APPARATUS FOR MEASURING PARAMETERS OF AN ELECTRONIC DEVICE

Abstract: A first terminal (791) of a device-under-measurement (DUM) is connected to a buffer amplifier (758) and to the inverting input (747) of an operational amplifier (754). A second terminal (792) of the DUM is connected to a buffer amplifier (756) having its output (796) connected to the non-inverting input (746) of the operational amplifier (754). The non-inverting input is also connected to the output (793) of a signal generator (760). The second terminal (792) is also connected to ground (767) through a programmable load circuit (762). A circuit (779) creates a feedback loop from the DUM and the load circuit (762) to the operational amplifier (754) to enable the amplifier to maintain a predetermined voltage across the DUM and to isolate the amplifier from current flow from the load circuit (762). An oscilloscope (766) is connected across the outputs (727, 796) of the buffer amplifiers (758, 757) and a computer (764) controls the signal generator (760) in response to a signal from the oscilloscope and also controls the load (762) via a load control circuit (765).

Description: APPARATUS FOR MEASURING PARAMETERS OF AN ELECTRONIC DEVICE BACKGROUND OF THE INVENTION 1. Field of the Invention The invention in general relates to apparatus for measuring electronic properties of an electronic device, and in particular apparatus for measuring the current or charge which flows through an electronic device in response to an applied voltage. 2. Statement of the Problem It is very important in research laboratories to be able to accurately measure the current or charge that flows through an electronic device in response to an applied voltage. For example, in research laboratories that develop advanced materials, such as ferroelectrics, for use in integrated circuit capacitors, it is crucial to be able to accurately measure the electronic parameters of large numbers of integrated circuit capacitors so that differences in materials can be correlated with differences in capacitance, leakage current etc. of the devices. In this disclosure we shall call a device, one or more electrical parameters of which are to be measured, a device-under-measurement, or DUM. The most common laboratory method of measuring the current or charge displaced within a ferroelectric device is the Sawyer-Tower circuit. In a Sawyer- Tower circuit, a voltage output by a waveform generator is applied to the input terminal of a device upon which the measurement of an electronic parameter is to be performed (DUM) and to one input of an oscilloscope. The output of the DUM is connected to another input of the oscilloscope and to ground through a load. This circuit does not properly terminate the transmission lines to the DUM, and relies on the assumption that the DUM has a constant high impedance. This assumption is not valid for non-linear materials and at high frequencies. Moreover, for almost all DUMs, there is some unknown voltage drop across the DUM, and thus the Sawyer-Tower circuit can often only provide an inadequate approximation to the true current and charge flowing through a DUM. The above problems with the Sawyer-Tower circuit have lead to a modification of the Sawyer-Tower circuit by the addition of a virtual ground. That is, instead of being directly connected to ground, the output of the DUM is connected to the inverting input of an operational amplifier (op amp) that has its non-inverting input connected to ground and its output connected to the second input of the oscilloscope. The load is then connected between the inverting input of the operational amplifier and its output. Low speed op amps typically have low input offset currents and voltages, and thus the voltage drop across the load can accurately be related to the current/charge flowing through the DUM. However, high speed op amps typically have large input offset currents and voltages, and thus, at high frequencies, the voltage across the load is not accurately related to the current/charge flowing through the DUM. Moreover, if the DUM has a low and/or a variable impedance, the conventional virtual ground does not in and of itself yield accurate results even at low frequencies. 3. Solution to the Problem The present invention solves the above problem by providing a measurement circuit that allows the voltage driving the DUM to vary. Feedback is provided from the load side of the DUM to the circuit that applies a voltage to the DUM. The drive circuit then applies whatever voltage that is necessary to create a predetermined voltage across the DUM no matter what its impedance, and at all frequencies, so long as the circuit is operated within its current compliance and bandwidth limitations. An operational amplifier, with its inverting input connected to its output and ground through resistors, and its non-inverting input connected to the feedback line and a signal generator, also through resistors, provides the DUM voltage drive circuit. The invention also permits the load to be selected so that the electrical parameters across it are related to the DUM parameters in an unambiguous manner and can easily and accurately be measured. This is done by referencing the non-inverting input to ground through the load. Put another way, the present invention provides a DUM/load circuit that is essentially independent of the signal generation circuit. Buffer amplifiers connect the DUM/load circuit to the inputs of the operational amplifier, thus preventing any current flow between the DUM/load circuit and the signal generation circuit. Thus, the signal across the DUM and the voltage drop across the load may be separately determined due to the essential independence of the signal generation circuit and the DUM/load circuit. The above problems are also solved by providing a programmable load. A desired load is selectable via a computer and a load control circuit. This allows the load to be manually or automatically adjusted to prevent the signal across the DUM to be too large or too small, either of which can lead to inaccuracies in measurement. The invention provides a measurement circuit comprising: a first electrical terminal for connection to a device upon which the measurement of an electronic parameter is to be performed (DUM); a second electrical terminal for connection to the DUM; a load circuit having a load circuit input and a load circuit output; a signal generation circuit including an operational amplifier having a first input, a second input and an output; the second terminal connected