Abstract:
A signal acquisition instrument, such as an oscilloscope, having an input stage that is referenced to a user&#39;s ground is disclosed. Information gathered by the input stage is stored in a storage element powered by a floating power supply that is referenced to the user&#39;s ground. After storage, the storage element is disconnected from the floating power and from the user&#39;s ground and switched to a power supply referenced to the remainder of the system. FET switching is beneficial, and information can be stored either in an analog format or in a digital format.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This patent application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/428,494, filed on Nov. 22, 2002 and entitled, “MEANS FOR IMPLEMENTING ISOLATED CHANNELS,” which is incorporated herein by reference in its entirety. 

   TECHNICAL FIELD 
   The present invention relates generally to signal acquisition instruments and, more specifically, to signal acquisition instruments having isolated input channels. 
   BACKGROUND OF THE INVENTION 
   Modern signal acquisition instruments typically include an analog-input section for receiving signals being acquired, an analog processor such as an amplifier or filter, a digitization system for digitizing processed analog signals, and a memory for storing the digitized signals. For example, U.S. Pat. No. 5,986,637, which issued to Etheridge et al. on Nov. 16, 1999, describes a high speed digital storage oscilloscope (DSO) having such features. 
   While generally successful, modern signal acquisition instruments can have problems in some applications, e.g., when acquiring signals from switched-mode power supplies, in locations with significant ground loops, or when small signals ride on large voltages. In such applications isolating the analog input stage so that it can utilize a user&#39;s ground can be beneficial. However, AC line-driven signal acquisition instruments typically must be electrically grounded relative to input AC power lines for safety and to comply with applicable electrical codes. Thus a conflict can exist between acquiring signals referenced to a user&#39;s ground and transferring the acquired information to the remainder of the signal acquisition instrument. 
   One approach to transferring information acquired by an isolated input stage to the remainder of an AC powered system is to use optical, capacitive, and/or inductive coupling. While such coupling can transfer analog information across grounds, this approach has problems because the gain-bandwidth product of the coupler often must be high to maintain linearity, because feedback mechanisms are generally unreliable, and because data quality is problematic. Another approach is to use optical, capacitive, and/or inductive coupling to couple digitized signals from logic referenced to the user&#39;s ground to logic referenced to the instrument&#39;s ground. However, this approach is relatively costly and complex and can require a significant amount of power. 
   Therefore, a new technique of coupling information gathered by an isolated input stage that is referenced to a user&#39;s ground to the remaining instrumentation that is referenced to instrument&#39;s ground would be beneficial. 
   SUMMARY OF INVENTION 
   The principles of the present invention provide for architectures, apparatuses, and methods of coupling information acquired by an isolated input stage that is referenced to a user&#39;s ground to the remainder of the system instrumentation that is referenced to an earth ground (which typically connects to the ground line of AC input power). Those principles can be implemented by acquiring signal information using an isolated input stage that is referenced to a user&#39;s ground, storing the acquired information either in an analog format or a digital format in a storage element that is powered by a floating power supply that is referenced to the user&#39;s ground, disconnecting the storage element from the floating power and the user&#39;s ground, and then connecting the storage element to a power supply referenced to the earth ground. Because of their speed and high voltage-handling capability, FET switches are useful devices for connecting and disconnecting the storage element. 
   In one embodiment of the invention, digital memory devices are used. In another embodiment analog memory, e.g., FISO (fast in-slow out) memory is used. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  depicts a high level block diagram of a signal acquisition system according to a first embodiment of the invention when that system is in a first state; 
       FIG. 2  depicts the signal acquisition system of  FIG. 1  when that system is in a second state; 
       FIG. 3  illustrates the use of FET switches; 
       FIG. 4  depicts a high level block diagram of a signal acquisition system according to another embodiment of the invention when that system is in a first state; 
       FIG. 5  depicts the signal acquisition system of  FIG. 4  when that system is in a second state; and 
       FIG. 6  depicts a block diagram of an oscilloscope that incorporates the principles of the present invention. 
   

   To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The subject invention will be primarily described within the context of a general signal acquisition instrument, and then in the context of a digital storage oscilloscopes (DSOs). It will be appreciated by those skilled in the art that the invention may be advantageously employed in many different systems where acquiring information referenced to one ground and then switching that information to another ground is desirable. 
