When GND isn’t GND single-ended circuits become differential
The system ground return, or GND, symbol is often taken for granted when drawing schematics. GND symbols are placed all over the schematic with the assumption that the different GNDs will all be at the same electrical potential on the printed circuit board (PCB). In reality, current flow through the GND impedance can create voltage differences between the GND connections on the PCB. Single-ended dc circuits are particularly sensitive to these GND voltage differences because the expected single-ended circuit is converted to a differential circuit causing output errors.
Let’s use the standard non-inverting amplifier circuit shown below as an example. When the GND potentials of the input source, VIN, and the input resistor, RI, are equal, the familiar circuit gain of 1+RF/RI applies. Therefore, the 100 mV input signal is multiplied by a gain of 10 V/V and the output is 1V.
In the circuit shown in the figure below, a voltage source, VGND2, has been inserted between the input source GND and the RI GND connection. The result is a modified transfer function with the addition of the VGND2 voltage multiplied by the inverting circuit gain of – RF/RI. A 10 mV difference in GND potentials reduces the desired 1V output by 90mV to 0.91V. This corresponds to a 9% relative error compared to the desired 1V output.
In the circuit shown below the circuit transfer function is further affected when the output voltage is referenced to a third GND potential, VGND3. The VGND3 voltage will directly subtract from the previous output transfer function. Therefore, a 20 mV VGND3 voltage further reduces the output voltage to 890 mV, corresponding to an 11% error compared to the desired 1V output.
The issues shown in the two examples above can be reduced by using proper PCB layout techniques to keep GND potentials of the circuit input source, input resistor, and output voltage at the same potential. The best solution is to place critical GND connections physically close to each other using a common “star” GND method. This will reduce the PCB impedance developed between the GND connections and will therefore reduce any voltage potential differences between them. In the example circuit schematic and layout shown below, the distances between the GND connection of the input source, output voltage, and input resistor are all located close together on the top layer of the PCB. This should prevent the single-ended circuit from becoming a differential one!
In summary, next time you’re having issues with dc circuit performance verify that the voltage potentials of all important GND connections are equal.
The Precision Hub is a multi-author source of contributed articles from Texas Instruments that focus on analog matters. This article first appeared on EE Times’ Planet Analog website.