to the load circuit input and to one of the inputs of the operational amplifier. In another aspect, the invention provides a measurement circuit comprising: a DUM/load circuit; a signal generation circuit; a first buffer amplifier connected between the DUM load circuit and the signal generator circuit, thereby preventing current flow through the connection between the DUM/load circuit and the signal generation circuit. Preferably, the signal generation circuit includes an operational amplifier having an input, the buffer amplifier includes an input and an output, and the buffer amplifier input is connected to the DUM/load circuit and the buffer amplifier output is connected to the operational amplifier input. Preferably, the DUM/load circuit includes a first terminal for connecting to a device upon which the measurement of an electronic parameter is to be performed (DUM), a second terminal for connecting to the DUM, a load circuit including a load circuit input, the operational amplifier includes an inverting input and a non-inverting input, the second terminal is connected to the load circuit input and the buffer amplifier input, the first buffer amplifier output is connected to the non-inverting input, the circuit further including a second buffer amplifier having an input and an output, and wherein the first terminal is connected to the operational amplifier output and the operational amplifier inverting input. Preferably, the measurement circuit includes an oscilloscope connected between the output of the first buffer amplifier and the output of the second buffer amplifier. Preferably, the measurement circuit includes a ground, the signal generation circuit includes a signal generator, and the measurement circuit further including a first resistor connected between the inverting input and the ground, a second resistor connected between the output of the second buffer amplifier and the inverting input, a third resistor connected between the signal generator and the non-inverting input, and a fourth resistor connected between the output of the second buffer amplifier and the non-inverting input. Preferably there is a computer connected between the oscilloscope and the signal generator. In a further aspect the invention provides an electrical measurement circuit comprising; a DUM/load circuit comprising terminals for connection to a device upon which an electronic measurement is to be performed (DUM) and an input for connection to a load; a signal generation circuit for applying an electrical signal to the DUM; and a feedback circuit for providing a feedback signal from the DUM/load circuit to the signal generation circuit; and the signal generation circuit includes an amplifier circuit responsive to the feedback signal for adjusting the signal applied to the DUM. Preferably, the DUM/load circuit terminals comprise a first electrical terminal for connection to the device upon which the measurement of an electronic parameter is to be performed (DUM) and a second electrical terminal for connection to the DUM, the amplifier circuit comprises an operational amplifier having a first input and an output; and the second terminal is connected to the first input of the operational amplifier and the output of the operational amplifier connected to the first terminal. In yet another aspect, the invention provides an electrical measurement circuit comprising: a first electrical terminal for connection to a device upon which the measurement of an electronic parameter is to be performed (DUM); a second electrical terminal for connection to the DUM; a ground, a first resistor, a second resistor, a third resistor, and a fourth resistor; a signal generator; an operational amplifier having an inverting input, a non-inverting input, and an output; the first terminal connected to the operational amplifier output, and the first terminal also connected to the inverting input through the first resistor; the inverting input also connected to the ground through the second resistor; the signal generator connected to the non-inverting input through the third resistor; and the second terminal connected to the non-inverting input through the fourth resistor. Preferably, the electrical measurement circuit further includes a first buffer amplifier and a second buffer amplifier, the first buffer amplifier is connected between the first terminal and the first resistor, and the second buffer amplifier is connected between the second terminal and the fourth resistor. The measurement circuit according to the invention not only permits the measurement signal across the DUM to be independently selected from the signal across the load, but also does this simply and with little expense. Further, the measurement circuit according to the invention is accurate at a wide range of frequencies limited only by the frequency response of the operational amplifier. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an exemplary embodiment of a measurement circuit according to the invention; FIG. 2 shows an exemplary embodiment of a measurement system according to the invention, including an alternative embodiment of a measurement circuit according to the invention; FIG. 3 shows a graph of a polarization in microcoulombs per square centimeter versus the time in seconds for a polarization switching charge measurement made with the prior art Sawyer-Tower circuit; FIG. 4 shows a graph of the polarization in microcoulombs per square centimeter versus the time in seconds for a polarization switching charge measurement made with the measurement circuit of FIG. 2; FIG. 5 shows a graph of voltage in volts versus time in microseconds for an input waveform pulse measured for the prior art Sawyer-Tower circuit; FIG. 6 shows a graph of voltage in volts versus time in microseconds for an input waveform pulse measured for the measurement circuit of FIG. 2; FIG. 7 shows the preferred embodiment of a measurement system according to the invention, including the preferred embodiment of a measurement circuit according to the invention; and FIG. 8 shows details of the measurement system of FIG. 7. DESCRIPTION OF THE PREFERRED EMBODIMENT Directing attention to FIG. 1, an exemplary measurement system 10 according to the invention is shown. Measurement system 10 includes a measurement circuit 40 including device under measurement (DUM) 12, operational amplifier 14, load capacitor 18, and resistors 20-23, plus signal generator 16 and oscilloscope 26. A ground is indicated at 30. Oscilloscope 26 measures the current or charge that flows into load