     FIG. 1  depicts a high level block diagram of a signal acquisition system  100  according to an embodiment of the present invention. The signal acquisition system  100  receives at an input port  102  an input signal that is referenced to a user ground  110 . The input signal is amplified and/or otherwise processed (e.g. filtered) by an analog network  104 . The output of the analog network  104  is applied via a closed switch  106  to a digitizer  108  that includes a memory. The digitizer  108  is powered by +F and −F voltages from a floating power supply (not shown in  FIG. 1 , but see  FIG. 6  for such a power supply) that is referenced to the user ground  110 . 
   The digitizer  108  converts the analog processed signal from switch  106  into digital values that are stored in its memory. At this time the digitizer  108  output is applied to an open switch  112  (or switches). The signal acquisition device  100  further includes an earth ground  134  referenced processor  130 , which is connected to the switch  112 , and an earth ground  134  referenced display  132 . The processor  130  and the display  132  are powered by voltages G+ and G− from an earth grounded referenced power supply (not shown in  FIG. 1 , but see  FIG. 6  for such a power supply). For convenience all of the devices that are constantly powered by voltages G+ and G− can be generically referred to as an instrumentation network. The earth ground  134  is, in one embodiment, connected to a ground input of AC input power. 
   As shown in  FIG. 1 , the +F voltage is connected to the digitizer  108  via a switch  140 , the −F voltage is connected to the digitizer  108  via a switch  142 , and the user ground  110  is connected to the digitizer  108  via a switch  144 . Thus, in  FIG. 1  the digitizer  108  is electrically isolated from the instrumentation network. Because the input port  102  is referenced to user ground  110  the input signal is not impacted by ground loops, high voltage differentials, noise, or other factors that impact the earth ground  134 . For example, the earth ground  134  will usually be shared by other devices powered by a common AC power line, and those devices can produce ground loop voltage drops that will appear on the earth ground  134 . 
   Referring now to  FIG. 2 , after the digitizer  108  has digitized the signal from the analog network  104 , a set of switch-changes occurs. Specifically, the switch  106  opens, which disconnects the digitizer  108  from the analog network  104 . Then, the switch  144  disconnects the digitizer  108  from the user ground  110  and connects it to the earth ground  134 , and the switches  140  and  142  disconnect the +F and −F voltages from the digitizer  108  and connect the digitizer  108  to the +G and −G. Finally, the switch  112  closes, connecting the digitizer  108  to the processor  130 . 
   As shown in  FIG. 2 , the user ground  110  is no longer connected to the digitizer  108 . The switching of user ground  110  to earth-ground  134  is performed in a manner that avoids damage from differences between user and earth grounds, and thus possible damage to the input stage and/or the signal source while also providing the signal acquisition system  100  with the protection afforded by a common earth ground. 
   It should be noted that in various embodiments switches  140 ,  142 , and  144  operate in a break-before-make fashion. Furthermore, while the switches  106 ,  112 ,  140 ,  142 , and  144  are shown in  FIGS. 1 and 2  as mechanical switches, in practice high voltage analog switches, e.g., bipolar transistor, FET, diodes, or any other non-linear devices, are beneficial. For example,  FIG. 3  illustrates generic FET switches  160 - 174 , which may be any type of FET such JFET, MOSFET, P-Channel, N-Channel, etc. Such FET switches are faster, more reliable, and cheaper than mechanical switches. While FET switches are a good choice, again, other types of devices can also be used. As shown in  FIG. 3 , switches  160  and  162  switch user ground  110  and earth ground  134 , switches  164  and  166  switch +F and +G, switches  168  and  170  switch −F and −G, switch  172  switches analog inputs to memory, and switch  174  switches the output of the memory to the remainder of the system. The driving of the FET switches is controlled by logic, such as from a processor (reference  FIG. 6  for a processor). 
   While  FIGS. 1 and 2  illustrate switching a user ground  110  to earth-ground  134  after the acquired signal has been digitized, this is not required. Switching of analog signals is also possible. For example,  FIG. 4  depicts a high level block diagram of a signal acquisition system  200  according to a second embodiment of the present invention. The signal acquisition system  200  receives an input signal that is referenced to a user ground  210  on an input port  202 . The input signal is amplified and/or otherwise processed by an analog network  204 . The output of the analog network  204  is applied via a closed switch  206  to an analog fast-in-slow-out (FISO) memory  208 . 
