Erik:

I believe you are thinking from the wrong end. The sensor is connected by only two wires and the current of the sensor module better be in the range of 4 to 20mA. The reason for the minimum of 4mA is that this is the current needed to bias and operate the sensor. The 20mA loop current would be the maximum on the loop to correspond to a full scale reading of the sensor.

The target end for the sensor at Raghunathan's USB device does not necessarily need to be powered from the sensor loop supply. As a matter of fact it is probably already powered from either the USB 5V or from 3.3V local supply. A more robust USB data acquisition module would use a local supply such as +3.3V so that it could filter and control the voltage carefully. The 5V coming from the USB cable is likely not very well controlled and may even be a little over 5V.

On 4-20mA loop devices it may very well not be correct to show minimum reading on an open and maximum reading on a loop short. There is plenty of opportunity for the 4-20 respondent device to detect < 4mA and report an open condition and > 20mA and show a short or overrange error condition.

Michael Karas