DE102013113598A1 - Arrangement for transmitting digital signals via a galvanically isolating interface and field device comprising such an arrangement - Google Patents

Abstract

The invention relates to an arrangement (1) for transmitting digital signals via a galvanically separating interface (3) from a primary side to a secondary side, comprising a printed circuit board (PCB) having a primary coil (L1) and a secondary coil (L2), wherein the primary coil (L1) is arranged on the primary side on at least a first layer of the printed circuit board, wherein the secondary coil (L2) is arranged on the secondary side on at least one second layer of the printed circuit board (PCB), so that the primary coil (L1) and the secondary coil ( L2), wherein the primary coil (L1) and the secondary coil (L2) form the galvanically separating interface (3) for transmitting digital signals, and wherein in the printed circuit board (PCB) between the primary coil (L1) and the secondary coil (L2) a Ableitstruktur (2) is arranged, which the primary coil (L1) and / or the secondary coil (L2) before by capacitive coupling between the primary coil (L1) and Seku Outer coil (L2) shields interference currents, in particular by the fact that the interference currents are discharged to the discharge structure (2). The invention further relates to a field device.

Description

The invention relates to an arrangement for transmitting digital signals via a galvanically isolating interface and a field device, in particular a transmitter, comprising such an arrangement.

It is known that data can be transferred via so-called print coils integrated in printed circuit boards (printed circuit board, or PCB). In each case, on one side of a printed circuit board - often on a surface layer - there is at least one printed conductor which is configured such that it functions as an electrical coil. The coil is designed approximately helically in a circle or rectangular arrangement.

In this case, arranged on opposite sides of the printed circuit board interconnect inductively with each other and can be used to transmit digital signals. This is also referred to as Printübertragern.

In this way, the requirements of explosion protection Exi (type of protection intrinsically safe) to a sufficient dielectric isolation between the two sides of the galvanic isolation can be easily ensured without the need for specially specified and designed according to the requirements of the explosion protection standards signal transformers. By eliminating transformers, the system costs can be reduced.

The following problems, however, exist when using the print transformer: The signal strengths are very weak due to the poor inductive coupling. The pairs of print coils are not only inductive, but also highly capacitively coupled with each other, which generates EMC problems due to the capacitive coupling of interference signals. In order to achieve a sufficiently good signal-to-noise ratio in the presence of EMC interference, the print coils must have a comparatively large printed circuit board surface, which is thus not available for component assembly. Because the ohmic resistance of the print coils is very high in relation to their inductance, the coils can not drive high impedances. This is z. B. for the secondary side, the input capacitance of a connected receiving circuit very significant.

The invention has for its object to make the known print transformer more resistant to interference and to modify so that they can be used even in a limited space.

The object is achieved by an arrangement comprising a printed circuit board with a primary coil and a secondary coil, wherein the primary coil is arranged on the primary side on at least a first layer of the printed circuit board, wherein the secondary coil is arranged on the secondary side on at least one second layer of the printed circuit board in that the primary coil and the secondary coil are opposite one another, the primary coil and the secondary coil forming the galvanically separating interface for transmitting digital signals, and wherein in the printed circuit board between the primary coil and the secondary coil, a diverting structure is arranged, which the primary coil and / or the secondary coil shields caused by capacitive coupling between the primary coil and secondary coil interference currents, in particular by the fact that the interference currents are dissipated to the Ableitstruktur.

If the respective discharge structures are connected as mentioned above, then they divert the predominant portion of the forming interference currents. Thus, these interference currents are now bypassed the primary coil and secondary coil. In particular, the weaker secondary-side signals of the secondary coil used for the reception are now influenced by far weaker interference components.

For an easy and inexpensive production, the primary coil and / or the secondary coil is designed as a conductor on / in the circuit board in an advantageous embodiment, and the Ableitstruktur is configured as a conductor in the circuit board.

Advantageously, the primary coil, secondary coil and diverting structure each comprise two layers of the printed circuit board, wherein the diverting structure comprises at least one primary winding structure and a secondary conductive structure and the primary winding structure of the primary coil and the secondary winding structure of the secondary winding are assigned.