   As shown in  FIG. 4 , the FISO memory  208  is powered by +F and −F voltages from a floating power supply (which is not shown in  FIG. 4 , but reference  FIG. 6 ) that is referenced to the user ground  210 . The user ground  210  is also connected to the input port  202 . The FISO memory  208  retains an analog version of the input signal. The output of the FISO memory  208  is applied to an open switch  212 . The signal acquisition device  200  further includes an earth-referenced processor  230 , which is connected to the switch  212 , and a display  232 . The earth-referenced processor  230  and the display  232  are referred to an earth ground  234  and are powered by +G and −G voltages from an earth-grounded power supply (which is not shown in  FIG. 4 , but reference FIG.  6 ). The devices that are continuously connected to the +G and −G voltages can be referred to as an instrumentation network. 
   As shown in  FIG. 4 , the +F voltage is connected to the FISO memory  208  via a switch  240 , the −F voltage is connected to the FISO memory  208  via a switch  242 , and the user ground  210  is connected to the FISO memory  208  via a switch  244 . Thus, in  FIG. 4  the FISO memory  208  is electrically isolated from the Earth-referenced processor  230  and the display  232 . Because the analog signal input on input port  202  is referenced to user ground  210  the input signal is not impacted by ground loops, high voltage differentials, noise, or other factors that might impact the earth ground  234 . 
   Referring now to  FIG. 5 , after the FISO memory  208  has captured the signal from the analog network  204 , a set of switch-changes occurs. Specifically, the switch  206  opens, which disconnects the FISO memory  208  from the analog network  204 . Additionally, the switch  244  switches the FISO memory  208  from the user ground  210  to the earth ground  234 . At the same time, the switches  240  and  242  switch the FISO memory  208  from the +F and −F voltages to the +G and −G voltages. Finally, the switch  212  closes, connecting the FISO memory  208  to the earth-referenced processor  230 . 
   As in the embodiments illustrated in  FIGS. 1 and 2 , the switches  240 ,  242 , and  244  operate in a break-before-make fashion and all switches are beneficially high voltage analog (FET) switches (see FIG.  3 ). If bipolar transistor switches are used DC level changes might have to be corrected for. 
     FIGS. 1 through 5  illustrate generic signal acquisition systems  100  and  200  that can be used for many purposes in many different systems. However, such signal acquisition systems are particularly useful in oscilloscopes. For example,  FIG. 6  illustrates a block diagram of an oscilloscope  600  that benefits from the principles of the present invention. As shown, the oscilloscope  600  includes an input  602  that is referenced to a user ground  604 . A signal on the input  602  is passed to an acquisition system  606 . The acquisition system  606  includes a user-selectable gain amplifier and an analog-to-digital converter (ADC). The ADC of the acquisition system  606  samples and quantizes the amplified signal and supplies the acquired information via closed switch  608  to an acquisition memory  610 . It is also possible for the acquisition system  606  to store an analog representation of the input signal in a FISO memory. However, for convenience, the oscilloscope  600  will be assumed to use an ADC and a digital memory. During data acquisition, and as shown in  FIG. 6 , the acquisition memory  610  is powered by +F and −F voltages from a floating power supply  611  that is referenced to user ground  604 . The +F and −F voltages are applied via switches  612  and  613 , respectively, and the user ground  604  is applied by a switch  614 . It should be understood that the acquisition system  606  is directly powered by the floating power supply  611  and is directly wired to the user ground  604 . The output of the acquisition memory  610  is applied to a switch  615  which is open during data acquisition. 
   After data acquisition is complete, a processor  616  causes the switch  608  to open and switch  615  to close. Contemporaneously, the processor  616  also causes switches  612 ,  613 , and  614  to switch such that the acquisition memory  610  is powered by +G and −G voltage from an earth ground  617  power supply  618  and such that the acquisition memory  610  is connected to earth ground  617 . 
   With switch  615  closed, the output of the acquisition memory  610  passes to a display memory  622  that stores the acquisition memory  610  output. The contents of the display memory  622  are employed to generate a waveform display on a raster scan display device  626 . The processor  616  may provide additional information, such as the amplification factor and a waveform time-base to the display memory  622  for display. After the display memory  622  has stored the output of the acquisition memory  610  the processor  616  causes switch  615  to open and switch  608  to close. Additionally, the processor  616  causes switches  612 ,  613 , and  614  to connect the acquisition memory  610  back to the floating power supply  611  voltages +F and −F and to the user ground  604 . It should be understood that the earth grounded power supply  618  supplies power to the display  626 , to the processor  618  and to the display memory  622 . Furthermore, the processor  616  causes the various switches to switch in a break-before-make fashion. In one embodiment, instead of mechanical switches high-voltage FET switches are used (see FIG.  3 ). All devices that are directly connected to the earth grounded power supply  618  and to earth ground  617  can be generically referred to as an instrumentation network. 
   While the foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.