tdehaeze/digital-brain
1
0
Fork 0
Code Issues Pull Requests Releases Wiki Activity

Update Content - 2021-05-30

Browse Source
This commit is contained in:
Thomas Dehaeze
2021-05-30 19:15:14 +02:00
parent 772483b405
commit 4c096a392a
19 changed files with 848 additions and 863 deletions
Download Patch File Download Diff File Expand all files Collapse all files

71
content/book/morrison16_groun_shiel.md
View File

@@ -1,6 +1,8 @@
+++
title = "Grounding and shielding: circuits and interference"
title = "Grounding and Shielding: Circuits and Interference"
author = ["Thomas Dehaeze"]
description = "Explains in a clear manner what is grounding and shielding and what are the fundamental physics behind these terms."
keywords = ["Electronics"]
draft = false
+++
@@ -8,7 +10,7 @@ Tags
: [Electronics]({{< relref "electronics" >}})
Reference
: ([Morrison 2016](#orgc3a94fb))
: ([Morrison 2016](#org7a49345))
Author(s)
: Morrison, R.
@@ -51,7 +53,7 @@ This displacement current flows when charges are added or removed from the plate
### Field representation {#field-representation}
<a id="orgbb971cb"></a>
<a id="orga3615d0"></a>
{{< figure src="/ox-hugo/morrison16_E_field_charge.svg" caption="Figure 1: The force field lines around a positively chaged conducting sphere" >}}
@@ -64,18 +66,18 @@ This displacement current flows when charges are added or removed from the plate
### The force field or \\(E\\) field between two conducting plates {#the-force-field-or--e--field-between-two-conducting-plates}
<a id="org0a58e51"></a>
<a id="org82b88ec"></a>
{{< figure src="/ox-hugo/morrison16_force_field_plates.svg" caption="Figure 2: The force field between two conducting plates with equal and opposite charges and spacing distance \\(h\\)" >}}
### Electric field patterns {#electric-field-patterns}
<a id="org2812c15"></a>
<a id="org16f20a9"></a>
{{< figure src="/ox-hugo/morrison16_electric_field_ground_plane.svg" caption="Figure 3: The electric field pattern of one circuit trace and two circuit traces over a ground plane" >}}
<a id="orge3117ef"></a>
<a id="org38210cb"></a>
{{< figure src="/ox-hugo/morrison16_electric_field_shielded_conductor.svg" caption="Figure 4: Field configuration around a shielded conductor" >}}
@@ -88,7 +90,7 @@ This displacement current flows when charges are added or removed from the plate
### The \\(D\\) field {#the--d--field}
<a id="orgd76a948"></a>
<a id="org5a4329e"></a>
{{< figure src="/ox-hugo/morrison16_E_D_fields.svg" caption="Figure 5: The electric field pattern in the presence of a dielectric" >}}
@@ -148,9 +150,9 @@ In a few elements, the atomic structure is such that atoms align to generate a n
The flow of electrons is another way to generate a magnetic field.
The letter \\(H\\) is reserved for the magnetic field generated by a current.
Figure [6](#org198efb1) shows the shape of the \\(H\\) field around a long, straight conductor carrying a direct current \\(I\\).
Figure [6](#org9b0e888) shows the shape of the \\(H\\) field around a long, straight conductor carrying a direct current \\(I\\).
<a id="org198efb1"></a>
<a id="org9b0e888"></a>
{{< figure src="/ox-hugo/morrison16_H_field.svg" caption="Figure 6: The \\(H\\) field around a current-carrying conductor" >}}
@@ -167,7 +169,7 @@ Ampere's law states that the integral of the \\(H\\) field intensity in a closed
\boxed{\oint H dl = I}
\end{equation}
The simplest path to use for this integration is the one of the concentric circles in Figure [6](#org198efb1), where \\(H\\) is constant and \\(r\\) is the distance from the conductor.
The simplest path to use for this integration is the one of the concentric circles in Figure [6](#org9b0e888), where \\(H\\) is constant and \\(r\\) is the distance from the conductor.
Solving for \\(H\\), we obtain
\begin{equation}
@@ -179,7 +181,7 @@ And we see that \\(H\\) has units of amperes per meter.
### The solenoid {#the-solenoid}
The magnetic field of a solenoid is shown in Figure [7](#org7535570).
The magnetic field of a solenoid is shown in Figure [7](#orgd3a9cf9).
The field intensity inside the solenoid is nearly constant, while outside its intensity falls of rapidly.