In a preferred embodiment, the primary baffle structure and the secondary baffle structure are located on different layers of the printed circuit board, in particular on two different inner layers of the printed circuit board. Alternatively, the primary baffle structure and the secondary baulk structure may be disposed on the same inner layer of the printed circuit board.

If the primary baffle structure and the secondary baffle structure are located on different layers of the printed circuit board, it is preferable to choose a distance between the corresponding layers which takes into account the regulations of the "type of protection intrinsically safe". It can thus be achieved a galvanic separation that meets these requirements. For example, the distance is about 1 mm with appropriate printed circuit board material.

According to a preferred embodiment, the primary coil and the secondary coil each comprise a surface layer, and an inner layer of the printed circuit board. The primary coil or the secondary coil is thus formed from a conductor track on each of a surface layer and an inner layer of the printed circuit board.

In an advantageous embodiment, the discharge structure comprises no flat sub-segments but only contiguous, as thin as possible individual lines, in particular the discharge structure is fan-shaped, meandering, or helical configured. It is particularly important that the structure in the mathematical-topological sense forms a simply coherent surface, d. H. that the thin individual conductors do not surround any "empty spaces", for example in the form of a conductor loop. For example, a lattice-shaped structure would not be optimal because conductor loops can form around the meshes of the grid, which can act as a short-circuit winding for the primary and secondary coils.

Particularly preferably, the dissipation structure is connected to ground. Furthermore, it is particularly preferred that the primary baffle structure are connected to ground of the primary side, and the secondary baffle structure to ground of the secondary side. If the respective Ableitstrukturen connected to the respective ground potentials, so they derive the vast majority of the forming interference currents. Thus these disturbing currents are now bypassed the coils.

In an advantageous embodiment, the digital signals are transmitted in the form of pulses at level changes, in particular a digital level change 0 -> 1 corresponds to a positive pulse and a level change 1 -> 0 a negative pulse, or vice versa. So the signals can be transmitted with relatively low power.

In order to realize this, the arrangement preferably comprises a driver circuit for coupling the digital signals into the primary coil, wherein the driver circuit has an output impedance and is connected to the primary coil via a coupling capacitor. With suitable dimensioning of coupling capacitor and output impedance, it is possible to suppress unwanted series resonance vibrations advantageous.

In an advantageous embodiment, the coupling capacitor and the primary coil form a series resonant circuit with a resonance and a period duration, and the damping time constant formed by the output impedance and the coupling capacitor is greater than or equal to 10% of the period duration with t1 = R1 * C1. To realize this, the coupling capacitor is selected to be correspondingly large. In an additional embodiment, the damping time constant t1 is less than the reciprocal of the transmission frequency of the digital signals. The actual digital signals are thus not disturbed by overlapping resonances of the resonant circuit.

Preferably, the arrangement comprises a receiving circuit for coupling the digital signals from the secondary coil, wherein the receiving circuit comprises a Schmitt trigger, wherein a contact of the secondary coil is connected to the input of the Schmitt trigger, and wherein the receiving circuit comprises a subcircuit, the signal level the output of the Schmitt trigger constant in the absence of pulses on the secondary coil, in particular the output of the Schmitt trigger is connected as feedback to the input of the Schmitt trigger. As mentioned, the transmission of the digital signals takes place in pulses. With the described receiving circuit can be ensured that even in the absence of pulses, a signal is applied to the output.

In an advantageous embodiment, the secondary side contains an evaluation circuit which is operated with a first supply voltage and processes the digital signals transmitted via the galvanically isolating interface, and the Schmitt trigger connected to the secondary coil with a second supply voltage deviating from the first supply voltage is operated, and the signal level at the output of the Schmitt trigger is converted by means of a level shifter to the signal level of the first supply voltage. Thus, the signal level at the output can be set to the desired value, in particular the signal level is increased. Due to the described galvanic isolation and the transmission via the primary coil and secondary coil, the signal loses amplitude, which is raised again by the level converter for subsequent components.

In a preferred embodiment, the subcircuit comprises at least one attenuator, in particular at least one resistor, wherein the attenuator is arranged in particular in the feedback between the output and the input of the Schmitt trigger. Thus, an oscillation occurring at the secondary coil can be damped.