capacitor 18 with each measurement pulse. In the embodiment shown, the DUM is a two-terminal device, such as a capacitor, but it may be other electrical devices also. It should be understood that the words "electrical" and "electronic" are intended to be equivalent herein. That is, the word "electronic" herein includes everything included in both the conventional term "electronic" and also everything included in the conventional term "electrical". Likewise, the term "electrical" herein includes everything included in both the conventional term "electrical" and also everything included in the conventional term "electronic". DUM 12 includes a first terminal 34 and a second terminal 36. The output of operational amplifier 14 is connected to the first terminal 34 of DUM 12, while the second terminal 36 is connected to ground through a load 18, which in the embodiment shown is a capacitor, but may also be a resistor, a combination of a resistor or capacitor, or one or more other electronic devices. The first terminal 34 of DUM 12 is also connected to the inverting input of operational amplifier 14 through resistor 21. The inverting input of operational amplifier 14 is also connected to ground through resistor 20. The second terminal 36 of DUM 12 is also connected to the non-inverting input of operational amplifier 14 through resistor 23. The non-inverting input of operational amplifier 14 is also connected to the output 37 of signal generator 16 through resistor 22. Oscilloscope 26 is connected between the second terminal of DUM and ground. Note that the connection to the oscilloscope is generally a coaxial cable which has its outer conductor grounded, which ground is represented in the drawing by a rectangle 27. Signal generator 16 is also connected to ground. In the preferred embodiment of exemplary measurement circuit 40, resistor 20 has a value of R, while resistor 21 has the value aR, and resistor 22 has the value of r while resistor 23 has the value ar. In this configuration, the circuit including the operational amplifier 14 and resistors 20-23 forms a variable reference amplifier with a gain of "a". When the signal generator 16 provides an AC output signal with an amplitude of "V", the circuit 40 will apply an AC signal across DUM 12 with an amplitude of "a x V". As long as the signal applied to the DUM 12 is within the voltage and current compliance and operational bandwidth of the operational amplifier 14, the shape and amplitude of the signal applied to the DUM 12 will be maintained with no distortion, regardless of the impedance of the DUM or the voltage across the load capacitor. As long as the impedance of the load capacitor 18 is small compared to the combined impedance of the resistors 22 and 23, the voltage across the load capacitor 18 will be, to a very good approximation, proportional to the charge displaced within the DUM. Thus, in comparison to the prior art measurement circuits, the measurement of the voltage across the load 18 by oscilloscope 26 gives a more accurate measurement of the charge that a signal pulse of predetermined amplitude displaces across the DUM. FIG. 2 shows an embodiment of a measurement system 100 according to the invention, including an alternative embodiment of a measurement circuit 50 according to the invention. Measurement circuit 50 includes DUM 52, operational amplifier 54, buffer amplifiers 56 and 58, load 62, resistors 71-75, 50 ohm cables 84-86, and twisted pair 82. Measurement system 100 includes the circuit 50 plus signal generator 60, oscilloscope 66, computer 64, and connecting cables 97 and 98. Like circuit 40, circuit 50 includes a resistor 72 having a value "R" connected between ground and the inverting input of operational amplifier 54, a resistor 71 having the value "aR" connected between the inverting input of operational amplifier 54 and the first terminal 91 of DUM 52, a resistor 74 having the value V connected between the output 93 of signal generator 60 and the non-inverting input of operational amplifier 54, a resistor 75 having the value "ar" connected between the second terminal 92 of the DUM 52 and the non-inverting input of operational amplifier 54, and a load circuit 62 having its input 61 connected to the second terminal 92 of DUM 52 and its output 63 connected to ground. Thus, the circuit 50 of FIG. 2, is essentially the same as the circuit 40 of FIG. 1 , except that it includes buffer amplifier 58 having its input connected to the first terminal 91 of DUM 52 and its output 95 connected to resistor 71 , and buffer amplifier 56 having its input connected to the second terminal 92 o f DUM 52 and its output 96 connected to resistor 75. In addition, the system 100 is more sophisticated in that channel 1 of oscilloscope 66 is connected to the output 95 of buffer 58 via coaxial cable 86 and resistor 70 in series, channel 2 of oscilloscope 66 is connected to the output 96 of buffer 56 via coaxial cable 85 and resistor 77 in series, the output 93 of signal generator 60 is connected to resistor 74 via coaxial cable 84, the node between coaxial cable 84 and resistor 74 is connected to ground through resistor 73, DUM 52 is connected to the rest of circuit 50 via a twisted pair 82, computer 64 receives the output of oscilloscope 66 via cable 97 and controls the output of signal generator 60 via cable 98. In FIG. 2, load 62 is shown generally, as a circuit 62 including a resistor 76 and a capacitor 80. It should be understood that load 62 can include just a resistor, just a capacitor, or any other appropriate load circuit, depending on the DUM and the electronic parameter of the DUM to be measured. Buffer amplifiers 56 and 58 provide proper feedback to the operational amplifier 54 while at the same time preventing current from flowing from the DUM to ground through the buffers, thus ensuring that the only effective path from the DUM to ground is through load 62 and that the current flowing through the load 62 is the same as the current flowing through the DUM 52. The buffer amplifiers 56 and 58 also serve as line drivers for the transmission cables 85 and 86, respectively, carrying signals to the oscilloscope 66. This allows cables 85 and 86 to be properly terminated, eliminating the ringing which occurs at high frequencies as a result of impedance mismatch. Connecting the feedback legs through the buffer amps 56 and 58 also provides short circuit protection. Twisted pair 82 is preferably made of a very fine wire, such as magnet wire. Using such a twisted pair to carry signals to DUM 52 limits any stray inductance which could