Abstract:
The invention relates to an analog isolation device ( 100 ) comprising a primary part ( 102 ) and a secondary part ( 104 ) separated by an electrical isolation barrier, these parts including a high-frequency channel configured to produce a high-frequency component in the secondary part and a low-frequency channel configured to produce a low-frequency component in the secondary part, in order to form the output signal from the high-frequency and low-frequency components, the device further including a control circuit (D 1, 132 ) configured to receive, in the primary part, a setpoint signal (S ic ) and a so-called image signal (S oim ) representative of the output signal (S o ) and to apply in the high-frequency and/or low-frequency channel a correction signal V COR  as a function of the difference between the image signal and the setpoint signal in order to cause that difference to tend toward zero.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an electrical isolation device capable of coupling together two circuits at frequencies from direct current (DC) up to high frequencies, which two circuits are at different electrical potentials, and the invention notably applies to electrical measuring instruments such as voltmeters and oscilloscopes. 
     A problem arises when the source of a signal to be measured and the measuring instrument itself do not share a common ground reference. 
     Floating-ground operation of the measuring instrument is not a satisfactory solution, firstly because the accuracy of the measurement may be affected by the presence of ground currents, and secondly because there is a risk of the potential of the measuring instrument reaching a dangerous level. 
     Proposals have therefore been made to transmit a signal that is to be measured across an electrical isolation barrier disposed between the source of the signal and the measuring instrument, which source and instrument may then retain their respective ground references, with floating-ground operation of the measuring instrument thus being avoided. 
     An analog isolation device incorporating such an electrical isolation barrier and shown diagrammatically in  FIG. 1  is described in the document EP 0 875 765. 
     That known device  10  comprises a primary part  12  and a secondary part  14  isolated from each other by an isolation barrier  16 . As shown in  FIG. 1 , the primary and secondary parts are connected to respective independent grounds. 
     The device  10  receives an input signal at an input A of the primary part  12  and is designed to deliver an isolated output signal at an output B of the secondary part  14 , reproducing the input signal  16 . To this end, the device  10  must have a flat frequency response, that is to say it must supply an output signal of amplitude that is identical (ignoring any multiplier coefficient) to the amplitude of the input signal, and it must do so across the whole of the range of usable frequencies. 
     To this end, the device comprises two parallel channels, a high-frequency (HF) channel conveying the HF component of the input signal and a low-frequency (LF) channel conveying the LF component of the input signal, the output signal being obtained by summing the LF and HF components that reach the secondary part. 
     The HF channel includes a transformer  18  having its primary P connected to the input A. On its secondary winding S, the transformer  18  faithfully reproduces the HF component of the input signal, but its frequency response deteriorates rapidly at low frequencies. 
     The LF channel includes an opto-coupler  20  comprising an electro-optical emitter  20   a , such as a light emitting diode (LED), connected to the input A and coupled on the secondary side to an opto-electric receiver, such as a photodiode  20   b , that delivers current that is converted to a voltage by a circuit  22 . The opto-coupler  20  further includes a second opto-electric receiver, such as a photodiode  20   c  similar to the photodiode  20   b  and also coupled to the electro-optical emitter  20   a , but situated on the primary side, the current from the photodiode  20   c  being converted into a voltage by a circuit  24 . The output voltage of the circuit  24  is applied to the inverting input of an amplifier  26  receiving the input signal on its non-inverting input, the circuit  24  and the amplifier  26  forming a feedback loop for linearizing the response of the opto-coupler  20 . The opto-coupler  20  faithfully reproduces the LF component of the input signal, but its frequency response deteriorates rapidly at high frequencies. 
     The transformer  18  and the opto-coupler  20  form the electrical isolation barrier  16 . The output signal is obtained by summing the HF component on the secondary of the transformer  18  and the LF component at the output of the circuit  22  by means of a circuit  28 . 
       FIG. 2  is a Bode diagram showing the frequency responses H 1 ( f ) and H 2 ( f ) of the LF and HF channels, respectively. 
     To obtain a flat overall frequency response, that is to say for the output signal faithfully to reproduce the input signal throughout the range of useful frequencies, it is necessary for there to be a corresponding relationship between the high cut-off frequency F LF1  of the LF channel (opto-coupler  20 ) and the low cut-off frequency F HF1  of the HF channel (transformer  18 ). 