It is critical that at least the secondary baffle structure be present, offering a signal path that is more low-impedance than the noise path across the secondary coil. Advantageously, in the case of two-layer coils (ie a surface layer and an inner layer of the printed circuit board) then the input of the Schmitt trigger connected to the inner layer of the circuit board, so that from outside interfering electric field lines can only directly reach the non-critical with respect to the interference effect conductor structures, which are not directly connected to the sensitive Schmitt trigger input.

Preferably, at the input of the Schmitt trigger, a first diode with cathode is connected against the second supply voltage and anode against the input of the Schmitt trigger, and a second diode is connected with cathode against the input of the Schmitt trigger and anode against ground of the secondary side. With such interconnections can be optionally formed parasitic vibrations, z. B. due to unavoidable parallel capacitances, within the coils advantageously suppress.

The object is further achieved by a field device, in particular a transmitter, the process automation technology comprising an arrangement as described above.

In one embodiment, the digital signals are supplied to a sensor.

The invention will be explained in more detail with reference to the following figures. Show it

1 the field device according to the invention comprising an arrangement according to the invention, including a sensor,

2a the arrangement according to the invention in cross section,

2 B the arrangement according to the invention in an embodiment in cross section,

2c the inventive arrangement 2 B in a plan view of the primary coil with dissipation structure, and

3 the driver circuit and receiving circuit of the inventive arrangement.

Unless otherwise described, the same features are identified by the same reference numerals in the figures.

The arrangement according to the invention in its entirety has the reference numeral 1 and is in 1 as part of the field device according to the invention 20 shown.

First, it is intended to indicate a field device according to the invention 20 be received in which the inventive electronic circuit 1 is used. Via a first interface 23 communicates a consumer, such as a sensor 21 with the field device 20 So for example a transmitter. At the transmitter 20 is a cable 24 provided, at the other end to the first interface 23 complementary second interface 22 is provided. The interfaces 22 . 23 are designed as galvanically isolated, in particular as inductive interfaces which can be coupled to one another by means of a mechanical plug connection. About the interfaces 22 . 23 be data (bidirectional) and energy (unidirectional, ie from transmitter 20 to the sensor 21 ) Posted. The transmission of the data takes place in digital form, which is described in the description of 3 will be discussed in more detail.

The field device 20 is mainly used in process automation. At the sensor 21 it is therefore about a pH, redox potential, also ISFET, temperature, conductivity, pressure, oxygen, in particular dissolved oxygen, or carbon dioxide sensor; an ion-selective sensor; an optical sensor, in particular a turbidity sensor, a sensor for optically determining the oxygen concentration, or a sensor for determining the number of cells and cell structures; a sensor for monitoring certain organic or metallic compounds; a sensor for determining a concentration of a chemical substance, for example a particular element or a particular compound; or a biosensor, e.g. B. a glucose sensor. Similarly, use in pressure, level, flow or temperature measuring stations is conceivable.

The inventive arrangement 1 is in the transmitter 20 , The transmitter 20 is designed with the type of protection intrinsically safe, ie as Exi, (English: intrinsic safety). In these devices, current and voltage are limited to values at which no flammable energy occurs, so potential sparking does not trigger an explosion of explosive fuel-air mixtures. The intrinsic safety is achieved by the fact that possible sparks during switching or in the event of a short circuit release so little energy that no ignition can take place. The internal resistance of the supplying current sources is particularly high. In such circuits energy storage such. B. capacitors only within certain limits.

The transmitter 20 is designed as a two-wire device. For example, the applicant sells such products under the name "Endress + Hauser Liquiline M CM42".

2a shows an arrangement according to the invention 1 in cross section. The order 1 includes a printed circuit board PCB from an electrical insulating base material, such as FR4. There are printed conductors on the printed circuit board PCB. The tracks are etched from a thin layer of copper. The top layer (English expression: layer) should be called PCB.top, the bottom layer PCB.bottom. Both PCB.top and PCB.bottom are layers on the surface of the PCB PCB, with PCB.top and PCB.bottom arranged on opposite sides. "Opposite" is not to exclude in this context that the two coils L1, L2 are offset from one another, ie that their centers are shifted from each other.

