In the continuation of our blogs on current sense amplifiers, we will discuss how to configure the NCS21xR and NCS199AxR current sense amplifiers so that they output a precision current. In some applications the system data readout board is remotely located from the circuit that monitors and measures the system current.
The problem with this scenario is twofold: 1) a long transmission line length could cause large undesirable voltage drops between the output of the current sense amplifier and the input to the system data readout board and 2) stray ground resistance between the two boards will cause voltage errors. The precision output current measurements are read more accurately as it overcomes the errors due to ground drops between the boards and voltage loss across the length of the transmission line.
Figure 1. Simplified schematic for remote current sensing
As shown in Figure 1, the RIOUT resistor is added between the OUT pin and the REF pin to convert the voltage output to a current output which is taken from the REF pin to the readout board. This circuit is intended to function with low potentials between the boards due to ground drops or noise. The current output is simply the relationship of the normal output voltage of the NCS21xR:
A resistor value of 1 kΩ for RIOUT is always a convenient value as it provides 1 mA/V scaling.
On the readout board, for simplicity, RITOV can be equal to RIOUT to provide identical voltage drops across both. It is important to take into consideration that RITOV and RIOUT add additional voltage drops in the current measurement path. The current source can provide enough compliance to overcome most ground voltage drop, stray voltages, and noise. However, accuracy will degrade if noise or ground drops exceed 1 V.
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, it does not have to be 1 kΩ or match the RIOUT resistor value.
One other enhancement is possible for those familiar with design using difference amplifiers. Often the effect on accuracy and common-mode rejection using this method is minimal, but in any event, buffering the reference input as shown in Figure 2, will preserve the high performance of the NCS21xR current sense amplifier.
Figure 2. Unity gain buffer for the voltage divider created between the NCS21xR output and readout board
The circuit discussed in Figure 1 provides an easy way to convert the output voltage of your current sense amplifier to a current for remote sensing and so the next time your readout board is out of reach, no need to fret, convert that output voltage to a current with a few simple components.
Read our first blog in this series Current Sense Amplifiers: Input and Output Filtering or for more information on current sense amplifiers, please visit:
