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I always regard VSS as gnd when there is only one exists in the schematic, but if they both exist, like this schematic, in the schematic, can I just regard VSS as gnd? Because VSS and ground should connect to the ground, but if they all connect to the ground, why are there both VSS and GND in one schematic?

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    \$\begingroup\$ There are signal grounds(analog and digital), power grounds and Earth ground, and they are not the same. They may meet at a controlled point directly or by an RC connection. Engineers choice. \$\endgroup\$ – Sparky256 Apr 26 '18 at 3:21
  • \$\begingroup\$ In this particular case the pin labeled as "Vss" seems to be connected to Vbat power of the battery. Connecting it to ground might start some smoke and even flames... \$\endgroup\$ – Ale..chenski Apr 26 '18 at 3:26
  • \$\begingroup\$ @AliChen you said "Vss" seems to be connected to Vbat power of the battery,well,that symbol should be "VDD" \$\endgroup\$ – XM551 Apr 26 '18 at 12:08
  • \$\begingroup\$ @Sparky256 I don't understand,so according your saying,can they just all connect gnd? \$\endgroup\$ – XM551 Apr 26 '18 at 12:09
  • \$\begingroup\$ Signal and power grounds often connect at a power connector where you may find large capacitors and voltage regulators, but the connection to Earth ground is not always direct. Sometimes a 'high impedance' connection is used with a resistor and capacitor. Chief Engineers choice. \$\endgroup\$ – Sparky256 Apr 26 '18 at 20:52
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If an engineer is pedantic enough to provide both VSS and GND on the same schematic, you can be assured that there is a reason. In this case, your best guide is in the top left corner of the suggested use in the schematic, where the VSS pin of the chip is connected to a 36nF capacitor to... GND. That indicates that the two are not the same.

OK, so what are they? As @Sparky256 mentioned, different grounds are labelled different things on a circuit to describe their uses. His best example is analogue versus digital grounds. If you treat them the same throughout the circuit, then digital noise can seriously affect the analogue waveform (think distortion in an audio amp). The two should be treated as separate, and only unified at one point in the circuit - usually with all sorts of filtering. So why unify them at all? Safety. If they stayed completely independent, it could theoretically be possible for audio ground to be tens (if not hundreds!) of volts different from the rest of the circuitry, providing all sorts of chances for sparks.

In the provided schematic, there are three distinct grounds shown - some inside the chip, others outside. I'll call them "triple-line" triangle, "outline" triangle (lower left), and "low bar" - which is the opposite of "high bar", the positive supply (upper right). Different parts of the chip's insides use the different grounds: my favourites are both DRVN and DRVP, which show "low bar" and "triple-line" as distinct inputs right next to each other.

But take a look at the "outline" triangle in the lower left (GNDA). It is only (apparently) used inside the chip as a ground for the Peak Detector, and the schematic shows that it is OK to tie it to circuit GND. Note the chip explicitly shows (top right) that "triple-line" triangle inside the chip (GNDP) should also be tied to circuit GND - "triple-line" inside the chip corresponds to "triple-line" outside the chip. So why bother with a separate "outline" triangle symbol? Why not use "triple-line" there too?

Maximum flexibility. By keeping the separate grounds separate, and providing pins for each of them, the same chip can be used in a variety of different circuits, where GNDP, VSS and indeed GNDA are distinct, and the chip is documented to guarantee their separation. Note that this means your circuit needs to connect to all the different versions of ground pins, even if to the same GND, to ensure proper operation of the chip.

But what does this mean for you? Think of this schematic diagram as a suggested use. The part inside the grey box (inside the chip) of course remains static. But the support circuitry outside can be changed as necessary for your application. It explicitly demonstrates that GNDA can be tied to circuit GND, as well as GNDP. It doesn't say the same for VSS though - that's why there's a 36nF capacitor to circuit GND.

Can you treat VSS and GND as the same thing though? For this circuit, probably - but then the 36nF capacitor is unneeded and a waste (GND-to-GND does nothing). But as a general rule (your title question is a general one) the answer is "you shouldn't, without careful analysis of the difference" (like I did above).

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In a more general case, circuit designers commonly use Vss to mean “the most negative voltage the IC will ever be subjected to.” Which for typical IC technologies will be the substrate connection.

This IC is no exception. Notice the substrates of the two mosfets at the top.

In many designs, the most negative voltage is ground. But that is not always the case.

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  • \$\begingroup\$ I like that way of thinking about it! I hadn't thought of it like that before - but I will now! I assume VDD implies "the most positive voltage"? The (confusing) definition that VSS means "source" and VDD means "drain" always bothered me - they're talking electron, not conventional, directions. \$\endgroup\$ – John Burger Nov 9 '18 at 6:21
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    \$\begingroup\$ @JohnBurger in my mind they lost that meaning a long time ago. Just like Vcc did. Strangely enough, nobody seems to use Vee anymore. \$\endgroup\$ – Edgar Brown Nov 9 '18 at 6:24

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