The printed conductors PCB.top and PCB.bottom are designed such that they each form a coil L1 or L2. The coils L1 and L2 are designed approximately annular or rectangular helical, as in 2 B recognizable in the plan view. On a first layer of the printed circuit board, a printed circuit board PCB.top is guided from the outside into a center like a spiral to form a coil. Via a centrally arranged through-hole, the conductor track is interconnected (not visible) in a second, below arranged layer and led to the outside.

2 B shows an embodiment. In 2 B the coil L1 is formed by two individual coils, that is, the coil is also guided helically to the outside after the via in a second, below arranged layer PCB.inner.L1. The conductor structures formed in these two layers PCB.top, PCB.inner.L1 thus act as a coil.

The two layers PCB.top and PCB.inner.L1 thus form the primary coil L1 on a first side of the printed circuit board PCB. On a second side of the printed circuit board PCB, the secondary coil L2 is formed from the layers PCB.bottom and PCB.inner.L2. These two coils L1, L2 form a print transformer 3 as galvanic separating interface. The coils L1 and L2 can as in 2a from a layer of windings or as in 2 B consist of two layers of windings. Needless to say, even more layers on windings are conceivable. The primary coil L1 is on the primary side, while the secondary coil L2 is on the secondary side.

An advantageous embodiment consists in that one uses for the print coil copper layers (ie about PCB.top and PCB.bottom and their respective underlying layer) in the circuit board an increased copper thickness, z. B. 36 microns and more, while for the Ableitstrukturen 2 only uses standard copper thicknesses (eg 18 μm).

The diameter of the coils L1 and L2 is about 5 mm.

The capacitive coupling of the directly opposite internal conductor structures leads to this print transformer 3 significant interference signals can be coupled, provided that there is a potential difference between the circuits of the primary and secondary side. The result is an electric field, wherein this electric field with time change displacement currents, so-called noise currents, cause, which manifest themselves in a superimposed on the useful signal noise.

To avoid this, a fan-shaped, meandering or helical discharge structure is used 2 used. In the example shown, the umbrella structure is fan-shaped. However, it is only crucial for the functioning of the principle that the structure used for shielding consists of thin individual lines, for. B. of a width of at most 150 microns, composed and in the mathematical sense forms a "simply coherent area", ie that the individual lines of the Ableitstruktur 2 do not form any conductor loop, which could act on the coils L1 and L2 as Kurschlusswindung. This can be done in the dissipation structure 2 induced unwanted eddy currents do not form loops that enclose large areas. The eddy current loops that form at most as much as possible thus form along the edges of the thin individual conductors, so that flow components flowing back and forth are thus, for. B. 150 microns are spaced. Thus, the area enclosed by eddy currents within the Ableitstruktur 2 in a spatial direction on z. B. limited to 150 microns, so that the primary coils are inductively not significantly affected. By required to avoid eddy currents gaps within the z. B. fan-shaped topology of the dissipation structure 2 possibly worsens. the dissipation effect. Advantageously, therefore, the distance of the individual conductors within the Ableitstruktur 2 chosen as small as technologically secure controllable, z. B. also 150 microns. The distance of the dissipation structure 2 of the coils L1 and L2 should therefore advantageously be chosen at least as large as the width of the gaps within the Ableitstruktur due to the gaps within the Ableitfläche 2 so that the dissipation structure 2 On the one hand with respect to the capacitive Ableitwirkung is almost as effective as a continuous Ableitfläche and on the other hand with respect to the inductive reaction (as a short circuit winding) on the coil pair L1 / L2 is negligible.

The dissipation structure 2 can be like in 2 B shown also from two layers, ie a primary lead structure 2.1 and a secondary tapping structure 2.2 , exist, wherein the primary lead structure 2.1 with the mass of the primary side and the secondary circuit structure 2.2 connected to the ground of the secondary side. In a preferred embodiment the primary lead structure and the secondary lead structure - as just described - on different layers of the circuit board, in particular on two different inner layers of the circuit board. Alternatively, the primary baffle structure and the secondary baulk structure may be disposed on the same inner layer of the printed circuit board. If the primary baffle structure and the secondary baffle structure are located on different layers of the printed circuit board, it is preferable to choose a distance between the corresponding layers which takes into account the regulations of the "type of protection intrinsically safe". The distance between the primary baffle structure and secondary baffle structure is then about 1 mm.

