How to deal with AGND and DGND on ADCs

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#1

In most ADC (Analog to Digital Converter) applications, you may end up confused about how to deal with analog ground (AGND) and digital ground (DGND) pins. Should you split these 2 grounds in circuit design?

In most situation, splitting your ground plane can cause more harm than good. Splitting the ground plan can increase inductance values for your return current. By the equation, V = L (di/ dt), when the inductance is increases, the voltage noise is increasing as well. Therefore it is better to keep both analog and digital grounds connected unless you have a reason to split them.

In common applications, you may also find that AGND and DGND should be tied together outside the chip with the lowest impedance connection and minimum lead lengths as possible. These 2 pins cannot be connected with the chip, because the noise present in the digital channel may enter the analog circuitry and affect its performance.

But when the system digital supply is very noisy or the switching digital signal is too fast, such as ADI’s AD977 powered by a noisy supply, it’s recommended that the ADC’s digital ground pin DGND should be connected to the system digital ground and be separate from its AGND pins.

For more grounding technical information, please check out further posts here:

  1. Grounding Data Converters and Solving the Mystery of “AGND” and “DGND” - Analog Device