynthia has already mentioned. TI has precision Delta-Sigma ADCs designed specifically for bridge type sensor applications (that would include load cells). 24-bit resolution devices include the ADS1231, ADS1232, ADS1220, and ADS124S06 to name a few. All of these devices are capable of using a 5V reference that can also be the excitation source for the load cell (which would then provide for a ratiometric measurement). The devices also include an internal PGA so that an external amplifier is not necessary.

    You should be able to use 5V instead of the 10V as your reference/excitation source and still achieve good results. There is a good design example in the ADS1220 datasheet for bridge measurements that can be used as a guide for calculating your desired performance. You should be able to follow the same calculation by inserting your sensitivity and capacity to determine if the scale resolution you desire can be achieved. Noise will be the determining factor. In particular look at equation 42 on page 59 of the ADS1220 datasheet.

    It is not clear what desired scale resolution is required, but based on the lowest value you show it is 10 gram on a 10kg scale. From equation 42 and supplying the values determined for a 2mV/V sensitivity and 10kg load cell, you should be able to achieve better than 1g resolution using the settings shown in this application.

    The other devices will be calculated the same way using the appropriate noise tables for the calculations.

    Best regards,
    Bob B
  • In reply to Bob Benjamin:

    Hi Cynthia, hi Bob:

    First, thanks for the link of books. I found there some good books.

    I took a look at those ADCs before, but the 10V for the load cell is a requirement.

    Then, I guess the best option is to use a precise voltage reference of 10V for the bridge, and another precise voltage reference of 5V or 2.5V for the ADC... Am I right?

    For the ADC I could use a REF5050 or a REF5025.
    For the bridge, which has a 400 Ohms +-20ohms input resistance..., it is needed around 27mA. Maybe I can use something like ATL431LI?

    I need an ADC with AREF - , because I have a small offset in the INA REF- because of the voltage swing.

    So, If there is no a better option for this application.... which ADC and voltage reference could you recommend me?

    Thank you.

    Best regards,
    Daniel C
  • In reply to Daniel Casale Linde:

    Hi Daniel,

    There are some great advantages using a ratiometric measurement as the noise and drift of the excitation source cancels for load cell measurement.  Also, when using the internal PGA of an ADC the input referred noise lowers with applied gain.  Using an external amplifier will not do this.

    When you add an external amplifier with a a non-ratiometric measurement, you add external noise, drift (both offset and gain) of the amplifier the excitation source and the ADC reference.  It is unfortunate that the 10V excitation is a requirement as this complicates things considerably.

    Your request for components to use is difficult to answer without some more specific information.  It is like asking for a good soup without knowing what you like for flavors or ingredients.  You mentioned the load cell sensitivity and the requirement for 10V excitation, but how often do you want to receive new conversion data (or the data rate output of the ADC) and what scale resolution are you trying to achieve (or minimum stable scale resolution)?

    Best regards,

    Bob B

  • In reply to Bob Benjamin:

    Hi Bob,

    Conversion time is not a huge problem: 1 time per second is okay. The scale resolution: 1 grame.

    I may ask something stupid, but with this low conversion frequency... if I use a two resistors and a buffer to get the Vref+ of the ADC, it would be a radiometric measurement also, right?
  • In reply to Daniel Casale Linde:

    Hi Daniel,

    You are looking for 20000 noise-free scale counts to achieve 1g for a 20kg load cell. You could use a voltage divider to create your reference from the 10V excitation supply, but it will not be truly ratiometric as there will be noise and drift of the resistors, and if you use a buffer there will be additional noise, offset and gain drift of the amplifier.

    Are you absolutely sure you are required to use 10V excitation? The circuit becomes much less complicated if 5V is used. No voltage dividers or external amplifier is needed. You should be able to use 5V and the ADS1231 to achieve the desired resolution.

    One of the important considerations when measuring the output of the load cell is repeatability of the measurement. Every device you add to the system will add error and reduce repeatability.

    With 10V excitation (referenced to ground), you will have a common-mode voltage of 5V (mid-excitation supply). The ADC will have a measurement range from 0 to 5V for an analog supply of 5V. This will require the amplifier to have a common-mode input range that allows for the 5V+/-10mV output of the load cell. The amplifier will need to shift the output into the common-mode range of the ADC. This will require another supply voltage.

    Or you could use 5V for excitation, analog supply and reference and the internal PGA of the ADC. This greatly reduces complexity and cost while increasing the level of precision.

    Best regards,
    Bob B
  • In reply to Bob Benjamin:

    Hi Bob, 

    After a few days of studying the problem we concluded that is going to be easier and cheaper to change the 10V requirement. 

    I have been comparing several ADS and the ADS1231 is a good recommendation.

    Thanks for your help.

    Sincerely, 

    Daniel C

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