The consequence of the installation of the dissipation structure 2 is that the primary and secondary side are still capacitively coupled and thus continue to flow capacitive interference currents between the primary and secondary side. If the respective arrester structures are connected to the respective ground potentials as mentioned above, then these divert most of the interfering currents that form. Thus, these interference currents are now bypassed the coils L1, L2. In particular, the weaker secondary-side signals of the secondary coil L2 used for the reception are now influenced by far weaker interference components.

The dissipation structure 2 is formed from an inner layer PCB.inner.2 in the printed circuit board PCB. In the above embodiment with two Ableitstrukturen 2.1 and 2.2 the dissipation structure is formed by the two inner layers PCB.inner.2.1 and PCB.inner.2.2.

For the effectiveness of the shield 2 It is especially important that the secondary coil L2 does not pass any interference currents. Since the signal levels on the primary side are much stronger, embodiments are also conceivable in which only one, then advantageously secondary side connected Ableitstruktur 2.2 is available.

It is crucial that at least one secondary-side dissipation structure 2.2 is present, which offers a signal path which is less impedance than the Störsignalpfad via the secondary coil L2. Advantageously, therefore, the first terminal L2.1 of the secondary coil, which is connected to the input 7.in the Schmitt trigger 7 (see below), tapped on the inner layer PCB.inner.L2 PCB PCB and led the less critical port L2.2 in the surface layer PCB.bottom.

A disadvantage of the installation of the dissipation structure 2 is that the coils L1, L2 are more capacitive coupled to the respective (primary side or secondary side) earth or ground potential. The dissipation structure 2 acts as a kind of capacitor plate, which is opposite to a second capacitor plate formed by the internal conductor structure. In combination with the high ohmic resistance, this results in a reduction of the generated signal levels. When installing the shield structure, higher demands are placed on the evaluation of the electrical signals.

The driver circuit 4 and receiving circuit 5 for control and evaluation for the print transformer 3 sees itself confronted with the following two challenges: power consumption of the circuit as well as low signal levels and very fast signal transients on the secondary side.

When used in the explosion-proof area, a typical feature of electrical circuits is that they have to provide themselves from an extremely limited power budget. The immediate consequence is z. Example is that very low-power operational amplifier or comparators must be used. However, there is the immediate physical connection that low-noise and fast analog circuits inevitably have higher power consumption than more noisy slow circuits.

The digital signals are to be considered below for illustration without limitation in the example as UART signals (English: Universal Asynchronous Receiver Transmitter, short UART). Here is a UART is an electronic circuit that serves to realize digital serial interfaces. This can be either an independent electronic component (a UART chip or component) or a functional block of a component with greater integration (eg a microcontroller). A UART interface is used to send and receive data over a data line. The data is transmitted as a serial digital data stream with a fixed frame consisting of a start bit, five to a maximum of nine data bits, an optional parity bit for detecting transmission errors and a stop bit.

The transmission method is divided into three functional parts. First, the control of the transformer 3 on the primary side via a driver circuit 4 , the transmitter 3 itself, and the reconstruction of the signal on the secondary side via a receiving circuit 5 ,

For signal transmission with only low power as in the example described above to a sensor 21 , it is particularly useful to use pulses for transmission. The edges of the input signal dig.in are converted into short pulses. A positive edge corresponds to one positive pulse, negative edge negative pulse (or vice versa).

About the driver circuit 4 the digital signals dig.in be coupled into the primary coil L1. The driver circuit 4 includes an output impedance R1 and a coupling capacitor C1 connected to the primary coil L1. For example, the driver circuit generates 4 a square wave signal, e.g. B. by means of a bridge circuit in the form of a half or full bridge. The advantage of a full bridge formed from two half-bridges driven in antiphase is that at the cost of more circuit complexity compared to the half-bridge in the driver circuit 4 the double voltage level of the square wave signal can be generated. Advantageously, the driver circuit includes digital gate z. 74LVC family, which combine the advantages of low power consumption, low cross-flow losses and low output impedance. The half bridges would then be formed by the p and n channel transistors of the digital gate outputs.

