But in any circuit design we should have ground or reference.
This is NOT a true statement. The use of "ground" whether it be an "Earth ground" or a power-line "neutral ground" or a "reference ground" or any other kind of ground has absolutely no purpose in any circuit design other than as
one of a pair of points between which electrical potential can be measured... for whatever purpose the designer desires or for no purpose at all.
Your incorrect statement can be disproved with just one counter-example, although there are many other counter-examples. The first one that comes to my mind is a simple hand-held flashlight consisting of a case, a battery, a lamp or LED, and a switch to turn the lamp or LED on and off. The battery, lamp or LED, and the switch are connected in a series circuit design with nary a "ground" anywhere in sight.
At this point in your learning, the ONLY thing you need to know about "ground" is this: ALL points identified as being the SAME "ground" will have ZERO potential between them. Such a collection of "ground" points may or may not be identified as such on a schematic, but any one of them is a reference point for measuring potentials in a circuit with respect to "ground."
In the real world there are many types of "ground". The terminology is ancient, going to at least as far back as the days when land-line telegraphs had just one wire strung between telegraph poles and a second wire (to complete the telegraph circuit) buried in the ground at each end of the single telegraph wire. This saved a fortune in not having to string a second copper wire along with the first one, just to complete the telegraph circuit. However, if money were no object, a second wire could have been strung and the telegraph would work just fine
without a connection to ground.
In modern electronics, significantly different currents can flow in different parts of a circuit. Analog circuits typically use much smaller currents than digital circuits, and digital circuits typically create large current transients as a result of fast switching activities. These currents create voltage drops in conductive board traces that are nominally all at the same potential. The high current traces are sometimes identified as power supply common, power supply ground, or digital common... all of these expressions being used to mean a high-current common.
For the purpose of measuring voltages (potentials) in circuits with respect to power supply common, power supply ground, or digital common, the small voltage drops (millivolts) in the board traces can often be ignored. However, these voltage drops can wreak havoc in measuring sensitive (microvolt) analog signal circuits. To prevent this, the sensitive analog circuits have their own power supply common, analog common, that is separate from the higher current traces. The analog circuits may even have their own isolated analog power supply, but eventually the analog signals (after amplification and other conditioning) may need to interface with the higher-powered circuits. The simplest way to allow this to happen is to connect the analog circuit common to the digital circuit common at a single point (called a "star" connection). Usually the schematic will separately identify analog common from digital common, usually with a triangle symbol with a number inside the triangle. The "star" connection is where the two meet.
You may ignore the three previous paragraphs until you encounter circuits that actually NEED two or more "grounds."