become problematic at high frequencies. The resistors 70 and 80 essentially create a voltage divider that divides the voltage output by the buffer amplifiers to a level more suitable for input into oscilloscope 66. Computer keyboard 88 is used to input data into a software program that controls signal generator 60 and which receives, displays, and prints out data from oscilloscope 66. In taking the measurements discussed below, resistors 70 through 75 were 450 ohms, 2.2 Kohms, 1.1 Kohms, 50 ohms, 500 ohms, and 1 Kohm, respectively. Load 62 was a 650 picofarads capacitor. That is, "a" was equal to 2, "R" was equal to 1.1 Kohms, and "r" was equal to 500 ohms. The above value of resistor 70 made the input to channel 1 of oscilloscope 66 a 10 to 1 divider and the above value of resistor 77 made the channel 2 input to oscilloscope 66 a 2 to 1 divider. Buffer amplifiers 56 and 58 were BUF04 buffer amplifiers from Analog Devices and operational amplifier 54 was an AD 811 op amp also from Analog Devices. In the embodiment of FIG.2, signal generator 60 was an HP8115A from Hewlett Packard Company, oscilloscope 66 was an HP54504A also from Hewlett Packard Company, computer 64 was an IBM compatible 486 computer, and cables 40 and 98 were HPIB communication busses, sometimes referred to as GPIB busses or IEEE 488 standard interfaces. Coaxial cables 84, 85 and 86 were standard coaxial cables with a 50 ohm characteristic impedance. Other values of all these components may, of course, be used. For example operational amplifier 54 and buffer amplifiers 56 and 58 may be any other suitable off-the-shelf or custom amplifiers, either of the integrated circuit or discrete variety. In the measurements discussed below, the DUM was a ferroelectric capacitor, and load element 62 was a 650 picofarad linear capacitor. In the case of the DUM being a ferroelectric capacitor, the load element 62 may be a capacitor, a resistor, or a combination of the two. If only a resistor is used, the voltage across the load resistor is proportional to the current flowing through the DUM 52. The charge can then be calculated by integrating the current with respect to time. If only a toad capacitor is used, the voltage across the load capacitor is proportional to the charge flowing through the DUM 52, and the current can be calculated by differentiating the charge with respect to time. If a combination of a capacitor and a resistor, or any other combination of elements are used as a load 62, the displaced charge and current can be calculated from the voltage across the load 62 with mathematical equations known in the art. By applying an AC voltage signal to DUM 52 and measuring the resultant current within the DUM, the impedance of the DUM as a function of voltage and frequency can be calculated from Ohm's law. Preferably, a resistor is used as the load 62 in such a measurement. By applying a DC bias to the DUM 52, the DC leakage current of the DUM 52 can be measured. By varying the DC voltage, the leakage current as a function of voltage may be obtained. The following electrical parameters of thin film capacitors as a function of voltage and frequency have been measured with the measurement circuit 50 according to the invention: 1) the resistance, capacitance and inductance; 2) the complex dielectric constant; 3) the ferroelectric, paraelectric, and dielectric polarization and current; and 4) the time- dependent dielectric breakdown. From the above, it is clear that these and many other electrical parameters may be measured for not only capacitors, but also for a wide variety of other electrical and electronic devices. FIGS. 3 and 4 compare PUND type measurements of the ferroelectric polarization of a ferroelectric capacitor made with a prior art Sawyer-Tower circuit (FIG. 3) and with the preferred embodiment of the measurement circuit according to the invention (FIG. 4). In both figures the measured ferroelectric polarization in microcoulombs per centimeter squared is plotted as a function of time in seconds. In the measurement performed with the prior art Sawyer-Tower circuit, the measurement oscillates about a maximum that is only about half the measured polarization for the circuit according to the invention. The measurement taken with the circuit of FIG. 2 has no ringing and reveals a polarization that is much higher than that measured with the conventional circuit. FIGS. 5 and 6 compare the waveforms input to the DUM for a prior art Sawyer-Tower circuit (FIG. 5) and the measurement circuit of FIG. 2 according to the invention (FIG. 6). In both figures, the input voltage was measured as a function of time in microseconds for a five volt voltage pulse of 1 microsecond duration. (The scales are slightly different in the two figures.) The input pulse for the Sawyer- Tower circuit rises and falls more slowly, includes an oscillation in the rise, and measured pulse length, from the half-rise point to the half-fall point, is not actually one microsecond. The input pulse for the circuit according to the invention is much more square and fits very close to the optimum pulse shape for a 1 microsecond duration pulse. Turning now to FIG. 7, the preferred embodiment of a measurement system 700 according to the invention is shown. Measurement system 700 includes the preferred measurement circuit 750. System 700 can perhaps be most easily understood as being essentially the same as the system 100 of FIG. 2, with the following modifications: the buffer amplifier 56 of FIG. 2 is replaced by a more sophisticated buffer amplifier 756 (enclosed by the dotted line) which provides both a small input bias current and a large bandwidth, which buffer amplifier 756 includes amplifiers 757 and 740, capacitor 742, and resistors 743-745; the provision of electrostatic discharge protection via diodes 731-736 and the associated circuitry; the replacement of resistors 70 and 77 with variable resistors 770 and 777 and the addition of variable resistor 778 in series with resistor 774 to provide more convenient and finer control of the input and output impedances; the replacement of load 62 with a programmable load 762 and a load control circuit 765, which permits the load to be selected by computer 764; and a relocation of buffer amplifier 58 to provide more bandwidth in driving the output 787 to oscilloscope channel 1. Measurement circuit 750 includes DUM 752, operational amplifier 754, buffer amplifiers 756 and 758, load 762, variable resistors 770, 777 and 778, resistors 771-775, diodes 731-736, and twisted pair 782. Measurement system 700 