     To this end, a fraction of the output voltage of the circuit  24  as determined by a divider circuit  30  is applied to the non-inverting input of an amplifier  32  having its inverting input receiving the input signal and having its output connected to primary P of the transformer  18 . The output voltage of the circuit  24  is an image of the LF component transmitted to the secondary via the opto-coupler  20 . The division ratio of the circuit  30  is adjusted to subtract from the input signal a fraction of the LF component such that the cut-off frequency F HF  is aligned with the cut-off frequency F LF . 
     The Applicant has nevertheless determined that that technique of compensating misalignment of the cut-off frequencies of the LF and HF channels does not guarantee a totally satisfactory result, that is to say the absence of any significant distortion of the output signal compared to the input signal. It is difficult to adjust the division ratio of the circuit  30  in an optimal fashion. Moreover, there is no compensation of asymptotic response errors in the LF channel resulting from the presence of orders higher than 1 in the cut-off frequency of that channel. Moreover, since the compensation is effected by aligning the cut-off frequency of the HF channel with that of the LF channel, the LF channel operates at full bandwidth, and it is necessary for the output from the opto-coupler going respectively to the secondary part via the photodiode  20   b  and to the primary part via the photodiode  20   c  to have the same bandwidth and the same gain, which requires delicate adjustments and components to be chosen that have small differences between their characteristics. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The aim of the invention is to remedy the drawbacks referred to above by proposing an analog isolation device capable of faithful reproduction of an input signal without notable distortion in a very wide range of frequencies. 
     To obtain a transfer function totally free of amplitude and phase distortion, three conditions must be combined:
         equality of the HF and LF gains in each bandwidth;   cut-off frequencies equal to within −3 dB; and   cut-off orders that are the same of the high-pass (HF) cut-off is of the first order, then the low-pass (LF) cut-off must also be of the first order).       

     The above aim is achieved by an analog isolation device comprising a primary part having an input adapted to receive an input signal and a secondary part separated from the primary part by an electrical isolation barrier and having an output for delivering an output signal reproducing the input signal;
         the primary part and the secondary part including a high-frequency channel having a low cut-off frequency and configured to receive the input signal in the primary part and to produce a high-frequency component in the secondary part from the input signal, and a low-frequency channel having a high cut-off frequency and configured to receive the input signal in the primary part and to produce a low-frequency component in the secondary part from the input signal, in order to form the output signal from the high-frequency and low-frequency components;   the device being provided with a control circuit configured to receive, in the primary part, a setpoint signal representative of the input signal and a so-called image signal representative of the output signal, and to apply in the high-frequency and/or low-frequency channel a correction signal as a function of the difference between the image signal and the setpoint signal in order to cause that difference to tend towards zero.       

     This device is advantageous in that it applies closed loop control to compensate distortion of the output signal relative to the input signal dynamically and regardless of its causes. 
     The correction signal may be applied to the low-frequency channel only, to the high-frequency channel only, or to both channels. 
     According to one feature of the device, the primary part includes a summing element adapted to produce the image signal by summing a first signal representative of said low-frequency component and a second signal representative of said high-frequency component. 
     The first signal and the second signal may be in the form of currents or voltages. 
     According to another feature of the device, the low-frequency channel comprises an electro-optical coupler forming part of the electrical isolation barrier and having an electro-optical emitter, a first opto-electrical receiver situated in the secondary part and coupled to the electro-optical emitter to produce a current enabling said low-frequency component to be generated, and a second opto-electrical receiver situated in the primary part and coupled to the emitter to produce a current enabling said first signal representative of said low-frequency component to be generated. The correction signal V COR  may be applied to a circuit for linearizing the opto-coupler connected to the electro-optical emitter. 
     According to another feature of the device, the high-frequency channel includes a transformer forming part of the electrical isolation barrier and having a primary situated in the primary part and a secondary situated in the secondary part to obtain said high-frequency component from a voltage taken from the secondary of the transformer and to obtain said second signal representative of said high-frequency component from a voltage taken from the primary of the transformer. The correction signal V COR  may be in the form of a voltage applied to the primary of the transformer. 
