JP3799949B2 - Printed board - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a printed circuit board on which analog circuits and digital circuits are mixedly mounted, and more particularly to a printed circuit board that reduces high-frequency noise.
[0002]
[Prior art]
In recent years, development of digital circuits has progressed, and printed circuit boards in which digital circuits and analog circuits are mixedly mounted are widely used. In such a printed circuit board, each analog signal has its frequency component continuously changing according to the original signal every moment, and it is necessary to transmit it faithfully. Therefore, the analog circuit needs to prevent the signal from the digital circuit from being mixed. Therefore, a printed circuit board in which a digital circuit and an analog circuit are separately mounted is widely used.
[0003]
[Problems to be solved by the invention]
However, in such a printed circuit board, there is a limit to the separation between the analog circuit and the digital circuit due to the high density of the circuit, and the high frequency signal handled by the digital circuit and its high frequency component are mixed in the analog circuit as high frequency noise. There is. Here, a circuit component or the like is mounted on the printed wiring board, but the area of the loop formed by the signal line through which the high-frequency signal of the digital circuit flows and its return line may increase depending on the arrangement of the component. When a high-frequency signal flows through such a circuit, this loop serves as an antenna, and high-frequency noise is radiated from the surface of the printed circuit board. Such noise is called normal mode noise. As the area of the loop increases, the high frequency noise radiated from the printed circuit board surface to the outside increases. When the analog circuit and the digital circuit cannot be completely separated, such noise generated from the digital circuit is mixed in the analog circuit, and the high-frequency noise is radiated to the outside through the cable from the analog input / output terminal. In addition, the potential of the analog ground with respect to the ground potential fluctuates, and high-frequency noise is radiated mainly through the cable. Such noise is called common mode noise. Therefore, in order to reduce noise radiated from the printed circuit board, it is necessary to reduce both normal mode noise and common mode noise.
[0004]
In addition, a method is used in which a digital circuit and an analog circuit are separated and mounted on separate layers on the outer layer using a multilayer substrate, and the inner layer is a shielding layer. When circuit components are mounted with high density on such a multilayer substrate, the power supply pattern must be finely arranged on the outer layer, and the circuit may malfunction due to a voltage drop associated therewith.
[0005]
The present invention has been made to solve the above-described conventional problems, and an object of the present invention is to provide a printed circuit board in which high-frequency noise can be extremely reduced despite the presence of digital circuits and analog circuits. There is to do.
[0006]
[Means for Solving the Problems]
The printed circuit board according to the present invention includes a first surface layer having an analog circuit, an analog ground, a digital circuit, and an external connection portion connected to the outside, a second surface layer, and a digital ground plane layer. An intermediate layer, and a first capacitor connected between the analog ground and the digital ground.
[0007]
In a preferred embodiment, the first capacitor is connected between the analog ground and the digital ground in the vicinity of the external connection portion.
[0008]
In a preferred embodiment, the semiconductor device further includes a second capacitor connected between the analog circuit and the analog ground.
[0009]
In a preferred embodiment, a noise removal filter connected to the external connection unit is further provided.
[0010]
Hereinafter, the operation of the printed circuit board according to the present invention will be described.
The printed circuit board according to the present invention includes a first surface layer on which an analog circuit, an analog ground, and a digital circuit are provided, an intermediate layer including a digital ground plane layer, and a second surface layer. By connecting the analog ground mixed with high frequency noise to the digital ground plane via the first capacitor, the high frequency noise is shunted to the digital ground via the first capacitor. In the digital ground plane, the loop area through which the high frequency noise divided by the first capacitor flows can be reduced. Therefore, normal mode high frequency noise radiated from the printed circuit board surface can be reduced.
[0011]
In a preferred embodiment, the first capacitor is connected to analog ground and digital ground in the vicinity of the external connection. By connecting the first capacitor in the vicinity of the external connection portion, the impedance to the high frequency noise mixed in the analog circuit can be reduced. For high frequency noise, the external connection portion is connected from the connection point between the analog ground and the digital ground. It can be considered that the route leading to is shortened. Therefore, the analog ground connection terminal of the external connection section and the plain digital ground with extremely low impedance have approximately the same potential, so not only normal mode high-frequency noise radiated from the printed circuit board surface but also cables from the external connection section. The common-mode high-frequency noise that is radiated can also be reduced.
[0012]
In the preferred embodiment, since the analog circuit is connected to the analog ground via the second capacitor, high-frequency noise mixed in the analog circuit is mainly transmitted to the analog ground via the second capacitor. And since the high frequency noise transmitted to the analog ground is shunted to the digital ground via the first capacitor, the high frequency noise radiated to the outside through the cable is further reduced from the case where there is no second capacitor. it can.