The coupling capacitor C1 and the primary coil L1 form a series resonant circuit having a resonance and a period T1. The damping time constant t1 is calculated with t1 = R1 * C1 and the components are chosen such that the damping time constant t1 is greater than or equal to 10% of the period T1. The series resonance between C1 and L1 is so heavily attenuated that unwanted oscillations are so strongly suppressed that they can not lead to a malfunction of the circuit. For example, L1 and C1 can be dimensioned to give an effective current pulse duration of about 200 ns. In one embodiment, the damping time constant t1 is less than the reciprocal of the transmission frequency of the digital signals.

The conversion of the digital signals into pulses is in 3 by the reference numeral 6 symbolizes. There are various design options for this transformation.

For example, the digital signal, possibly amplified, can be conducted to a first contact L1.1 of the primary coil L1 and inverted to a second contact L1.2 of the primary coil L1. As a result of this wiring, compared with wiring without an inverting amplifier, a double voltage is produced at the primary coil L1 and thus an increased signal on the secondary side.

For the evaluation of about 200 ns pulses you need fast evaluation circuits, z. B. a comparator with a propagation time of <100 ns and external circuitry for a Schmitt trigger, which realizes the desired hysteresis. However, such fast analog comparators require very high operating currents, which in practice can not be accommodated in the budget of two-wire devices.

Instead of fast analog comparators, digital gates are used 7 used with Schmitt trigger input, in which hysteresis and switching threshold, however, poorly defined and in particular is not adjustable. In particular, the Schmitt trigger 7 designed as a low-power gate, for example, from the 74AUP series. The Schmitt trigger 7 has low input capacitances and thus loads the signals of the secondary coil L2 capacitively only comparatively weak.

In one embodiment, the Schmitt trigger 7 supplied with an adjustable operating voltage VDD2. About the detour of the operating voltage VDD2 can thus be at the input 7.in control effective hysteresis and switching threshold altitude.

To the output signals of the Schmitt trigger 7 To implement the logic levels required for further processing becomes a level shifter 8th used. The level converter 8th is supplied with a further, compared to the operating voltage VDD2 increased, operating voltage VDD1 to the level at the input 8.in set to a higher value. As a level converter, for example, a converter from the 74LVC family is used, which has higher input capacitances compared to logic families such as 74AUP, but can also drive higher output voltages and has installed lower-impedance output drivers. The level converter 8th has in particular a low-impedance output. It is an evaluation circuit 11 , z. As a microcontroller, provided that via the galvanically isolating interface 3 processed digital signals processed. The output is called dig.out.

It is still a subcircuit 9 provided that the signal level at the output 7.out the Schmitt trigger 7 constant in the absence of pulses. This includes the subcircuit 9 a feedback of the output 7.out the Schmitt trigger 7 on the entrance 7.in the Schmitt trigger 7 , It is at least an attenuator 10 provided that attenuates the vibrations of the pulses to the secondary coil L2, in particular potentially self-resonant vibrations of the coil L2. Such disturbing self-resonant vibrations can z. B. generated by parasitically effective capacitors between the terminals L2.1 and L2.2, which together with the inductance L2 can form a parallel resonant circuit.

Next are at the entrance 7.in the Schmitt trigger 7 two diodes D1 and D2 connected. D1 is connected to ground in the same direction against the first operating voltage VDD2, D2. The advantage of this circuit measure is apparent from the following example. It is assumed that both the output and the input signal are initially at a logic 0 level and the Schmitt trigger on the secondary side is operated with a supply voltage VDD2 of 1 V.

Initially, both the signals are located 7.in and 7.out at a voltage level of 0 V. If a level change 0 -> 1 occurs at the input signal of the primary side, then a positive current pulse is impressed into the primary coil L1. This pulse generates at the secondary coil L2 a voltage signal of, for example, 0.8 V. The input at the Schmitt trigger 7.in applied voltage thus exceeds the, for example, design-related, located at VDD2 / 2 = 0.5 V switching threshold and causes the output of the Schmitt trigger to the voltage VDD2 = 1 V switches. Since the secondary coil L2 between Schmitt trigger output 7.out and input 7.in is connected, would result after switching the output at the input of the Schmitt trigger without diode D1, a voltage of about 1 V + 0.8 V = 1.8 V at the output 7.in , However, the diode D1 derives this overvoltage, so that only an overvoltage of VDD2 + V _diode = 1 V + 0.4 V = 1.4 V is set at the input. Diode D1 protects the input of the Schmitt trigger on the one hand 7.in before overvoltage, on the other hand, it ensures that transient overvoltages do not lead to oscillations high voltage amplitude, which the Schmitt trigger 7 could cause unwanted switching to the 0 level.

