Customer Support Recommended – Appnote on Implementing the Force-Sense Kelvin Connection

The use of separate force (F) and sense (S) connections (often referred to as a Kelvin connection) is a common requirement in the PCB design. The separate force (F) and sense (S) connection at the load eliminates any errors resulting from voltage drops in the force lead. The Kelvin Sense connection is routed by separating the sensing signals (S) from the lines, and delivering the power to the load (F). This type of connection prevents noise related problems in a closed loop system because it allows for more accurate measurement of the sense voltage at the load.

Consider the following figure:

A long resistive PCB trace is still used to drive the input of a high resolution Analog-Digital Converter (ADC), with low input impedance. In this case, however, the voltage drop in the signal lead does not give rise to an error, as feedback is taken directly from the input pin of the ADC and returned to the driving source. This scheme allows full accuracy to be achieved in the signal presented to the ADC, despite any voltage drop across the signal trace.

The requirement is to implement this at the schematic created using Allegro Design Entry HDL (DEHDL) to drive the PCB board created using Allegro PCB Editor so that both Force and Sense signals can be identified and constrained independently, and still allowed to be physically shorted in layout.

Flow Overview

This flow is based on a special logical symbol, which is created and saved in a library. The force sense library symbol(s) has shorting schemes defined within the symbol definition, which allows the engineer to seamlessly define the nets to be force sense. When placed in a schematic, the shorting scheme will short at least two sense lines to a force line. While packaging the schematic, separate nets are generated for the Sense and Force lines which are passed on to the PCB board file. As shown in the image below, four sense lines are connected to a force line using the library symbol. Inside PCB Editor, a symbol gets placed, and defines the location of the short for force and sense signals.

The pins of the schematic symbol will have a unique property called PIN_SHORT whose value consists of the logical pin names. While packaging the schematic (running File > Export Physical), based on the <project>.CPM directive, the Packager-XL(PXL) acknowledges the PIN_SHORT property value and creates a NET_SHORT property with the value containing the physical net names connected to the logical pin names.

When you look at the PCB Editor DRA symbol for the footprint, you will see that the pins with different pad stacks are placed at the same location.

Constraint Assignment

This flow allows for individual net constraints to be assigned and used in the front to back flow. As an example, Max Propagation Delay and trace width can be defined.

Refer the following AppNote for the detailed procedure used to implement the Force-Sense (Kelvin) connection using Allegro Design Entry HDL (DEHDL) & Allegro PCB Editor.

Click here for the AppNote.

Note: The above link can only be accessed by Cadence customers who have valid login credentials for Cadence Online Support (https://support.cadence.com/).

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