     The control circuit preferably has a high cut-off frequency at least 100 times greater than the low cut-off frequency of the high-frequency channel. Thus, beyond the cut-off frequency of the control circuit, the input signal is transmitted virtually exclusively via the high-frequency channel and the problem of misalignment between the cut-off frequencies of the low-frequency and high-frequency channels no longer arises. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood on reading the following description given by way of non-limiting illustration only and with reference to the appended drawings, in which: 
         FIG. 1 , described above, shows a known prior art isolation device; 
         FIG. 2 , described above, shows the frequency responses of a transformer and of an opto-coupler of the  FIG. 1  device; 
         FIG. 3  shows an isolation device of a first embodiment of the invention; 
         FIG. 4A  shows a frequency response of a device such as that of  FIG. 3  and the variation of the correction signal when no correction signal is applied; 
         FIG. 4B  shows the frequency response of the device of  FIG. 3  and the variation of the correction signal when a correction signal is applied; and 
         FIGS. 5 ,  6 , and  7  show isolation devices of second, third, and fourth embodiments of the invention, respectively. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The isolation device from  FIG. 3  comprises a primary part  102  and a secondary part  104  isolated from each other by an electrical isolation barrier  106 . These two parts have respective independent grounds GND 1  and GND 2 . The device  100  is designed to receive an input signal S i  at the input of the primary part and to deliver at the output of the secondary part an output signal S o  reproducing the signal S i , where applicable with a multiplier coefficient not equal to 1. In the  FIG. 3  embodiment, the input signal S i  is a differential signal received at a bipolar input A 1 , A 2  and the output signal is a non-differential signal emitted at a unipolar output O 1 . 
     The isolation device  100  comprises two parallel channels, namely a high-frequency (HF) channel and a low-frequency (LF) channel. 
     The HF channel includes a transformer  108  forming part of the isolation barrier  106 . In the primary part  102 , the transformer has a primary with two half-windings P 1 , P 2  having first ends connected to the input ports A 1 , A 2  via respective resistors R 1 , R 2  and second ends connected in common to the ground GND 1 . In the secondary part  104 , the transformer  108  has a secondary with two half-windings S 1 , S 2  having first ends connected to a summing circuit  110  and second ends connected to the ground GND 2 . Resistors R 3 , R 4  respectively connected between the first ends of the half-windings S 1 , S 2  and the ground GND 2  serve both to adjust the fractions of the voltages that are taken from the half-windings S 1 , S 2  and that are applied to the summing circuit  110  and also to load the secondary of the transformer  108 . It should be noted that grounding the half-windings on the primary side and on the secondary side provides good common mode rejection without requiring the use of a screen in the transformer. 
     The transformer  108  has a low cut-off frequency F HF  (cut-off frequency of the HF channel). Accordingly, from the input signal, the HF channel enables the secondary part  104  to produce a high-frequency component that, in the  FIG. 3  embodiment, is applied to the summing circuit  110  in differential form. 
     The LF channel includes an opto-coupler  114  forming part of the isolation barrier  106 . The opto-coupler  114  includes an electro-optic emitter, typically an LED, optically coupled both to a first opto-electric receiver D p  situated in the secondary part and also to a second opto-electric receiver situated in the first part. The receivers D s  and D p  are typically photodiodes having respective bias voltages V ees  and V eep  applied thereto. The LED is connected to one of the input ports (A 1  in the example shown), notably via a linearization circuit  116  as described below. The current from the receiver D s  is converted into a voltage by means of a converter circuit  118  including an operational amplifier AO 1  having an inverting input connected to the receiver D, and a non-inverting input, connected to the ground GND 2 . A circuit formed by a resistor R 5  shunting a capacitor C 5  is connected between the output of the amplifier AO 1  and its inverting input. In a similar fashion, the current from the receiver D p  is converted into voltage by means of a converter circuit  120  including an operational amplifier AO 2  having an inverting input connected to the receiver D p  and a non-inverting input connected to the ground GND 1 . A circuit formed by a resistor R 6  shunting a capacitor C 6  is connected between the output of the amplifier AO 2  and its inverting input. 