[0013]
In a preferred embodiment, since the printed circuit board further includes a noise removal filter, high-frequency noise leaking from the external connection portion to the cable can be attenuated.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail, but the present invention is not limited to these embodiments. In this embodiment, a multilayer substrate having two intermediate layers will be described as an example, but the number of intermediate layers is not limited to two.
[0015]
FIG. 1 is a schematic cross-sectional view showing a printed circuit board according to a preferred embodiment of the present invention. The printed circuit board 1 has a first surface layer 2, a second surface layer 3, a first intermediate layer 4, and a second intermediate layer 5. The printed wiring board is not limited, and a normal four-layer wiring board is used. FIG. 2 is a block diagram illustrating a printed circuit board according to a preferred embodiment of the present invention. Hereinafter, a description will be given with reference to FIGS. 1 and 2.
[0016]
An analog circuit 6, an analog ground 7, and a digital circuit 8 are attached to the first surface layer 2. That is, the first surface layer 2 is used as a layer for actually attaching circuit components and wiring signal lines.
[0017]
The first surface layer 2 is further attached with an external connection portion 9. The external connection unit 9 is, for example, an analog input / output terminal or the like, and is connected to an external device or another printed circuit board via a cable or the like. The external connection unit 9 is connected to an analog circuit 6 (specifically, a signal line connected to the analog circuit) and an analog ground 7. (Alternatively, a digital input / output terminal may be used as the external connection unit 9).
[0018]
The second surface layer 3 is not particularly limited, but typically, the analog circuit 6, the analog ground 7, and the digital circuit 8 are attached similarly to the first surface layer 2. Here, the first surface layer 2, the second surface layer 3, the first intermediate layer 4, and the second intermediate layer 5 are connected through a through hole 10.
[0019]
A power supply unit 11 is used to supply power to the analog circuit 6 and the digital circuit 8 from the outside. The power supply unit 11 is connected to the analog circuit 6, the analog ground 7, the digital circuit 8, and the digital ground 12. In the present embodiment, power is supplied to the digital circuit 8 via the second intermediate layer 5 as will be described later.
[0020]
The mounting of the analog circuit 6, the analog ground 7, and the digital circuit 8 is not limited, and can be arbitrarily mounted on the first and second surface layers according to applications. For example, by connecting a plurality of analog grounds 7 at any one point of the first and second surface layers (not shown), the common impedance between the analog grounds can be reduced. For example, as shown in FIG. 2, when the analog ground 7 and the digital ground 12 are connected to the power supply unit 11, the path from the connection point A between the two grounds to the power supply unit 11 is shortened. The common impedance between the ground and the digital ground can be reduced, and the potential of the analog ground can be stabilized.
[0021]
The first intermediate layer 4 is a plane. The plane is a surface formed entirely of a conductor except for the outer peripheral portion and the through hole 10 and its periphery. The first intermediate layer 4 is used as a digital ground 12. That is, the ground line from the digital circuit 8 attached to the first and second surface layers is connected to the first intermediate layer 4 which is the digital ground 12 through the through hole 10. Since the first intermediate layer 4 is a plane and no circuit components are mounted, the loop area can be reduced when a digital high-frequency signal is transmitted through the digital ground 12. When the loop area is reduced, the radiation of the high-frequency signal from the substrate surface can be reduced (details will be described later).
[0022]
The second intermediate layer 5 is not particularly limited in the present invention, but is preferably a plane. This is the same reason that a plane is used for the first intermediate layer 4. In the present embodiment, the second intermediate layer 5 is used as a digital power supply pattern. That is, the second intermediate layer 5 is connected to the power supply unit 11 and the digital circuit 8 through the through hole 10, and supplies the power from the power supply unit 11 to the digital circuit 8. The second intermediate layer 5 that is a digital power supply pattern layer may be formed in one pattern or may be divided into a plurality of patterns.
[0023]
The order of stacking the first intermediate layer 4 and the second intermediate layer 5 is arbitrary in the present invention, and can be determined according to the circuits mounted on the first surface layer 2 and the second surface layer 3. Good. For example, when a digital circuit 8 that transmits a high-frequency signal is installed in the first surface layer 2, the first intermediate layer 4 that is the digital ground 12 is adjacent to the first surface layer 2. preferable. This is because the digital ground uses an undivided plane pattern, so that the area of the loop through which the high-frequency signal flows can be reduced.
