SNAS781E October 2020 – July 2022 REF70
Current flowing through a PCB trace produces an IR voltage drop, and with longer traces, this drop can reach several millivolts or more, introducing a considerable error into the output voltage of the reference. A 3000-mil long, 15-mil wide trace of 1-ounce copper has a resistance of approximately 100 mΩ at room temperature; at a load current of 10 mA, this can introduce a full millivolt of error. In an ideal board layout, the reference must be mounted as close as possible to the load to minimize the length of the output traces, and, therefore, the error introduced by voltage drop. However, in applications where this is not possible or convenient, force and sense connections (sometimes referred to as Kelvin sensing connections) are provided as a means of minimizing the IR drop and improving accuracy.
Kelvin connections work by providing a set of high impedance voltage-sensing lines to the output and ground nodes. Because very little current flows through these connections, the IR drop across their traces is negligible, and the output and ground. The REF70 has kelvin connection capabilities due to its output force (OUTF) and input sense (OUTS) connection as shown in Basic Reference Connection. The output force voltage will vary upwards from the internal VREF voltage to ensure that at VOUT, which is where the OUTF and OUTS connect at the point-of-load, the voltage will be precisely VREF. The sense connection on the REF70 requires 4 mA due to its architecture.
It is always advantageous to use Kelvin connections whenever possible. However, in applications where the IR drop is negligible or an extra set of traces cannot be routed to the load, the force and sense pins for VOUT can simply be tied together close to the pins, and the device can be used in the same fashion as a normal 3-terminal reference.