Using Ampere's law \eqref{eq:ampere_law}:
@@ -188,7 +190,7 @@ Using Ampere's law \eqref{eq:ampere_law}:
\oint H dl \approx n I l
\end{equation}
<a id="org7535570"></a>
<a id="orgd3a9cf9"></a>
{{< figure src="/ox-hugo/morrison16_solenoid.svg" caption="Figure 7: The \\(H\\) field around a solenoid" >}}
@@ -196,10 +198,10 @@ Using Ampere's law \eqref{eq:ampere_law}:
### Faraday's law and the induction field {#faraday-s-law-and-the-induction-field}
When a conducting coil is moved through a magnetic field, a voltage appears at the open ends of the coil.
This is illustrated in Figure [8](#orgd2dee77).
This is illustrated in Figure [8](#org4b2f5c1).
The voltage depends on the number of turns in the coil and the rate at which the flux is changing.
<a id="orgd2dee77"></a>
<a id="org4b2f5c1"></a>
{{< figure src="/ox-hugo/morrison16_voltage_moving_coil.svg" caption="Figure 8: A voltage induced into a moving coil" >}}
@@ -237,7 +239,7 @@ The unit of inductance if the henry.
</div>
For the coil in Figure [7](#org7535570):
For the coil in Figure [7](#orgd3a9cf9):
\begin{equation} \label{eq:inductance\_coil}
V = n^2 A k \mu\_0 \frac{dI}{dt} = L \frac{dI}{dt}
@@ -483,39 +485,39 @@ For example, signals that overload an input stage can produce noise that may loo
### The basic shield enclosure {#the-basic-shield-enclosure}
Consider the simple amplifier circuit shown in Figure [9](#orgd60f7ec) with:
Consider the simple amplifier circuit shown in Figure [9](#org3286d62) with:
- \\(V\_1\\) the input lead
- \\(V\_2\\) the output lead
- \\(V\_3\\) the conducting enclosure which is floating and taken as the reference conductor
- \\(V\_4\\) a signal common or reference conductor
Every conductor pair has a mutual capacitance, which are shown in Figure [9](#orgd60f7ec) (b).
The equivalent circuit is shown in Figure [9](#orgd60f7ec) (c) and it is apparent that there is some feedback from the output to the input or the amplifier.
Every conductor pair has a mutual capacitance, which are shown in Figure [9](#org3286d62) (b).
The equivalent circuit is shown in Figure [9](#org3286d62) (c) and it is apparent that there is some feedback from the output to the input or the amplifier.
<a id="orgd60f7ec"></a>
<a id="org3286d62"></a>
{{< figure src="/ox-hugo/morrison16_parasitic_capacitance_amp.svg" caption="Figure 9: Parasitic capacitances in a simple circuit. (a) Field lines in a circuit. (b) Mutual capacitance diagram. (b) Circuit representation" >}}
It is common practice in analog design to connect the enclosure to circuit common (Figure [10](#org412bfcb)).
It is common practice in analog design to connect the enclosure to circuit common (Figure [10](#org9f3c9db)).
When this connection is made, the feedback is removed and the enclosure no longer couples signals into the feedback structure.
The conductive enclosure is called a **shield**.
Connecting the signal common to the conductive enclosure is called "**grounding the shield**".
This "grounding" usually removed "hum" from the circuit.
<a id="org412bfcb"></a>
<a id="org9f3c9db"></a>
{{< figure src="/ox-hugo/morrison16_grounding_shield_amp.svg" caption="Figure 10: Grounding the shield to limit feedback" >}}
Most practical circuits provide connections to external points.
To see the effect of making a _single_ external connection, open the conductive enclosure and connect the input circuit common to an external ground.
Figure [11](#org5d67d92) (a) shows this grounded connection surrounded by an extension of the enclosure called the _cable shield_.
Figure [11](#orgc4242ae) (a) shows this grounded connection surrounded by an extension of the enclosure called the _cable shield_.
A problem can be caused by an incorrect location of the connection between the cable shield and the enclosure.
In Figure [11](#org5d67d92) (a), the electromagnetic field in the area induces a voltage in the loop and a resulting current to flow in conductor (1)-(2).
In Figure [11](#orgc4242ae) (a), the electromagnetic field in the area induces a voltage in the loop and a resulting current to flow in conductor (1)-(2).
This conductor being the common ground that might have a resistance \\(R\\) or \\(1\,\Omega\\), this current induced voltage that it added to the transmitted signal.
Our goal in this chapter is to find ways of keeping interference currents from flowing in any input signal conductor.
To remove this coupling, the shield connection to circuit common must be made at the point, where the circuit common connects to the external ground.
