• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 1810 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS circuit for load cell application

VT
Genius 3300 points

Other Parts Discussed in Thread: ADS1230

1. I have a application for measuring load cell output. Selected ADS1230 for this. Attached find the circuit diagram. Can you review if its ok?

2. Entire ckt is powered from 3.3V supply. 

I have pulled down CLKIN by 10k since i m using internal clock. REF design uses 47k, I have put 10K, is it ok? 

3. I have placed a 100ohm/0.01uF RC network at inputs, AINP & AINN, is it necessary or should I remove it?

4. Any other point

3666.ckt.pdf

  • 0
    TI__Guru** 113765 points

    Hi Vindhyachal,

    Regarding (1), your schematic appears to be ok. (2) The 47k resistor was used in case an external clock was to be applied, so the larger value of resistance was used to limit any possible clock loading.  If you will always use the internal clock then the CLKIN pin can be connected directly to DGND...or you can use the resistor you are currently using.  (3) In many cases you might need some input filtering to remove external noise prior to the gain stage.  I would leave them in.

    You might consider using the 5V source for the analog supply and as the excitation/reference source.  This will increase the output of the load cell and possibly increase scale resolution.  It really depends on how noisy the 5V source is relative to the 3.3V source.  Also,  you may want to consider adding some RC filtering to the REFP/REFN inputs to more closely match the RC of the analog inputs to keep the measurement ratiometric for noise as well as for the excitation drift as it is now.

    Layout consideration is very important as you must keep any digital signals away from the analog input signals.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Genius 3300 points

    Bob,

    1. I have removed the clkin resisotr & attached the circuit.

    2. What type of RC filter to add at REF pins. Do you have any example circuit for this?

    3. Any C sample code for interfacing it with microcontroller?

    ads.pdf

  • 0 in reply to VT
    TI__Guru** 113765 points

    Hi Vindhyachal,

    Regarding the filter structure for the reference, it would look similar to the analog input filter.  What may differ are the values which greatly depend on the load cell you are using as the resistance of the load cell is also a part of the filter.  Unfortunately we don't as of yet have a good document to cover the filtering for bridge circuits.  I would suggest you look at SBAA201 that discusses ratiometric filtering for RTDs.  Many of the same considerations apply. 

    Here is the link to the code used for the ADS1230REF.  Keep in mind this code uses freeRTOS so it may be a little hard to follow.  The ADS files are most pertinent.

    ftp://ftp.ti.com/pub/data_acquisition/ADS123xREF/ADS123x_CDROM/Firmware/Firmware_Source_Code/Source_for_123XREF_firmware-1.1.0d.zip

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Genius 3300 points

    Hi Bob,

    There is slight modification in the specs. I have attached the updated schematic. Now supply of load cell is different. It is fixed +10V & max differential output is +-36mV.

    Now I cannot short the ground of both the supplies. Because from load cell system only two wires are coming & this is signal +-36mV.

    How do I measure the voltage using ADS1230 in this case? As without ground shorting there will be no common voltage to measure signal.

    But I checked the multimeter operation. In this black probe is not shorted with baterry ground? Then how does it measure the signal when there is no common ground.
    On other hand while measuring with CRO, we essentially short earth with one of signal reference.

    ads2.pdf

  • 0 in reply to VT
    TI__Guru** 113765 points

    Hi Vindhyachal,

    You are attempting to increase the output level of the load cell by increasing the excitation voltage.  This will not work for a couple of reasons.  On reason is the analog inputs relative to ground exceed the absolute maximum ratings of AVDD + 300mV.  The load cell when balanced will be 1/2 of the excitation voltage (10V/2 = 5V).  Even though the difference voltage may be 0V, the input relative to ground will be 5V. 

    The second reason this will not work is the common-mode is violated with 5V.  The DC input relative to ground must be AVDD-1.5V to AVSS+1.5V.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Genius 3300 points
    Hi Bob,

    Since supply of load cell (10V) and ads(3.3V) are different. Can I level shift the signal using 3.3V supply before feeding it to ADS input?
  • 0 in reply to VT
    TI__Guru** 113765 points

    Hi Vindhyachal,

    You can level shift the output, but the process of doing so will add additional noise. Another option would be to use a bipolar supply arrangement for the excitation where the output of the load cell is within the correct common-mode.  Something like EXC+ is +6V and EXC- is -4V.  That is not exactly the correct values, but you can see the general idea.

    Best regards,

    Bob B