Current and power measurement

MonoDAQ-U-X allows convenient current and power measurement thanks to the internal current sensings resistors (05R and 1K) on the CUR+ and CUR- pins. Combination of shunt values and ADC input ranges provides a wide selection of current measurement ranges from 100 uA to 2 A. This article will explain the internal wirings and limitation of the CUR+ and CUR- pins. Examples of a current loop and battery wiring will be shown for each input type.

Voltage limitations

It is important to note that CUR+ and CUR- pins have a limited voltage potential range with respect to GND within which the current measurement will be successful. Each of the pins should always be within the -0.1 … +5 V potential against the GND pin. If the voltage of the pins is out of that range, the measurement will not be correct (there will be high leakage currents).

The voltage limitations mean that there can be higher current measured in the positive direction then in negative direction. The table below gives the highest measurable currents depending on the shunt resistance value (providing that a high enough ADC range is selected):

	0R5	1K
Maximum positive current	500 mA continuous, 2 A peak	4 mA
Maximum negative current	200 mA	0.1 mA

The exact wiring of the application circuit to the U-X should be planned according to those limitations. Some examples will be given in this article.

Absolute maximum voltage for the CUR+ and CUR- pins is +24/-0.7 V w.r.t. the GND pin. However, absolute maximum voltage rating defines the maximum voltage that can be applied without any damage to the U-X and it is not the maximum voltage for successful measurement.

0R5 shunt

0R5 is meant to be used for the continuous currents of up to 500 mA. When 500 mA range is selected, the ADC range is in fact +-1 V, which allows +-2 A of current to be captured without saturation, but this should only be used to analyze current peaks. Continuous current operation should be limited to 500 mA. When the 200 mA range is selected, the ADC operates at the 100 mV range and there is no special limit on the continuous current.

There are two options provided by the U-X internal wiring for the 0R5 shunt: floating and non-floating. Whenever the floating option is selected under the Type column, it is advisable to connect the GND pin to the part of the measured circuit to define the potential of the CUR+ and CUR- pins. Non-floating shunt option already has an internal 1k resistor between CUR+ and GND, so the two options can be used depending on the primary direction of the current loop, as shown below.

0R5 shunt, floating, current loop

0R5 shunt, floating, battery power measurement

0R5 shunt, current loop

0R5 shunt, battery power measurement

Note that all of the above battery wiring diagrams include differential voltage measurement of the battery. Once the potential of one of the CUR pins is defined, it is straightforward to use the voltage input pin in single-ended mode if we are looking at the voltage w.r.t. the GND pin. It is also possible to wire the battery and load the other way around, keeping in mind the voltage limitations of the CUR pins.

1k shunt

1 kOhm shunt is meant to be used for very low currents in the uA range. There are four possible input ranges of +-100 uA, 1 mA, 2 mA and 4 mA. 1k shunt input is not available in the floating configuration, since the shunt is internally wired between the CUR+ and GND pins.

1k shunt, current loop