4-Wire Current Transmitter on (Earth) GND-referenced 3-Pin Receiver?

HI AP,

On a 4-wire power - isolated transmitter (as shown below), the sensor and transmitter are isolated from the power supply, and the receiver can sense the differential voltage across the resistor.  Since the power supplies are isolated, the difference in voltage between the receiver supply and transmitter supply is not an issue. Is your question discussing a 4-wire power-isolated transmitter?  Can you provide an schematic/diagram of the 3-pin receiver and the schematic/diagram of the 4-wire current loop transmitter you are discussing? 

Thank you and Regards,

Luis 

Hi Luis,

Thanks for taking a look at this. The figure in your post is fine for my question. The question is about the ADC device specification regarding it's IN+ and IN- inputs (which are still voltage inputs). It seems that the voltage difference between the isolated transmitter ground and ADC ground is unknown/undefined. I guess internal diodes to the ADC supply rails would not be an issue because of the isolated transmitter. But what about the ADC common mode voltage? As an example, the ADC datasheet may specify a maximum input common mode voltage [(IN+ + IN-)/2] of let's say 10V relative to the ADC ground(!). If we assume a current of 20mA and a sense resistor value of 100R we would get 2V accros the resistor (i.e. the differential ADC input voltage). But what about the voltage between IN+ and ADC ground (which we need to look at for checking if we are still within the ADC input common mode voltage range)? Do we need an additional (external) connection between IN- and AND ground (since otherwise the voltage difference of isolated transmitter ground and ADC ground would be unknown/undefined)?

Regards,

... it should have been: Do we need an additional (external) connection between IN- and ADC ground?

Hi Paul,

If your ADC allows common-mode voltage to GND,  would it be possible place a weak 1MOhm pull down resistor from RSENSE to GND?  If you need the RSENSE common-mode at mid voltage between the ADC supplies, is it possible to place a 1MOhms pull-up and 1MOhm pull-down resistor divider between the ADC  supplies connected to the terminals of the RSENSE resistor?

Thank you and Regards,

Luis

Hi Luis,

the ADC supplies are typically not available at the receiver I/O connector, while the receiver = ADC GND signal typically is.
So it would be possible to add an (external) resistor between the ADC IN- side of the sense resistor and the ADC GND.
But why a resistor at all? What about the idea of connecting a direct wire between ADC IN- and ADC GND? Anything bad that might happen here?
For a transmitter current of 20mA and a sense resistor of 100R, the differential measurement voltage between ADD IN+ and IN- should still be 2V (as an example) and the common mode voltage at the ADC inputs would be (2V + 0V)/2 = 1V, which should be fine for most ADCs. So the additional direct wire seems to be a simple solution for my understanding. Anything bad about this additional direct connection that I did not notice?

Regards,