This connection is shown in Figure [11](#org5d67d92) (b).
This connection is shown in Figure [11](#orgc4242ae) (b).
This connection keeps the circulation of interference current on the outside of the shield.
There is only one point of zero signal potential external to the enclosure and that is where the signal common connects to an external hardware ground.
@@ -540,7 +542,7 @@ It is an issue of using the _right_ ground.
</div>
<a id="org5d67d92"></a>
<a id="orgc4242ae"></a>
{{< figure src="/ox-hugo/morrison16_enclosure_shield_1_2_leads.png" caption="Figure 11: (a) The problem of bringing one lead out of a shielded region. Unwanted current circulates in the signal lead 2. (b) The \\(E\\) field circulate current in the shield, not in the signal conductor." >}}
@@ -552,7 +554,7 @@ The power transformer couples fields from the external environment into the encl
The obvious coupling results from capacitance between the primary coil and the secondary coil.
Note that the secondary coil is connected to the circuit common conductor.
<a id="orgb45b4f3"></a>
<a id="org5995e31"></a>
{{< figure src="/ox-hugo/morrison16_power_transformer_enclosure.png" caption="Figure 12: A power transformer added to the circuit enclosure" >}}
@@ -564,7 +566,7 @@ Note that the secondary coil is connected to the circuit common conductor.
The basic analog problem is to condition a signal associated with one ground reference potential and transport this signal to a second ground reference potential without adding interference.
<a id="org75ed03f"></a>
<a id="org3228c82"></a>
{{< figure src="/ox-hugo/morrison16_two_ground_problem.svg" caption="Figure 13: The two-circuit enclosures used to transport signals between grounds" >}}
@@ -580,7 +582,7 @@ The basic analog problem is to condition a signal associated with one ground ref
### The basic low-gain differential amplifier (forward referencing amplifier) {#the-basic-low-gain-differential-amplifier--forward-referencing-amplifier}
<a id="orge28ae4f"></a>
<a id="org4f33add"></a>
{{< figure src="/ox-hugo/morrison16_low_gain_diff_amp.svg" caption="Figure 14: The low-gain differential amplifier applied to the two-ground problem" >}}
@@ -623,11 +625,11 @@ Here are a few rule that will help in analog board layout:
### Feedback theory {#feedback-theory}
<a id="orgbf57c39"></a>
<a id="org4a09d89"></a>
{{< figure src="/ox-hugo/morrison16_basic_feedback_circuit.svg" caption="Figure 15: The basic feedback circuit" >}}
<a id="org795e24d"></a>
<a id="orgf414d06"></a>
{{< figure src="/ox-hugo/morrison16_LR_stabilizing_network.svg" caption="Figure 16: An LR-stabilizing network" >}}
@@ -665,7 +667,7 @@ If the resistors are replaced by capacitors, the gain is the ratio of reactances
This feedback circuit is called a **charge converter**.
The charge on the input capacitor is transferred to the feedback capacitor.
If the feedback capacitor is smaller than the transducer capacitance by a factor of 100, then the voltage across the feedback capacitor will be 100 times greater than the open-circuit transducer voltage.
This feedback arrangement is shown in Figure [17](#org964dc8b).
This feedback arrangement is shown in Figure [17](#org74f6090).
The open-circuit input signal voltage is \\(Q/C\_T\\).
The output voltage is \\(Q/C\_{FB}\\).
The voltage gain is therefore \\(C\_T/C\_{FB}\\).
@@ -679,11 +681,11 @@ It converts a charge signal to a voltage.
</div>
<a id="org964dc8b"></a>
<a id="org74f6090"></a>
{{< figure src="/ox-hugo/morrison16_charge_amplifier.svg" caption="Figure 17: A basic charge amplifier" >}}
<a id="orgdd200ce"></a>
<a id="orgb9f996c"></a>
{{< figure src="/ox-hugo/morrison16_charge_amplifier_feedback_resistor.svg" caption="Figure 18: The resistor feedback arrangement to control the low-frequency response" >}}
@@ -1031,6 +1033,7 @@ To transport RF power without reflections, the source impedance and the terminat
### Shielded and screen rooms {#shielded-and-screen-rooms}
## Bibliography {#bibliography}
<a id="orgc3a94fb"></a>Morrison, Ralph. 2016. _Grounding and Shielding: Circuits and Interference_. John Wiley & Sons.
<a id="org7a49345"></a>Morrison, Ralph. 2016. _Grounding and Shielding: Circuits and Interference_. John Wiley & Sons.