[0024]
The printed circuit board 1 further includes a first capacitor 13. The first capacitor 13 has one end connected to the analog ground 7 and the other end connected to the digital ground 12 of the first intermediate layer 4 through the through hole 10. That is, the analog ground 7 and the digital ground 12 are connected via the first capacitor 13. The first capacitor 13 typically uses a chip-type ceramic capacitor having a low impedance at a high frequency (for example, several tens to several hundreds of MHz), and has a capacity of several hundred pF, for example. FIG. 3A is a schematic diagram showing a state in which the first capacitor 13 is connected between the analog ground 7 and the digital ground 12.
[0025]
Hereinafter, the operation of the printed circuit board 1 when the first capacitor 13 is provided will be described with reference to FIG. 2 and FIG. The high frequency noise mixed in the analog circuit 6 flows through the analog ground 7 and is transmitted to the digital ground 12 through the first capacitor 13. In the present invention, the digital ground 12 is a plane, and no circuit components or the like are installed. Therefore, in the digital ground 12, the area of the loop through which the high frequency noise is transmitted can be made extremely small as compared with the case where the first surface layer 2 is transmitted. Therefore, since the area of the loop can be made extremely small, the high frequency noise radiated from the printed circuit board surface can be reduced extremely well. Furthermore, by reducing the area of the loop, the effective inductance can be reduced and the high frequency noise voltage can be extremely reduced.
[0026]
In the preferred embodiment, the first capacitor 13 is connected to the analog ground 7 and the digital ground 12 in the vicinity of the external connection portion 9. The vicinity of the external connection portion 9 is a position where the analog ground potential at the external connection portion 9 is substantially the same as the potential of the digital ground 12 that is the reference potential of the printed circuit board. Therefore, it is preferable to connect the first capacitor 13 as close as possible to the external connection portion 9, but practically, the first capacitor 13 is connected to a position of 2 mm to 10 mm from the external connection portion 9. Is done.
[0027]
By connecting the first capacitor 13 in the vicinity of the external connection portion 9, the point B that is a connection point between the analog ground 7 and the digital ground 12 is in the vicinity of the external connection portion 9. Here, when the first capacitor 13 is not provided, the connection point between the analog ground 7 and the digital ground 12 is only the point A, and for high frequency noise, the path from the point A to the external connection unit 9 is High impedance. However, since the path from the point B to the external connection portion 9 is extremely short as compared with the point A, the impedance to the high frequency noise is extremely small even if the first capacitor 13 is taken into consideration. Therefore, the fluctuation of the analog ground potential due to the flow of high frequency noise from the point B which is the reference potential of the printed circuit board to the external connection portion 9 is approximately 0, and the external connection portion 9 and the digital ground 12 are approximately the same potential. . The reason why the digital ground 12 is used as the reference potential is that the digital ground 12 is a plane and the potential fluctuation can be regarded as approximately zero. Since the external connection unit 9 and the digital ground 12 have substantially the same potential, high-frequency noise radiated through the cable can be extremely reduced.
[0028]
In the preferred embodiment, the analog circuit 6 is connected to the analog ground 7 via the second capacitor 14. By connecting the second capacitor 14, high-frequency noise transmitted through the analog circuit 6 is transmitted to the analog ground 7 through the second capacitor 14. The high frequency noise transmitted to the analog ground 7 is transmitted to the digital ground 12 via the first capacitor 13.
[0029]
The capacitance of the second capacitor 14 varies depending on the frequency range of the analog signal to be transmitted. For example, when the analog circuit handles a low frequency up to 20 kHz, a capacitor of about several hundred to several thousand pF is used. The number and connection positions of the second capacitors 14 are not limited, but at least one end of the second capacitor 14 is connected between the external connection unit 9 and the analog circuit 6. This is to prevent high frequency noise from being transmitted to the external connection unit 9. Further, it is preferable to connect the second capacitor 14 to the connection point of the first capacitor 13 in the analog ground 7. This is because the distance over which the high-frequency noise is transmitted through the analog ground 7 can be minimized.
[0030]
In a preferred embodiment, a noise removal filter 15 is connected to the external connection unit 9. FIG. 3B is a schematic diagram showing a state in which the noise removal filter 15 is connected. For example, a high-frequency cutoff filter is used as the noise removal filter 15. The signal transmitted to the external connection unit 9 is attenuated by high frequency noise by the noise removal filter 15, and only the signal to be transmitted to the external connection unit 9 is transmitted. Therefore, the high frequency noise radiated | emitted via a cable can further be reduced.
[0031]
In this embodiment, a four-layer substrate is used as a multilayer substrate, but the present invention is not limited to a four-layer substrate. For example, it can be adopted for a six-layer substrate. In addition, the first intermediate layer is a digital ground. However, the first intermediate layer is not limited to the digital ground layer.
[0032]
【Example】
Examples of the present invention will be specifically described below, but the present invention is not limited to these examples.
