How to configure NCS21xR and NCS199AxR current amplifiers

Regarding current sense amplifiers , this article will talk about how to configure the NCS21xR and NCS199AxR current amplifiers to output accurate currents. In some applications, the system data reading board is far away from the circuit that monitors the system current.

There are two problems with this situation: 1) The long transmission line length causes a large, unwanted voltage drop between the output of the current sense amplifier and the input to the system data readout board; 2) The stray ground resistance between the two boards creates voltage errors. The precision output current measurement is read more accurately because it overcomes the errors caused by the ground voltage drop between the boards and the voltage losses in the transmission line.

Current Measurement Circuit Board: Current Measurement Circuit Board

System Data Readout Board: System data reading board

Line Receiver: Line Receiver

Stray ground resistance between boards: Stray ground resistance between boards

Figure 1. Simplified remote current sensing circuit diagram

As shown in Figure 1, the RIOUT resistor is added between the OUT pin and the REF pin to convert the voltage output into a current output that flows from the REF pin into the reading board. This circuit is designed to operate with a low potential between boards due to ground drop or noise. The current output is only related to the normal output voltage of the NCS21xR :

A 1kΩ resistor value is always an easy value to reach for RIOUT because it provides a 1mA/V scale.

On the readout board, for simplicity, RITOV can be equal to RIOUT to provide the same voltage drop. It is important to consider that RITOV and RIOUT add additional voltage drops in the current measurement path. The current source provides enough flexibility to overcome most ground drops, stray voltages, and noise. However, if the noise or ground drop exceeds 1V, accuracy will be degraded.

To increase the dynamic range of the application, the RITOV resistor value on the readout board can be selected from a wide range of resistor values ​​and does not have to be 1 KΩ or match the RIOUT resistor value.

For those skilled in designing with differential amplifiers, further enhancements are possible. Typically, the impact on accuracy and common-mode rejection using this approach is minimal, but regardless, buffering the reference input as shown in Figure 2 will maintain the high performance of the NCS21xR current sense amplifiers.

Output pin: Output pin

REF pin: REF pin

Op Amp: Operational amplifier

Figure 2. Unity-gain buffer for voltage divider circuit between NCS21xR output and readout board.

The circuit of Figure 1 provides a simple method to convert the output voltage of a current-sense amplifier to a current for remote sensing, so the next time your readout board is far from the monitoring circuit, no need to fret; just use a few simple components to convert the output voltage to a current.