includes the circuit 750 plus signal generator 760, oscilloscope 766, computer 764, 50 ohm cables 784-786, and connecting cables 797, 798, 799, and 701. Like circuit 40 and 50, circuit 750 includes a resistor 772 having a value "R" connected between ground and the inverting input of operational amplifier 754, a resistor 771 having the value "aR" connected between the inverting input of operational amplifier 754 and its output, as well as the first terminal 791 of DUM 752, resistors 774 and 778 having the value V connected between the input line 793 from signal generator 760 and the non-inverting input of operational amplifier 754, a resistor 775 having the value "ar" connected between the second terminal 792 of the DUM 752 (through buffer amplifier 756) and the non-inverting input of operational amplifier 754, and a load circuit 762 having its input 761 connected to the second terminal 792 of DUM 752 and its output 763 connected to ground. In the preferred embodiment of this circuit "a" is again 2 but "R", and "r" are preferably 500 ohms. Buffer amplifier 758 has its input connected to the first terminal 791 of DUM 752, as well as the output of operational amplifier 754, and its output driving channel 1 of oscilloscope 766 though variable resistor 770 and coaxial cable 786. The node 755 between the output of operational amplifier 754 and the input 758 of buffer amplifier 758 is also connected to the positive voltage supply voltage, +Vs, through diode 731 , with the cathode of the diode toward the +Vs power supply, and is connected to the negative power supply voltage, -Vs, through diode 732, with the anode of the diode toward the -Vs power supply. Buffer amplifier 756 has its input 737 connected to the second terminal 792 of DUM 752 and its output 796 connected to the non-inverting input of operational amplifier 754 through resistor 775. The output 796 is also connected to the output line 788 through variable resistor 777. Output line 788 is connected to channel 2 of oscilloscope 76 through coaxial cable 785. Input line 793 is connected to signal generator 760 via coaxial cable 784 and to ground through resistor 773. DUM 752 is connected to the rest of circuit 750 via a twisted pair 782, computer 764 receives the output of oscilloscope 766 via cable 797, controls the output of signal generator 760 via cable 798, and provides output signals to control circuit 765 via cable 799. Control circuit 765 in turn operates adjustable load 762 via cable 701. In FIG. 7, adjustable load 762 is shown generally, as a circuit 762 including a plurality of resistors 776 and a plurality of capacitors 780. The details of control circuit 765 and adjustable load circuit 762 are shown in FIG. 8 and will be discussed below. As indicated above, buffer amplifier 756 includes amplifier 757, operational amplifier 740, capacitor 742, and resistors 743-745.The input 739 of buffer amplifier 756 is connected to the non-inverting input 746 of operational amplifier 740 through resistor 743 and to the input of amplifier 757 through capacitor 742. It is noted that optionally a resistor of about 100 ohms could be inserted between capacitor 742 and the input of amplifier 757, if it is desired to reduce bounce in the signal and make it more stable. However, for most applications, the signal is sufficiently stable without the additional resistor. The output 748 of operational amplifier 740 is connected to ground through resistor 745 and to the input of amplifier 757 through resistor 744. The output 796 of amplifier 757 provides the output of the buffer amplifier 756. The output 796 is also connected to the inverting input 747 of operational amplifier 740 and to channel two of oscilloscope 766 through variable resistor 777 and coaxial cable 785. output 796 is also connected to the anode of diode 735 and the cathode of diode 736. The cathode of diode 735 is connected to the positive voltage supply +Vs, and the anode of diode 736 is connected to the negative voltage supply -Vs. The input 737 of buffer amplifier 756 is connected to its output 796 through a parallel diode pair, 733 and 734, with the anode of diode 733 connected to the output 796, and the anode of diode 734 connected to the input 737. As indicated above, the embodiment of FIG. 7 provides a buffer amplifier circuit 756 that has both a small input bias current and a large bandwidth. This is accomplished by using two amplifiers, 757 and 740, in conjunction. The operational amplifier 740 has a small input bias current and a relatively small bandwidth. This amplifier 740 passes low frequency signals, since it is DC coupled to the input 737 through resistor 743. Amplifier 757 has a large input bias current and a large bandwidth. It passes high frequency signals, since it is AC coupled to the input 737 through capacitor 742. The output of amplifier 740 feeds input bias current to amplifier 757 through resistor 744. The output voltage of amplifier 757 is sensed by the inverting input of amplifier 740 so that no DC offset is developed between the buffer amplifier input 737 and the buffer amplifier output 796. The high frequency signals pass directly through amplifier 757 via capacitor 742. The net result is a buffer amplifier 756 that has the input bias current of amplifier 740 and all the bandwidth of amplifier 757, plus some additional low frequency bandwidth. In the preferred embodiment, amplifier 757 is an Analog Devices BUF-04 buffer amplifier, amplifier 740 is an Analog Devices AD-546 Op-Amp, capacitor 742 is a 1 nanofarad capacitor, resistors 743, 744 and 745 are 100 kiloohm (kohm), 1 megaohm, and 10 kohm, respectively, and the supply voltages are +15 volts and -15 volts respectively. With this configuration, the buffer amplifier 756 has at least 35 megahertz bandwidth and an input bias current of about 1 picoamp. The embodiment of FIG. 7 also provides ESD protection to the measurement circuit 750. The first DUM terminal 791 is connected to the supply rails, through diodes 731 and 732, which is a conventional way of providing protection against ESD that may occur when connecting the DUM. However, if conventional ESD protection were provided to the input 739 of buffer amplifier 756, leakage current through the diodes would disturb the measurements. In the embodiment shown in FIG. 7, the diodes 733 and 734 have no voltage bias across them in normal operation. Thus there is no leakage through them. However, when the second DUM terminal 792 is exposed to a source of ESD, a large voltage develops across diodes 733 and 734. This causes them to conduct the electrostatic