Author Topic: Measure differential output with different ground  (Read 921 times)

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Offline Vindhyachal.takniki

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Measure differential output with different ground
« on: September 28, 2016, 04:09:39 pm »
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.
 

Offline Assafl

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Re: Measure differential output with different ground
« Reply #1 on: September 28, 2016, 06:10:45 pm »
A differential measurement doesn't really need a ground (except for noise shielding). The voltage is referenced between the positive and negative input. Voltage that is on both lines (common mode) is cancelled out by the amplifier.

You have to make sure you don't exceed the voltage input of the amplifier (or use a resistor divider).

You have to ensure one of the sides is floating. Otherwise ground becomes connected via mains or somewhere else. DMMs and other meters are floating, Hence you can connect them to any grounded or ungrounded voltage (within the safety rating).
 

Offline Zero999

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Re: Measure differential output with different ground
« Reply #2 on: September 28, 2016, 10:28:56 pm »
The problem is, without the 0V line of the differential amplifier connected to the load cell circuit, the common mode of the amplifier will probably be exceeded. If it's impractical to run another wire to connect the 0V of the amplifier to the load cell, then connect two 1M resistors in series across the + & - outputs of the load cell and connect the middle of the 1M resistors to the amplifier's 0V rail.
 


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