The current drawn by the KEY, approximately 40mA, is interfering with the 4-20mA current loop calibration. How can I fix this?
A. In the 4-20mA applications the KEY must be isolated from the 4-20mA circuit when making a current measurement, so that current drawn by the KEY will not interfere with the 4-20mA current loop. There is a simple and most effective way to achieve this: use a separate 5V power supply to power the KEY, and use relays (not ICs) to isolate the KEY from the 4-20mA current loop when measuring the 4-20mA current. The GND of the KEY power supply and VSS of the signal conditioner must be shorted together (through a relay). The KEY’s GND and DIO lines must be switched to isolate the KEY from the 4-20mA circuit. The system will work in the following manner. During calibration, when communicating with the signal conditioner for writing to DACs or flash memory, close the relays for the KEY to communicate. Open the relays when measuring 4-20mA current so that the KEY is completely isolated from the circuit. Remember that the KEY must always be powered, and the VDDIN and VDDOUT lines from the KEY to
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