charge, which is then discharged through diodes 735 and 736. Turning to FIG. 8, there is shown the details of the adjustable load 762, the load control circuit 765, and the interface 810 between the control circuit 765 and the computer 764 (FIG. 7), and the connector 840 between the computer 764, the oscilloscope 766, the signal generator 760, and the measurement circuit 750 and the control circuit 765. In the preferred embodiment, adjustable load 762 comprises two ADG431 CMOS switches, 820 and 830, available from Analog Devices, resistors 776, capacitors 780, and two TQ255-L-5V relays 816 and 818 available from Aromat Corp. In the preferred embodiment, control circuit 765 comprises a P1C16C74 integrated circuit available from Microchip Technology, Inc., and interface 810 is a MAX232A RS-232 interface available from Maxim, Inc., or an Analog Devices ADM232A RS-232 interface. Connector 840 is a conventional pin type connector. The oscilloscope channel 1 , function generator, and oscilloscope channel 2 sockets of connector 840 are connected to the lines 787, 793 and 788 respectively of measurement circuit 750. The line 761 of measurement circuit 750 is connected to the No. 8 pin of relay 816. The RS-232 IN and RS232OUT sockets of connector 840 are connected to the No. 13 and 14 pins, respectively, of interface 810. The No. 6 pin of interface 810 is connected to ground through capacitor 811, the No. 4 and 5 pins of interface 810 are connected through capacitor 812, and the No. 1 and 3 pins of interface 810 are connected through capacitor 813, and the No. 2 and 16 pins of interface 810 are connected through capacitor 814. The No. 16 pin is also connected to the +5 volts power source, while the No. 15 pin is grounded. The inputs and outputs of integrated circuit (IC) 765 are given in terms of the internal signals, such as RC0, instead of the pin numbers, since the in outs of the integrated circuit are different for different package options, thus the pin numbers could be ambiguous. Those skilled in the art will unambiguously know how to set up the IC 765 from the description herein and the directions that come with the IC. The No. 10 and 11 pins of interface 810 are connected to the RC6 and RC7 signals, respectively, of IC 765. The drains of each of microswitches 821-823 in CMOS switches 820 and the drains of each of the microswitches 831 -824 in CMOS switch 830 are connected to ground 767. The IN line of microswitches 821-824 of CMOS switch 820 are connected to the RB7, RB6, RB5, and RB4 signals, respectively, of IC 765. The IN line of microswitches 831-834 of CMOS switch 830 are connected to the RB3, RB2, RB1 , and RB0 signals, respectively, of IC 765. The source of microswitch 821 is connected to the drain of microswitch 824 through resistor 851 and capacitor 871 connected in parallel. The source of microswitch 822 is connected to the drain of microswitch 824 through resistor 852 and capacitor 872 connected in parallel. The source of microswitch 823 is connected to the drain of microswitch 824 through resistor 853 and capacitor 873 connected in parallel. The drain of microswitch 824 is also connected to ground through resistor 854 and capacitor 874 connected in parallel. The source of microswitch 824 is connected to ground through resistor 855 and capacitor 875 connected in parallel. The No. 2 pin of relay 818 is connected to ground through resistor 856 and capacitor 876 connected in parallel. The No. 7 pin of relay 818 is connected to ground. The No. 9 pin of relay 818 is connected to ground through resistor 857. The No. 9 pin of relay 818 is also connected to the source of each of microswitches 831-834 through resistors 858-861, respectively. The ACLR signal of IC 765 is connected to the +5 volt voltage source through resistor 862. The V signal of IC 765 is connected to the +5 volt voltage source and the V ss signal is connected to ground. The OSC2 signal and the OSC1 signals are connected to ground through capacitors 877 and 878, respectively, and are also connected through oscillator 866. The RCO and RC1 signals of IC 765 are connected to the No. 1 and No. 10 pins, respectively, of relay 816. The RC2 and RC3 signals of IC 765 are connected to the No. 1 and No. 10 pins, respectively, of relay 818. The Nos. 7 and 9 pins of relay 816 are connected to the Nos. 3 and 8 pins, respectively, of relay 818. In the preferred embodiment of the circuit of FIG. 8, capacitors 811 through 814 are each 0.1 microfarads, and capacitors 871-878 are 1 microfarad, 100 nanofarads, 8.2 nanofarads, 1 nanofarads, 100 picofarads, 10 picofarads, 15 picofarads, and 15 picofarads, respectively. Resistors 851-862 are, preferably, 100 kiloohms, 1 megaohm, lO megaohms, lOO megaohms, 1 gigaohm, lO gigaohms, 100 kiloohms, 10 kiloohms, 1 kiloohm, 100 ohms, 10 ohms, and 10 kiloohms, respectively. Oscillator 866 is preferably a 4 millihertz crystal oscillator. The measurement system 700 operates as follows. The IC 765, in response to commands from computer 764, sets the output voltages on individual ones of output pins RC0-RC3 and RB0-RB7 to either a high voltage state (+5V) or a low voltage state (0 V). The No. 1 and No. 10 pins of relays 816 and 818 are connected across a coil 881, 882 within relay 816, 818. The coil is shown in ghost since it is within the package of the relay. The No. 3 and No. 8 pins of each relay are connected to a switch, such as 884. The switches 884-887 are shown in the drawing in their default state in which they connect pins Nos. 2 and 3 and pins Nos. 8 and 9. When the coil, such as 882, is energized, the switches, such as 886 and 887, switch to connect the Nos. 3 and 4 pins and the Nos. 7 and 8 pins respectively. Preferably, the relays are latchable, in that the relays stay in the switched position until they are switched back by a signal to the coils 881 and 882. Thus, when appropriate RCO and RC1 signals are output by IC 765, the line 761 is switched from being connected to the No. 8 pin of relay 818 to being connected to the No. 3 pin of relay 818. And when appropriate RC2 and RC3 signals are output by IC 765, the circuit from the No. 3 pin of relay 818 is switched from going through resistor 856 and capacitor 876 to going through resistor 855 and capacitor 876 and whatever ones of the other resistors and capacitors are connected to this circuit via switches 820, and likewise the circuit from pin No. 8 of relay 818 is switched from going through resistor 857 and whichever ones of the other resistors that are connected into the circuit via switches 830 to going directly to