Nonlinear active shunt ammeter

Abstract

A feedback ammeter that may be included in a source measurement unit or digital multimeter includes, for example, an operational amplifier having an input and an output and a feedback path electrically coupled between the output and the input of the operational amplifier. The feedback path includes a first non-linear device for allowing measurement of currents on the order of tens. The ammeter further includes an amplifier electrically coupled to the input of the operational amplifier and the output of the operational amplifier, a second non-linear device electrically coupled to the output of the amplifier, and a resistor electrically coupled between a second capacitor and the input of the operational amplifier. A constant resistance input impedance is established using the second non-linear device that may adjust the circuit gain.

Description

[0001] Priority

[0002] This disclosure claims the benefit of U.S. Provisional Application No. 62 / 666,510, filed on May 3, 2018, entitled "Diode Active Shunt", which is hereby incorporated by reference in its entirety. Technical Field

[0003] The disclosed technology generally relates to electrical measurement equipment, and more particularly, to an active shunt ammeter for measuring current. Background Art

[0004] Source measurement units (SMUs) are used for precise measurements in many fields, including the testing of semiconductor products. A typical SMU design includes a voltage or current source with integrated voltage and current measurement capabilities. A device under test (DUT) can be coupled to the SMU and then excited with a voltage or current source. There are several ways to measure the current through the DUT. For example, a shunt ammeter can be used to simply sense the voltage across a resistor. However, a small resistance must be maintained so as not to cause a large load voltage on the input signal. A low-noise gain stage is needed to amplify the load voltage so that it can be measured.

[0005] Feedback ammeters typically use a high-gain operational amplifier (op-amp) to pull the input circuit through a resistor. Due to the high DC gain of the operational amplifier (e.g., typically greater than one million), the op-amp maintains a low load voltage. This allows for a larger resistance and thus a larger output signal. However, the high gain of the op-amp starts to roll off at relatively low frequencies, resulting in an increase in the load voltage at higher frequencies. If the input is capacitive, it may cause the feedback ammeter to ring or even oscillate.

[0006] Embodiments of the present disclosure address these and other deficiencies of the prior art. Brief Description of the Drawings

[0007] Figure 1 Illustrates an example of a conventional shunt ammeter configured to simply sense the voltage across resistor R S .

[0008] Figure 2 Illustrates an example of a conventional feedback ammeter configured with a high-gain op-amp to pull the input circuit through resistor R S .

[0009] Figure 3 Illustrates a first example of a diode active shunt ammeter circuit in accordance with certain embodiments of the disclosed technology.

[0010] Figure 4 FIG. illustrates a second example of a diode active shunt ammeter circuit in accordance with certain embodiments of the disclosed technology.

[0011] Figure 5 FIG. illustrates an example of a diode active shunt ammeter having multiple ranges Figure 4 thereof.

[0012] Figure 6 FIG. illustrates a third example of a diode active shunt ammeter circuit in accordance with certain embodiments of the disclosed technology.

[0013] Figure 7 FIG. illustrates another example of an active shunt ammeter in accordance with some embodiments of the disclosed technology.

[0014] Figure 8 FIG. illustrates another example of an active shunt ammeter in accordance with some embodiments of the disclosed technology.

[0015] Figure 9 FIG. illustrates another example of an active shunt ammeter in accordance with some embodiments of the disclosed technology.

[0016] Figure 10 FIG. illustrates an example of an active shunt ammeter having a transistor as a non-linear device in accordance with some embodiments of the disclosed technology.

[0017] Figure 11 FIG. illustrates an example of an active shunt ammeter in accordance with some embodiments of the disclosed technology.

[0018] Figure 12 FIG. illustrates a block diagram of a test and measurement unit in accordance with some embodiments of the disclosed technology. DETAILED DESCRIPTION

[0019] Implementations of the disclosed technology generally relate to electrical measurement equipment and, in particular, to active shunt ammeter circuit designs for measuring current. Such ammeters are typically sub-components of measurement products that include digital multimeters (DMMs) and source measurement units (SMUs). There are several ways to measure the current through a device under test (DUT). Figure 1 is a basic diagram of a conventional shunt ammeter 100 that is used to simply sense the voltage across resistor R S 102. In this example, R S 102 must be kept small so as not to cause a large load voltage on the input signal. A low-noise gain-stage amplifier 104 amplifies this load voltage so that it can be measured. The impedance seen from the input is R S .

