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In the MAX1979 datasheet, I see two types of ground specified, analog and power ground. My question is: what exactly is the difference between these grounds (noise? if so, which is noisier?), and more specifically, is the power ground an input to the IC? If so, how do I keep it separate from the analog ground? Or is just an output that I am supposed to use, as it is said that it has internal synchronous rectifier ground connections?

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In circumstances where there are a number of components that need to make a "0V" reference connection, it's best to group these together and call it a different name (not power ground) and make one single connection to power ground from this collection of components.

This prevents power_ground_currents causing small mV differences in the 0V reference because the current will not pass through the reference connections. Normally, the common link for the 0V reference supply and the power ground will be made close to the incoming power feed.

Sometimes the two "ground" connections can be isolated without interconnection but I can't see that this is the case with your device however, neither can I see anywhere in the data sheet that tells you how to definitively make the connection between the two.

The power ground will always be the noisier because it is likely passing much, much larger currents than what I refer to as the reference 0V.

It's more common to have this problem in audio amplifiers where the speaker return wire makes its return connection through the 0V on the input stage instead of heading right back to the battery or power 0V connection. The current that flows in the speaker creates small ac volt drops on the sensitive input circuits and suddenly the whole thing just oscillates instead of being stable!

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The datasheet is recommending to make a star ground, and avoid power currents flowing through analog ground and offsetting analog signals. In practice, that means that PGND pins are connected to one copper pour (or ground plane), AGND pins are connected to another, and the copper pours are connected together at just one spot.

You can refer to the MAX1978 evaluation kit. Schematic on p.9 shows that the grounds are connected together (top left corner). You can also examine the layout on pp.11-12; it's proposed by Maxim themselves.

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Power ground and (digital) ground are used for analog and digital sources. I'm not the biggest expert at this but i can tell that analog ground most of the time must have a steady voltage (no noise)

If you have a lot of digital (fast-switching) componenents. The digital 'noise' from all switching components can influence the steady analog (power supply) voltage and add noise to it.

Both the digital and analog ground are the same ground plane. However, most of the time they only are connected at a single point (at input-power connector or something).

Therefore, they are seperated and can't interfere each other easily. But like i said before. They must be connected together at least one point to keep the same ground potential.

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