ground. Similarly, when appropriate RB0-RB7 signals are output from IC 765, desired ones of switches 820 and 830 are activated to include selected ones of the resistors 851-854 and 858-861 and capacitors 871-874 into the load circuit. A user inputs a command via keyboard 788 indicating the load desired for a particular measurement. Computer 764 refers to a table in its memory to determine the signals it needs to send to IC 765 to set the adjustable load 62 to the desired load, or the nearest available selectable load. In general, it does not matter if the load selected is exactly the desired load, since as long as the selected load is reasonably close to the desired load, a useful measurement can be made. The computer 764 then sends the signals as directed by the table, and indicates to the user via monitor 765 the load that will be set. The integrated circuit 765 is programmed so that in response to the selected commands from computer 764 sent via RS-232 interface 810 the IC 765 sets the voltages on its output pins RCO- RC3 and RB0-RB7 appropriately to open or close circuits of relays 816 and 818 and CMOS switches 820 and 830 to place the selected load between the measurement circuit 750 and ground. The computer then commands the signal generator 760 to provide the desired signal to measurement circuit 750, the measurement circuit provides outputs to channels 1 and 2 of oscilloscope 766, which sends the output to computer 764 which displays it on monitor 765, and prints it on a printer (not shown) as requested by the user. A sequence of loads and/or signals can be programmed into computer 764 so that computer 764 automatically runs a predefined series of measurements. The computer can also use input from the oscilloscope 766 to automatically adjust the load if the signal from the DUM is so low that it is difficult to pull it out of the background noise, or if the signal from the DUM is so large that it may unbalance the system and cause current flow or other undesirable feedback to the signal generation circuit. The measurement circuit 50, 750 of either FIG. 2 or FIG. 7 may be considered to be the combination of a DUM/load circuit 78, 728 comprising DUM 52, 752 and load 62, 762, and a signal generation circuit 68, 768 comprising operational amplifier 54, 754 and resistors 71-75, 771-775. A feature of the invention is that while the DUM/load circuit 78, 728 provides voltage feedback to the signal generation circuit, it is isolated from the signal generation circuit 68, 768 with respect to current. For example, in circuit 50 the buffers 56, 58, isolate the DUM/load circuit 78 from the signal generation circuit 68 thereby preventing current flow through the connection between the DUM/load circuit 78 and the signal generation circuit 68. Likewise, in circuit 750 the buffer 756 isolates the DUM/load circuit 728 from the signal generation circuit 768 thereby preventing current flow through the connection. It is another feature of the invention that feedback, via feedback circuit 79, 779 is provided from the DUM load circuit 78, 728 to the signal generation circuit, to enable the signal generation circuit to maintain a predetermined voltage across the DUM. It is a feature of the invention that the voltage across the load 18, 62, 762 provides the reference voltage into the operational amplifier 14, 54, 754. It is another feature of the invention that the first terminal 34, 91 , 791 of the DUM is allowed to float to the voltage that will maintain the desired predetermined voltage across the DUM. That is, the voltage on the second terminal 36, 92, 792 of the DUM is input as a reference voltage to the operational amplifier, and the operational amplifier holds its output at the voltage that will cause the voltage drop across the DUM to be the predetermined voltage. Thus, the measurement of the electrical parameter is always for an accurately known voltage applied across the DUM, and errors due to inaccuracies of the applied voltage are eliminated. The combination of the above two features permits the selection of a load such that the voltage, current, or other load parameter measured will always be much higher than the measurement tolerances, so that the measurements will always be accurate. That is, the load can be selected such that the electrical parameter measurements on it are optimized, knowing that the rest of the circuit will automatically apply the predetermined voltage across the DUM. It is a further feature of the invention that current that flows from or through the DUM 12, 52, 752 is the same as the current that flows through the load 18, 62, 762. In the circuit of FIG. 1, this is true to a good approximation provided the resistors 22 and 23 and the load are chosen properly. In the circuits of FIGS. 2 and 7, the buffer amplifiers 56, 756 and 58, ensure that this is true to within the tolerances of the buffer appliers, which generally are extremely small as compared to the inaccuracies of the prior art circuits. It is a related feature of the invention that the buffer amplifiers control the flow of current to the inputs of the operational amplifier. Thus, any offset current that derives from the portion of the circuit 50, 750 including the DUM 52, 752 and load 62, 762 is virtually eliminated. There has been described novel apparatus and methods for measuring the current, charge, and other electronic parameters in response to an applied voltage. It should be understood that the particular embodiments shown in the drawings and described within this specification are for purposes of example and should not be construed to limit the invention which will be described in the claims below. Further, it is evident that those skilled in the art may now make numerous uses and modifications of the specific embodiment described, without departing from the inventive concepts. For example, now that the advantage of a circuit, such as an operational amplifier, that assures that a predetermined voltage is applied across the DUM and that the current through the DUM is the same as the current through the load, other such circuits may be devised. A wide variety of load elements or circuits may be used. Further, the apparatus and methods of the invention may be combined with other electronic elements and process steps to provide variations on the apparatus and methods described. Or equivalent circuits and processes may be substituted for the various circuits and processes described. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in and/or possessed by the measurement apparatus and methods described.

