Measure differential output with different ground

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Sep 28, 2016.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    349
    6
    1. I have a application for measuring load cell differential output. Load cell is powered from 10V from a differnet supply while ADS1230 is powered from 3.3V which is different ground.

    2. On measuring output of load cell using multimeter, it comes out to be +-36mV.

    3. I have attached a circuit using ADS1230. Obviously it will not work since differential output have no common ground with ADS1230.
    What if I short negative differential output of load cell with ground of ADS1230. Can I do that.
    Again voltage will not be CMR range of ADS1230 which is AVSS+1.5V to AVDD-1.5V
    So I am planning to use ADS1248 for this purpose & will use +-supply for it. In this ADS1248 can be used.

    4. One thing I want to know, can I somehow use ADS1230 in this application. provided cannot short two different grounds.

    5.Other thing is how multimeter operation works. 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.

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  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,684
    2,747
    You are using a 20 bit A/D, yet you will never get close to this in terms of accuracy and drift. Why? Because you've got a sloppy Vref (the LDO) which is entirely disassociated with the excitation voltage of the load cell. You will have considerable noise (and drift) both due to the 10V cell supply and the 3.3V "reference".

    You'll do *much* better by driving the cell with the 3.3V (FSO will be 33% lower, of course), but reference noise and power supply drift will be entirely eliminated as sources of error. You should also use a divider on the 3.3V supply to lower the reference voltage so as to maximize the dynamic range.

    This will solve all of your problems as stated, and improve overall performance dramatically.
     
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