     The combination of the LED and the receiver D s  has a high cut-off frequency F LF  (cut-off frequency of the LF channel). Accordingly, starting from the input signal, the LF channel enables the secondary part  104  to produce a low-frequency component V LF  available at the output of the converter  118  and applied to the summing circuit  110 . 
     The summing circuit  110  includes an operational amplifier AO 3  having a non-inverting input connected to the first end of the half-winding S 1  via a resistor P 3  and connected to the output of the converter circuit  118  via an inverting circuit  122  and a resistor R 8 . The inverting input of the amplifier AO 3  is connected to the first end of the half-winding S 2  via a resistor R 9  and is connected to the output of the converter circuit  118  via a resistor R 10 . The output of the amplifier AO 3  is connected to the output O 1  of the secondary part. The inverter  122  enables a low-frequency component to be produced in differential form for summing with the differential high-frequency component, and the amplifier AO 3  delivers the output signal in non-differential (unipolar) form. 
     It would also be possible to obtain an output signal in differential form by using a second opto-coupler analogous to the opto-coupler  114  and connected to the second input port in order to obtain a low-frequency component that is in differential form, and to combine it with the high-frequency component in differential form, the isolation device then being entirely bipolar. 
     At the output of the converter  120  there is a voltage V LFi  that is equal or substantially equal to the low-frequency component V LF  in that the gains of the respective combinations formed by the LED and the receiver D and by the LED and the receiver D p  are substantially equal, and likewise their cut-off frequencies are substantially equal since the amplifiers AO 1  and AO 2  are identical. The resistor R 5  is advantageously a variable resistor in order to be able to equalize these gains, while the capacitors C 5  and C 6  enable the high cut-off frequencies to be equalized. 
     The linearization circuit  116  includes an operational amplifier AO 4  having its inverting input connected to the input port A 1  via a resistor R 11  and to the output of the converter  120  via a resistor R 12 . The output of the amplifier AO 4  is connected to the LED via a resistor R 13  and a circuit formed by a capacitor C 7  in series with a resistor R 14  is connected between the output of the amplifier AO 4  and its inverting input. The ratio between the resistances of the resistors R 11  and R 12  determines the gain of the LF channel. 
     To obtain an output signal S o  that faithfully reproduces the input signal S i , it is necessary to correct a misalignment between the cut-off frequencies F LF  and F HF  of the LF and HF channels. The invention does this by the primary part reproducing an image signal S oim  representative of the signal S o  and injecting a correction signal into the LF channel and/or into the HF channel, which correction signal is produced from the difference between the signals S i  and S cim . 
     In the  FIG. 3  embodiment, the correction signal V COR  is injected into the LF channel. 
     To produce the image signal S oim , a first signal S oi1  representative of the LF component transmitted into the secondary part  104  by the opto-coupler  114  and a second signal S oi2  representative of the HF component transmitted into the secondary part  104  are summed by the transformer  108 . The output of the converter  120  is connected to a summing node S 1  via a resistor R 15  to provide the signal S oi1  in the form of a current. The voltage at the first end of one of the half-windings of the primary of the transformer, here the winding P 2 , is the image of the voltage at the corresponding secondary half-winding. The first end C 2  of the half-winding P 2  (which is connected to the input port A 2  via the resistor R 2 ) is connected to the summing node S 1  via an operational amplifier AO 5  and a resistor R 16  to provide the signal S oi2  in the form of a current. In the example in which the transformer  108  and the opto-coupler  114  have the same gain and the amplifier AO 5  has unity gain, the values of the resistors R 15  and R 16  are chosen so that R 15 =2×R 16  to take account of the fact that the voltage at the point C 2  represents only half the overall HF component transmitted into the secondary part  104 . 
     The image signal (current) S oim  obtained at the point S 1  by summing the currents S oi1  and S oi2  is applied to a summing point D 1  that is also connected to the input port A 1  via a resistor R 17 . The summing point D 1  therefore receives the image signal S oim  and a setpoint signal S ic  representative of the input signal S i , which signals S oim  and S ic  have opposite polarities. The resistor R 17  is chosen so that the signals S oim  and S ic  in the form of currents represent the signals S o  and S i  in the same ratio so as to be meaningfully comparable (in the example where the output signal S o  reproduces the input signal S i  in a ratio equal to 1). 