[0020] Figure 2is a basic circuit diagram of a feedback ammeter 200 for an input circuit pulled by a resistor R through a high-gain operational amplifier (op-amp) 202. The op-amp 202 maintains a low load voltage due to its high DC gain (e.g., typically greater than one million). This allows for a larger resistance for resistor R S 204, thus allowing for a larger output signal. However, the high gain of the op-amp 202 starts to decline at relatively low frequencies. This causes the load voltage to increase at higher frequencies as well. If the input is capacitive, it may cause the feedback ammeter 200 to ring or even oscillate. The impedance seen at the input is R S / A; thus, when the gain drops at higher frequencies, the impedance rises and the input appears inductive. S / A; thus, when the gain drops at higher frequencies, the impedance rises and the input appears inductive.

[0021] Embodiments of the present disclosure address Figure 1 and 2 various problems of the ammeter in Figure 3 Illustrates a first example of a diode active shunt ammeter circuit 300 according to certain embodiments of the disclosed technology. The optional op-amp 302 may have a feedback path in which the non-linear device D s 304 and the capacitor C s 306 are electrically coupled between the output and the input of the op-amp 302. The non-linear devices D s 304 and the capacitor C s 306 are in parallel. The input of the differential amplifier 308 is also electrically coupled to the feedback path. The output of the differential amplifier 308 is connected to the optional capacitor C 2 312 and the non-linear device D 2 314 in parallel. The resistor R 0 316 and the amplifier 318 are both electrically coupled to the parallel capacitor 312 and non-linear device 314 and the input of the op-amp 302. The capacitors C s 306 and C 2 312 are the capacitances of the non-linear devices D s 304 and D 2 314, or may be additional capacitances added.

[0022] Although the non-linear devices 304 and 314 are illustrated as diode pairs in the Figure 3 embodiments, embodiments of the present disclosure are not limited to diode pairs, but any non-linear device may be used, such as but not limited to piecewise linear circuits and bipolar transistors. In some embodiments, only a single diode instead of a pair of diodes may be used to measure the current of one polarity.

[0023] In Figure 3In the embodiment of, two nonlinear devices D s 304 and D 2 314 will both have a large ΔV / ΔI (i.e., tangential resistance) when their currents are small, and a small tangential resistance when their currents are large. The tangential resistance of the nonlinear device D 2 314 divided by the resistance of the resistor R 0 316 sets the small-signal gain of the tangential resistance of the nonlinear device D s 304. This gain varies with the current flowing through the circuit 300. If R 0 C s is adjusted or selected to be equal to 1 at the gain bandwidth (G) of the op-amp 302, then the tangential resistance of the nonlinear device D s 304 will be equal to the impedance of the capacitor C s 306 at the same frequency, at which the gain of the op-amp 302 starts to decline from the gain set by the ratio of the tangential resistance of D 2 314 and R 0 316.

[0024] Therefore, when the op-amp loses gain at increasing frequencies and the feedback path of the op-amp 302, i.e., the nonlinear device D s 304, in parallel with C s 306, the impedance in it decreases at the same rate. Therefore, the input impedance R in across the op-amp's frequency band is constant, and this relationship applies to all functional values of the input current. The circuit 300 outputs the logarithm of the input current and can handle input currents in the tens while having a constant input impedance R in .

[0025] This circuit 300 is based on the diode models shown in equations (1) and (2):

[0026] (1)

[0027] (2).

[0028] As shown in the following equations, based on the diode models in equations (1) and (2) and the circuit shown in Figure 3 , the input resistance is approximately equal to R 0 , which means the input of the circuit is resistive, providing stability for the circuit.

[0029] (3)

[0030] (4)

[0031] (5).

[0032] During the operation of the circuit of Figure 3 , a current measurement can be obtained across the non-linear device D s 304.