Diode D2 acts in a similar way to suppress undervoltages at the input 7 , in, which can occur with level changes 1 -> 0 and limits these to a maximum of about -0.4 V.

Over- and undervoltage levels are particularly undesirable because the secondary coil L2 between the terminals L2.1 and L2.2 has a certain unavoidable coupling capacitance which, together with the secondary coil L2, has a parallel resonant circuit with a parallel resonance frequency. If an overvoltage or undervoltage level is initially present at the connection L2.1, a damped oscillation with approximately the parallel resonance frequency will form between the connections L2.1 and L2.2. The advantage of adding the resistor 10 is that this vibration is additionally damped and thus decays faster.

The advantage of the diodes D1 and D2 is that the maximum voltage amplitude of these unwanted oscillations is limited to the forward voltage of the diodes. If one uses z. As diodes with a forward voltage of about 0.4 V, this ensures that the switching voltage of the Schmitt trigger of about VDD2 / 2 = 0.5 V can not be exceeded by the unwanted parallel resonant vibrations.

LIST OF REFERENCE NUMBERS

1

arrangement

2

Ableitstruktur

2.1

Primärableitstruktur

2.2

Sekundärableitstruktur

3

galvanically isolating interface

4

driver circuit

5

receiving circuit

6

pulse conversion

7

Schmitt trigger

7.in

Entrance to 7

7.out

Exit to 7

8th

level converter

8.in

Entrance to 8th

9

Subcircuit of 5

10

attenuator

20

field device

21

sensor

22

First interface

23

Second interface

24

electric wire

dig.in

incoming digital signals

DIG.OUT

outgoing digital signals

C1

capacitor

L1

primary coil

L1.1

First contact from L1

L1.2

Second contact from L1

L2

secondary coil

PCB

circuit board

PCB.bottom

Bottom layer of PCB

PCB.inner.L1

Inner location of PCB

PCB.inner.L2

Inner location of PCB

PCB.inner.2

Inner location of PCB

PCB.inner.2.1

Inner location of PCB

PCB.inner.2.2

Inner location of PCB

PCB.top

Top location on PCB

R1

resistance

VDD1

First supply voltage

VDD2

Second supply voltage

Claims (17)