     The error signal S ERR  in the form of a current representing the difference between S ic  and S oim  and coming from the summing point D 1  is applied to the input of a PI (proportional/integral) corrector circuit  132 . In conventional manner, the circuit  132  includes an operational amplifier AO 6  having its non-inverting input connected to the ground GND 1 , its inverting input connected to the summing point D 1 , and its output connected to its inverting input via a circuit formed of a capacitor C 8  in series with a resistor R 18 . A correction signal is thus obtained at the output of the P 1  corrector circuit  132 , which correction signal, here a voltage V COR , is applied to the LF channel by being injected in the linearization circuit  116  into the non-inverting input of the amplifier AO 4 . The current injected into the LED of the opto-coupler is therefore modified as a function of the difference between the image S oim  of the output signal and the input signal S i . 
     Closed loop control is thus achieved, with the correction signal V COR  modifying the operation of the LF channel to cancel out the difference between the image S oim  of the output signal and the input signal S i , that is to say by aligning the cut-off frequency F LF  of the LF channel with the cut-off frequency F HF  of the HF channel and by aligning the asymptotic responses (cut-off order). 
     The control circuit, which is formed of the elements for producing the image signal S oim  and the error signal S ERR  and of the PI corrector circuit  132  delivering the correction signal V COR , has its own high cut-off frequency F CASS  and is preferably designed so that this high cut-off frequency is very much higher than the low cut-off frequency F HF  of the HF channel. It is advantageous if F CASS ≧100 F HF , or even F CASS ≧1000 F HF . Thus beyond the frequency F CASS , all or almost all of the input signal is transmitted by the HF channel only, with the result that the loss of the effective servo-control becomes inconsequential. The high cut-off frequency F CASS  may be set by operating on the values of the capacitor C 8  or of the resistors R 16 , R 17 , and/or R 15 . 
       FIG. 4A  shows the frequency response H′(f) of an isolation device like that from  FIG. 3  and the variation of the correction signal V COR  with no correction signal applied, that is to say with the connection open between the PI corrector circuit  132  and the linearization circuit  116 , its amplifier AO 4  having its non-inverting input connected to the ground GND 1 . High distortion of the output signal is observed in an intermediate frequency range by reason of a misalignment between the cut-off frequencies F LF  and F HF  and the corresponding variation of the correction signal V COR  (the distortion is lower with closed loop control). 
       FIG. 4B  shows the frequency response H(f) of the  FIG. 3  device with the correction signal V COR  applied and the corresponding variation of the correction signal. Distortion is found to be virtually absent in the output signal, closed loop control enabling compensation of various causes of distortion, notably a defective asymptotic response in the LF channel. Furthermore, by aligning the cut-off frequency of the LF channel with the substantially lower cut-off frequency of the HF channel, the LF channel is made to function in a reduced bandwidth, enabling the influence of spurious signals transmitted by capacitive coupling to be reduced, and thus enabling common mode rejection in the LF channel to be improved. 
       FIGS. 5 ,  6 , and  7  show other embodiments. 
     In the  FIG. 5  isolation device  200 , the input, of the isolation device is unipolar, the non-differential input signal S i  received at the terminal A 1  and the terminal A 2  being connected to the ground GND 1 . Elements common to the embodiment of  FIG. 5  and of  FIG. 3  carry the same references and are not described again. 
     In the primary part  202 , the transformer  108 ′ includes a single primary winding P′ 1  having a first end connected to the input port A 1  via a resistor R′ 1  and a second end connected to the ground GND 1 . In the secondary part  204 , the transformer  108 ′ includes a single secondary winding S′ 1  having a first end connected to a summing circuit  210  and a second end connected to the ground GND 2 . The first end of the winding S′ 1  is furthermore connected to the ground GND 2  via a resistor R′ 3 . The summing circuit  210  essentially comprises an operational amplifier having a non-inverting input connected to the first end of the winding S′ 1  and an inverting input connected to the output of the converter circuit  118  via a resistor R′ 10 , the resistors R′ 3  and R′ 10  enabling the ratio between the HF and LF components applied to the summing circuit  210  to be adjusted. 
