Monday, 6 March 2017

How to achieve common ground for a single ended input?


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I'm using a current loop transducer as in the set up picture I provided. The transmitter is regulating the current proportional to a physical quantity and regulating the current. It is a typical 4 to 20mA loop. We have a data acq. board which can read voltages so we read the voltage from 250 ohm resistor's ends. We use single ended input(red colored in my picture). Input amplifier is part of the data acquisition board. From R we use a BNC cable to connect to the daq board.


But in this link: http://www.ni.com/white-paper/7113/en/ (look at Figure 5) my setup seems to be not recommended. Isn't my setup common grounded?




Answer



Figure 5 is really saying that having two grounds on the system can cause problems. Those that do have two seperate grounds are fed into differential amplifiers which effectively circumvent the problem of multiple grounds.


Figure 5 has two columns and the diagram that has the cross through it is in the column marked "ground referenced signal sources". If your measurement device has galvanically isolated signals from it's metal casing (if it has a metal case) then there should be no problem - it falls under the column marked "floating signal sources".


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