Arrangement ( 1 ) for transmitting digital signals via a galvanically isolating interface ( 3 ) from a primary side to a secondary side a printed circuit board (PCB) having a primary coil (L1) and a secondary coil (L2), wherein the primary coil (L1) is arranged on the primary side on at least a first layer of the printed circuit board, wherein the secondary coil (L2) on the secondary side on at least one second Position of the printed circuit board (PCB) is arranged so that the primary coil (L1) and the secondary coil (L2) are opposite, wherein the primary coil (L1) and the secondary coil (L2), the galvanically isolating interface ( 3 form) for transmitting digital signals, and wherein in the printed circuit board (PCB) between the primary coil (L1) and the secondary coil (L2) a diverting structure ( 2 ), which shields the primary coil (L1) and / or the secondary coil (L2) from interference currents caused by capacitive coupling between the primary coil (L1) and the secondary coil (L2), in particular by the parasitic currents being applied to the dissipation structure ( 2 ) be derived. Arrangement ( 1 ) according to claim 1, wherein the primary coil (L1) and / or the secondary coil (L1) as a conductor track (PCB.top, PCB.bottom, PCB.inner.L1, PCB.internal.L2) on / in the printed circuit board (PCB). are configured, and wherein the discharge structure ( 2 ) is designed as a conductor track (PCB.inner.2, PCB.inner.2.1, PCB.inner.2.2) in the printed circuit board. Arrangement ( 2 ) according to claim 2, wherein the primary coil (L1), secondary coil (L2) and discharge structure ( 2 ) each comprise two layers of the printed circuit board, wherein the Ableitstruktur ( 2 ) at least one primary control structure ( 2.1 ) and a secondary billing structure ( 2.2 ) and the primary control structure ( 2.1 ) of the primary coil (L1) and the secondary conductive structure ( 2.2 ) are associated with the secondary coil (L2). Arrangement ( 1 ) according to claim 3, wherein the primary coil (L1) and the secondary coil (L2) each have a surface layer (PCB.top, PCB.bottom) and an inner layer (PCB.inner.L1, PCB.inner.L2) of the printed circuit board ( PCB). Arrangement ( 1 ) according to at least one of claims 1 to 4, wherein the dissipation structure ( 2 ) comprises interconnected individual lines, in particular the diverting structure is designed fan-shaped, meandering or helical. Arrangement ( 1 ) according to at least one of claims 1 to 5, wherein the discharge structure ( 2 ) is connected to ground. Arrangement ( 1 ) according to at least one of claims 3 to 6, wherein the primary baffle structure ( 2.1 ) with ground of the primary side, and the secondary conductive structure ( 2.2 ) are connected to ground of the secondary side. Arrangement ( 1 ) according to at least one of claims 1 to 6, wherein the digital signals are transmitted in the form of pulses at level changes, in particular corresponds to a digital level change 0 -> 1 a positive pulse and a level change 1 -> 0 a negative pulse, or vice versa. Arrangement ( 1 ) according to at least one of claims 1 to 8, wherein the arrangement comprises a driver circuit ( 4 ) for coupling (dig.in) the digital signals into the primary coil (L1), wherein the driver circuit has an output impedance (R1) and is connected to the primary coil (L1) via a coupling capacitor (C1). Arrangement ( 1 ) according to claim 9, wherein the coupling capacitor (C1) and the primary coil (L1) form a series resonant circuit having a resonance and a period duration (T1), and the damping time constant (t1) formed by the output impedance (R1) and the coupling capacitor (C1) t1 = R1 · C1 is greater than or equal to 10% of the period (T1). Arrangement ( 1 ) according to at least one of claims 1 to 10, wherein the arrangement comprises a receiving circuit ( 5 ) for coupling out (dig.out) the digital signals from the secondary coil (L2), wherein the receiving circuit comprises a Schmitt trigger ( 7 ), where a contact ( 12.1 ) of the secondary coil with the input ( 7.in ) of the Schmitt trigger, and wherein the receiving circuit has a subcircuit ( 9 ), which determines the signal level of the output ( 7.out ) of the Schmitt trigger remains constant in the absence of pulses on the secondary coil (L2), in particular the output ( 7.out ) of the Schmitt trigger as feedback to the input ( 7.in ) of the Schmitt trigger. Arrangement ( 1 ) according to claim 11, wherein the secondary side has an evaluation circuit operated with a first supply voltage (VDD1) ( 11 ), which are connected via the galvanically isolating interface ( 3 ) processed digital signals, and connected to the secondary coil (L2) Schmitt trigger ( 7 ) is operated with one of the first supply voltage (VDD1) different, in particular a lower, second supply voltage (VDD2), and the signal level at the output ( 7.out ) of the Schmitt trigger ( 7 ) by means of a level converter ( 8th ) on the Signal level of the first supply voltage (VDD1) is converted. Arrangement ( 1 ) according to claim 11 or 12, wherein the subcircuit ( 9 ) at least one attenuator ( 10 ), in particular at least one resistor, wherein the attenuator in particular in the feedback between the output ( 7.out ) and the entrance ( 7.in ) of the Schmitt trigger. Arrangement according to at least one of claims 11 to 13, wherein the input ( 7.in ) of the Schmitt trigger ( 7 ) is connected to the inner layer (PCB.inner.L2) of the printed circuit board (PCB). Arrangement according to at least one of claims 11 to 14, wherein at the entrance ( 7.in ) of the Schmitt trigger a first diode (D1) with cathode is connected against the second supply voltage (VDD2) and anode against the input of the Schmitt trigger, and a second diode (D2) with cathode against the input of the Schmitt trigger ( 7.in ) and anode is connected to ground of the secondary side. Field device ( 20 ), in particular transmitters, the process automation technology comprising an arrangement ( 1 ) according to at least one of claims 1 to 15. Field device ( 20 ) according to claim 16, wherein the digital signals (dig.out) a sensor ( 21 ).

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