     In the primary part  202 , the image signal S oim  of the output signal S o  is fed to the summing point S 1  that is connected to the output of the converter circuit  120  via the resistor R 15  and that is connected to the first end C′ 1  of the winding P via an operational amplifier AO′ 5  and a resistor R 16 , the ratio between the resistances of the resistors R 15  and R 16  being equal to 1 here. To be more precise, the amplifier AO′ 5  has its inverting input connected to the point C′ 1 , and its output and its non-inverting input connected to the ground GND 1 . The signal S oim  is fed to the summing point D 1 , which also receives the signal S ic  because it is connected to the input port A 1  via the resistor R 17 . The signals S oim  and S ic  (in the form of currents) have opposite polarities. The error signal S ERR  coming from the point D 1  is converted by the PI corrector circuit  132  into a correction signal V COR  applied to the linearization circuit  116 , as in the  FIG. 3  embodiment. 
     In the isolation device  300  from  FIG. 6 , the image signal S oim  is in the form of a voltage. Elements common to the isolation devices  300  and  100  carry the same references and are not described again. 
     The secondary part  304  of the isolation device  300  is similar to the secondary part  104  of the isolation device  100 . 
     The primary part  302  of the device  300  differs from the primary part  102  of the device  100  in that the summing point D 1  is replaced by a differential circuit D′ 1  with an operational amplifier A 07  having its inverting input connected to the summing point S 1  to receive the signal S oim  and having its non-inverting input connected to the port B 1  to receive the signal S ic . The error signal S ERR  in the form of a voltage at the output of the amplifier AO 7  is applied to the PI corrector circuit  132  via a resistor R 19  to generate the correction signal V COR  applied to the linearization circuit  116 . 
     In the isolation device  400  of  FIG. 7 , the input signal S i  is in unipolar form and the correction signal V COR  is applied to the HF channel. Elements common to the isolation device  400  and to the device  200  of  FIG. 5  carry the same references and are not described again. 
     The secondary part  404  of the isolation device  400  is similar to the secondary part  204  of the isolation device  200 . 
     In the primary part  402  of the isolation device  400 , the second end of the primary winding P′ 1  of the transformer  108  is connected to the output of the PI corrector circuit  132  delivering the correction signal V COR . In the linearization circuit  116 , the non-inverting input of the amplifier AO 4  is connected to the ground GND 1 . 
     Furthermore, the non-inverting input of the amplifier AO′ 5  is connected, not directly to the ground GND 1 , but to the mid-point of a voltage divider formed by resistors. R 20  and R 21  in series between the second end of the winding P′ 1  and the ground GND 1 . The values of the resistors R 20  and R 21  are chosen to obtain a signal. S oi2  at the output of the amplifier AO′ 5 , which signal S oi2  represents the HF component as produced at the secondary of the transformer without being influenced by the correction signal V COR  injected on the primary side (differential amplifier). 
     It should be noted that the particular features of the embodiments described above may be combined in various ways, for example by producing an isolation device with bipolar input and application of the correction signal in the HF channel or by generating an error signal in the form of a voltage as in the  FIG. 6  embodiment but with a bipolar input signal and/or application of the correction signal in the HF channel. Moreover, as already indicated, the LF channel may be produced in bipolar form. 
     What is more, in the diverse variants that may be envisaged, it is possible to replace the transformer of the HF channel and/or the opto-coupler of the LF channel by other components respectively providing the same functions. Accordingly, the transformer may be replaced by any other device able to produce a high-frequency component from the input signal in the secondary part. For example, the transformer  108  or  108 ′ may be replaced by a differential capacitor coupling. Similarly, the opto-coupler  114  may be replaced by any other device able to produce a low-frequency component from the input signal in the secondary part. For example, the opto-coupler  114  may be replaced by a transmission system employing modulation/demodulation of the pulse width or of the frequency of a signal (PWM, Sigma Delta, FM, etc. system). Transmission across the isolation harrier may then be effected via an analog coupler (radio channel, etc.) or via a digital coupler. 
     By virtue of one of the alternatives of the invention, it is also possible to add an additional winding to the primary of the transformer of the HF channel, that additional winding then supplying the signal S oi2  on the primary side.