[0033]
Example 1
A USB (Universal Serial Bus) audio processor was fabricated using the printed circuit board according to the present invention. The first capacitor is attached at a distance of 3 mm from the analog input / output terminal. However, a microphone (made by Hosiden), a personal computer, and a cable are connected to the analog input terminal, and a cable is connected to the analog output terminal. Using the obtained audio processor, the radiated high frequency noise was measured by a normal method. The measurement results are shown in FIG. 4 together with Example 2 and Comparative Example 1. In FIG. 4, the horizontal axis indicates the frequency (MHz), and the vertical axis indicates the noise radiation level (dBμV). The VCCI (Voluntary Control Council for Radio Wave Information Processing Equipment) Limit is a standard value.
[0034]
(Example 2)
A USB audio processor was produced in the same manner as in Example 1 except that the first capacitor was placed at a distance of about 4 cm from the analog input / output terminal on the printed board of the present invention. The noise radiated from the printed board was measured in the same manner as in Example 1 for the obtained audio processor. The measurement results are shown in FIG.
[0035]
(Comparative Example 1)
An audio processor was manufactured in the same manner as in Example 1 except that the first capacitor was not provided. The noise radiated from the printed board was measured in the same manner as in Example 1 for the obtained audio processor. The measurement results are shown in FIG.
[0036]
Example 3
The radiated high frequency noise was measured in the same manner as in Example 1 except that no microphone and cable were connected to the analog input / output terminal. The measurement results are shown in FIG.
[0037]
(Comparative Example 2)
The radiated high frequency noise was measured in the same manner as in Comparative Example 1 except that no microphone and cable were connected to the analog input / output terminal. The measurement results are shown in FIG.
[0038]
As shown in FIG. 4, according to Examples 1 and 2, noise is reduced very well particularly at frequencies of 96 MHz and 144 MHz as compared with Comparative Example 1. In particular, in Example 1, the noise is reduced to about 60 percent compared to Comparative Example 1. Moreover, in the frequencies of 144 MHz and 336 MHz, the noise exceeds the standard value of VCCI in Comparative Example 1, but the noise is improved to the standard value or less in Example 1. Further, comparing Example 1 and Example 2, it can be seen that the closer the first capacitor is to the analog input / output terminal, the smaller the potential fluctuation of the analog ground, and the less the radiated noise. Furthermore, in Example 3 and Comparative Example 2, since the cable and the microphone are not connected, only high-frequency noise from the surface of the general printed circuit board is measured. As shown in FIG. 5, when Example 3 and Comparative Example 2 are compared, the effect of reducing the loop by the first capacitor and reducing noise from the printed circuit board surface can be seen.
[0039]
【The invention's effect】
According to the printed circuit board of the present invention, the high frequency noise mixed in the analog ground is transmitted to the first intermediate layer that is the digital ground plane via the first capacitor, and the loop area is extremely small. Therefore, the high frequency noise is extremely reduced despite the presence of both digital and analog circuits.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing a printed circuit board according to a preferred embodiment of the present invention.
FIG. 2 is a block diagram illustrating a printed circuit board according to a preferred embodiment of the present invention.
FIG. 3A is a schematic diagram for explaining a state in which a first capacitor is connected. (B) is the schematic explaining the state which connected the noise removal filter in (a).
FIG. 4 is a graph showing measurement results in an example of the present invention.
FIG. 5 is a graph showing measurement results in an example of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Printed circuit board 2 1st surface layer 3 2nd surface layer 4, 5 Intermediate layer 6 Analog circuit 7 Analog ground 8 Digital circuit 9 External connection part 12 Digital ground 13 1st capacitor 14 2nd capacitor 15 Noise removal filter

Claims (4)

A first surface layer having an analog circuit, an analog ground, a digital circuit, and an external connection portion which is an analog input / output terminal connected to the outside;
A second surface layer;
An intermediate layer including a digital ground plane layer;
A printed circuit board Ru and a first capacitor connected between the analog ground and the digital ground plane layer,
The first capacitor is connected between the analog ground and the digital ground plane layer at a position where the analog ground potential at the external connection portion is equal to the potential of the digital ground plane layer, which is the reference potential of the printed circuit board . A printed circuit board connected in between. A second capacitor connected between the analog circuit and the analog ground;
The printed circuit board according to claim 1, wherein one end of the second capacitor is connected between the external connection portion and the analog circuit.   The printed circuit board according to claim 1, further comprising a noise removal filter connected to the external connection unit. A second intermediate layer that is a plane layer used as a power supply pattern;
The first surface layer further includes a power supply;
The printed circuit board according to claim 1, wherein the power supply pattern is connected to the power supply unit and the digital circuit.

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