# Resistor grounded at both ends

Reading the datasheet of the MAX17055, I found on page 11 the following example circuit: It clearly shows a ground symbol at the CSP pin. As far as I understand, the IC directly connects to an external load with SYSPWR to VCC and SYSGND to GND. Since this would imply that the sense resistor is also grounded at the other side, its voltage sits at 0V. The resistor's purpose is the measurement of current flow across itself. But how is this possible when both ends are already at 0V?

## 2 Answers

"SYSGND" is only ground by name (with respect to the power supply ground). Truth of the matter is that it isn't directly connected to the ACTUAL ground (of the power supply - the battery in this case). Instead it connects to the ACTUAL ground through the resistor which has a resistance of 0.010 ohms. This means that when current is flowing from the negative side of the battery into the circuit (electron flow, not conventional current flow) there is a very small voltage drop across the resistor proportional to the current being drawn by the circuit. This IC measures this tiny voltage drop and uses it to calculate the current. For all practical purposes, however, this voltage drop is so low that "SYSGND" is close enough to the actual GND that it is considered "GND".

• So when designing a PCB, do all the system's GNDs connect to the ground plane, and the ground plane connects through the sense resistor to the battery's negative terminal? – Overblade Mar 30 '18 at 20:52
• It depends on what the IC is measuring. If it's measuring total system current, then yes. – DerStrom8 Mar 31 '18 at 2:29

It isn't grounded at both ends. The intent is for the system to be grounded "through" the Rsense resistor, so that it can measure the device's current draw as a voltage across that resistor.