# What is signal ground

I encountered signal ground symbol while studying 555 IC Please explain why isn't a simple ground connection used and how does signal ground differ from normal ground. I am a first year undergraduate, kindly explain the most basics.This inverted triangle symbol

• Normal ground is where potatoes come from. and even in elecronics ground has many different meanings. what research have you done? – Jasen Mar 17 at 5:51
• I tried to read it on internet but they say too less about it and i couldn't comprehend. – Kutsit Mar 17 at 6:02
• I understand ground in physics means an extremely gigantic capacitor like earth where any practical amount of charge poured would produce no tangible voltage change for all practical purposes at undergraduate level. – Kutsit Mar 17 at 6:06
• in most electronic circuits, the ground is the reference point where you put the negative probe of your DMM or oscilloscope ...... that is usually the negative terminal of the power supply – jsotola Mar 17 at 7:18
• In this case the terms "signal ground" and "normal ground" are synonyms. Voltage is the difference between two electric potentials p0 and p1--i.e., V=p1-p0. When looking at an electronic circuit schematic diagram, the circuit's designer defines one circuit node as the 'zero electric potential' node, or "ground potential" node, or "circuit ground", or "signal ground", etc. The electric potential difference (voltage) at the circuit's other nodes is usually measured relative to the circuit's ground potential node. – Jim Fischer Mar 17 at 7:25

Figure 1. Various earth and ground symbols. Source: Ground, earth, chassis.

“Ground” is a reference point in an electrical circuit. It is used as a reference point for voltage measurements. As a result a voltage may be above ground (positive) or below ground (negative). This is very like a surveyor taking a reference point in a certain location and referencing all other points to that datum.

The most common reference is Earth itself. Power systems are usually “earthed” at some point to provide a reference for the system voltages. The earth symbol represents the parallel plates that were buried in the soil to ensure good conductivity. (The plates were connected by wire and early forms of the symbol show the vertical line connecting all the plates. The modern “cleaner” symbol omits the vertical.)

The ground symbols indicate the generic reference point. Even if there is no earth or chassis connection it is common to refer to one point or voltage in the circuit as “ground”. In equipment where electrical isolation is provided between sections of the circuit two or more ground symbols may be required to indicate which ground the components are connected to.

I have written further on the topic and given circuit examples in the linked article.

• Thanks a lot !!! – Kutsit Mar 18 at 10:29
• Don't forget to upvote useful answers and "accept" the best answer if there is one that answers your question. – Transistor Mar 18 at 10:32
• Is that your blog ? – Mitu Raj Mar 20 at 3:57
• Yes. See my user profile. – Transistor Mar 20 at 7:06

Lets examine this circuit/system, which has a SignalGround of length 13 feet, running from the 100 microvolt Return wire of the Moving Coil vinyl record playback cartridge, thru 5' coax, thru RIAA preamp, thru 3' of coax to the 100 watt Audio Power Amplifier, then thru 5' of Speaker Return wire to the speaker (which has about 100 volts PeakPeak, or 1,000,000X larger than the sensor signal).

simulate this circuit – Schematic created using CircuitLab

Signal Ground runs all the way from the MovingCoil sensor which is left-most 1uH inductor (bottom node of that inductor is the "ground") thru the 5 feet of cable to the J1 JFET, to the RIAA capacitors (C3 and C4), thru the rest of the low-noise RIAA preamplifier transistors, out the COAX CABLE to the 100 watt audio power amplifier, and finally to ONE of the leads to the speaker.

The non-ground wire of the speaker will have substantial signal voltage {100 watts and 8 ohms => ~ 30 volts RMS or 100 volts PeakPeak), and electric fields will couple to EARTH, with those currents needing to find a path back home.

The designer gets to DESIGN all these paths. Its an incremental process of learning. For now, start with viewing a sheet of copper as your ground.

The (charging, or displacement) current thru a capacitor is I = C * dV/dt.

The dV/dT of 100 volts at 60Hz is 100 * (60 * 2 * pi) = 100 * 377 = 37,700 volts per second.

Now we need the "capacitance".

What is the capacitance between you and the earth thru your shoes?

What is the capacitance between two wires? look up the wire-wire capacitance formul (Electric Fields between two wires)

What is the capacitance between a wire and a plate?

After you ask these questions, and you gather up just a few (4? 5?) formulas, you can perform your own analysis of electric-field-induced stray currents. You will begin to think about shielding, and you will be on the way to designing high-fidelity (high signal-noise-ratio) circuits and systems.