Claims: CLAIMS We claim: 1. An electrical measurement circuit (700) comprising: terminals (791, 792) for connection to a device (752) upon which an electronic measurement is to be performed (DUM); and a signal generation circuit (768) for applying an electrical signal to one of said terminals; said electrical measurement circuit characterized by: a feedback circuit (779) for providing a feedback signal from one of said terminals to said signal generation circuit; and said signal generation circuit including an amplifier (754) responsive to said feedback signal for adjusting the signal applied to said DUM. 2. An electrical measurement circuit for measuring charge flow across a ferroelectric device (752), said circuit comprising: terminals (791 , 792) for connection to said ferroelectric device and a signal generation circuit (768) for applying an electrical signal to one of said terminals; said electrical measurement circuit characterized by: a feedback circuit (779) for providing a feedback signal from one of said terminals to said signal generation circuit; and said signal generation circuit including an amplifier (754) responsive to said feedback signal for adjusting the signal applied to said ferroelectric device. 3. An electrical measurement circuit as in claims 1 or 2 wherein said terminals comprise a first electrical terminal (791 ) for connection to said device and a second electrical terminal (792) for connection to said device, said measurement circuit further characterized in that said amplifier comprises an operational amplifier (754) having a first input (746), a second input (747), and an output (748); said second terminal (792) is connected to said first input (746) of said operational amplifier and said output (748) and said second input (747) of said operational amplifier are connected to said first terminal (791 ). 4. An electrical measurement circuit as in claim 3 and further including a ground (767), and wherein said measurement circuit includes a load (762) and said signal generation circuit includes a signal generator (760), and wherein said load is connected between said second electrical terminal (792) and said ground and said signal generator is connected to said first input (746). 5. An electrical measurement circuit as in claim 3 and further characterized in that an oscilloscope (766) is connected to said second terminal (792) through a buffer amplifier (756) and a computer (764) is connected to said oscilloscope and said signal generator. 6. An electrical measurement circuit as in claims 1 or 2 and further characterized in that a first buffer amplifier (756) is connected between said device (752) and said signal generator circuit (768), thereby preventing current flow through said connection between said device and said signal generation circuit. 7. An electrical measurement circuit as in claim 6 and further characterized in that said amplifier comprises an operational amplifier (754) having an input (746, 747), said first buffer amplifier (756) includes an input (737) and an output (796), and said first buffer amplifier input is connected to one (791 , 792) of said terminals and said first buffer amplifier output is connected to said operational amplifier input. 8. An electrical measurement circuit as in claim 7 and further including a load circuit input (761) and wherein said terminals include a first terminal (791) for connecting to said device and a second terminal (792) for connecting to said device, said operational amplifier includes an inverting input (747) and a non- inverting input (746), said second terminal is connected to said load circuit input and said buffer amplifier input, said first buffer amplifier output is connected to said non-inverting input, said circuit further including a second buffer amplifier (758) having an input (726) and an output (727), and wherein said first terminal (791 ) is connected to said operational amplifier output (748) and said operational amplifier inverting input (747), and further including an oscilloscope (766) connected between said output (796) of said first buffer amplifier and said output (727) of said second buffer amplifier. 9. An electrical measurement circuit as in claim 8 and further including a ground, and wherein said signal generation circuit includes a signal generator (760), and further characterized in that a first resistor (772) is connected between said inverting input (747) and said ground, a second resistor (771) is connected between said first terminal (791 ) and said inverting input (747), a third resistor (774) is connected between said signal generator and said non-inverting input (746), and a fourth resistor (775) is connected between the output (796) of said second buffer amplifier and said non-inverting input. 10. An electrical measurement circuit as in claim 9 and further including a computer (764) connected between said oscilloscope and said signal generator. 11. An electrical measurement circuit as in claim 1 and further characterized in that said measurement circuit includes a programmable load circuit (762) including a plurality of loads (890), and an electronic apparatus (764, 765) communicating with said programmable load circuit for selecting one of said plurality of loads. 12. An electrical measurement circuit as in claim 11 further characterized in that said electronic apparatus comprises a computer (764). 13. An electrical measurement circuit as in claim 11 and further characterized in that said programmable load circuit includes a plurality of programmable microswitches (820), and a resistor (851 ) and a capacitor (871 ) connected in parallel between each of said microswitches and one of said terminals (792). 14. An electrical measurement circuit comprising: a DUM circuit (728) comprising terminals (791 , 792) for connection to a device (752) upon which an electronic measurement is to be performed (DUM) and a signal generation circuit (768) for applying an electrical signal to one of said DUM terminals; said electrical measurement circuit characterized in that it includes a programmable load circuit (762) including a plurality of loads (890) connectable to one of said DUM terminals; and an electronic apparatus (764, 765) communicating with said programmable load circuit for selecting one of said plurality of loads. 15. An electrical measurement circuit for measuring charge flow across a ferroelectric device (752) comprising terminals (791,792) for connection to said ferroelectric device and a signal generation circuit (768) for applying an electrical signal to one of said terminals; said electrical measurement circuit characterized in that it includes a programmable load circuit (762) including a pturality of loads (890) connectable to one of said terminals; and an electronic apparatus (764, 765) communicating with said programmable load circuit for selecting one of said plurality of loads. 16. An electrical measurement circuit as in claims 14 or 15 and further characterized in that said electronic apparatus comprises a computer (764). 17. An electrical measurement circuit as in claims 14 or 15 and further characterized in that said programmable load circuit includes a plurality of programmable microswitches (820), and a resistor (851 ) and a capacitor (871 ) are O 97/34156 PC17US97/04568 connected in parallel between each of said microswitches and one (792) of said terminals. 18. An electrical measurement circuit as in claims 14 or 15 and further characterized in that said programmable load circuit includes a plurality of programmable microswitches (830), and a resistor (858) is connected between each of said microswitches and one of said terminals (792). 19. An electrical measurement circuit as in claims 14 or 15 and further characterized in that said measurement circuit includes a buffer amplifier (752) connected between said signal generation circuit (768) and said one (792) of said terminals to which said signal generator is connected. 20. An electrical measurement circuit as in claims 14 or 15 and further characterized in that said measurement circuit includes an oscilloscope (766) and a buffer amplifier (758, 756), and said buffer amplifier is connected between one (791, 792) of said terminals and said oscilloscope.

Inventors: DEVILBISS ALAN, DEVILBISS, ALAN
Applicants: DEVILBISS ALAN, SYMETRIX CORP, DEVILBISS, ALAN, SYMETRIX CORPORATION
Application year: 1997
Main classification: G01R19
IPC codes: G01R19, G01R31