[0033] Figure 4 FIG. illustrates a second example of a diode active shunt ammeter circuit 400 in accordance with certain embodiments of the disclosed technology. In this example, the circuit 400 can be a piecewise linear circuit. The circuit 400 includes an op-amp 402, and the op-amp 402 can have a feedback path. The feedback path can include two resistors R 1 404 and R 2 406 in series. The resistor R 2 406 has a non-linear device D 1 408 in parallel therewith. The feedback path further includes a capacitor C 1 410 in parallel with the two resistors R 2 404 and R 1 406.

[0034] An amplifier 412 can be electrically coupled between the resistor R 1 404 and R 2 406 to determine the voltage at that point for current measurement. The voltage can also be measured at V 1 in the circuit.

[0035] The input of a differential amplifier (diff-amplifier) 414 is also electrically coupled to the feedback path. The output of the differential amplifier 414 is connected to the same circuit as the feedback path. That is, the circuit includes resistors R 1 416 and R 2 418, a non-linear device D 1 420 and a capacitor C 1 422, which are the same as and arranged identically to the resistors R 1 404 and R 2 406, the non-linear device D 1 408 and the capacitor C 1 410. The resistor R 0 422 and the amplifier 424 are both electrically coupled to the capacitor 422 and the resistor 418 and the input of the op-amp 402.

[0036] Similar to Figure 3 the circuit 300 in 0316 and amplifier 318, and thus, these components will not be referred to further herein Figure 4 Further discuss these components. Similar to Figure 3 the circuit 300 shown in

[0037] (6)

[0038] (7).

[0039] In circuit 400, the non-linear devices D 1 408 and 420 are both clamping devices (such as a pair of diodes as shown in Figure 4 ) or switches. Each of the resistors R 1 404 and R 2 406 has a different resistance. Generally, a diode turns on when the voltage is greater than the threshold. Thus, during the operation of circuit 400, if the current is large enough such that the non-linear device D 1 408 turns on, then the resistor R 2 406 saturates, and then the voltage across R 1 is an accurate measurement of the current. If the current is low enough such that the non-linear device D 1 408 does not turn on, then the voltage across R 2 is a more accurate measurement of the current. The voltages V 1 and V 2 can be read to determine the current.

[0040] Although the example circuit 400 only includes two ranges, the circuit can be extended to include multiple ranges, as shown in Figure 5 Circuit 500 can have a number N of ranges, where N is an integer. Figure 5 Components similar to those in Figure 4 are given the same reference numerals and will not be referred to further herein Figure 5 Further discussion.

[0041] In Figure 5 instead of having only two resistors R 1 404 and R 2 406, a number N of resistors can be provided, each resistor having a clamping device in parallel. For example, Figure 5 illustrates a resistor R N-1 502 with a non-linear device D N 504 in parallel. Since the circuit after the differential amplifier 414 is the same as the feedback path, a number N of resistors are also provided, each resistor having a clamping device in parallel and electrically coupled to the output of the differential amplifier 414. For example, Figure 5Illustrates a resistor NR N-1 508 having a non-linear device D N 506 in parallel. N-1 508 of resistor NR N 506.

[0042] Resistor R 1 404, R 2 406 and R N Each of 502 has a different resistance. For example, assume N is 3, then R 1 404 can be 10 ohms, R 2 406 can be 100 ohms, and R 3 can be 1000 ohms. For this example, also assume that the voltage limiting devices D 1 408 and D N-1 504 turn on at 0.6V. If the current is 1 mA, then there will be 1V across R 3 502, and the non-linear device D 2 504 will turn on, causing the current to be routed around the resistor R 3 502. Since the resistor R 1 404 is only 10 ohms, it is a noisier measurement, and the current measurement can be taken based on the voltage across R 2 406. If the current jumps to 100 mA, then R 1 404 gives a voltage, and the other two resistors R 2 406 and R 3 502 will saturate, and the voltage across R 1 404 can be used to determine the current. This creates a logarithmic characteristic for the circuit 500 and can create more accurate current measurements at all different magnitudes of current. In some embodiments, the resistances of the resistors in the circuit can be selected in decade multiples, as described above.

[0043] Figure 6 Illustrates another example of a diode active shunt ammeter circuit 600 according to certain embodiments of the disclosed technology. This example includes components similar to those discussed above with respect to Figure 3 the components discussed. Therefore, these components are given the same reference numerals and are not discussed further herein.

[0044] In the circuit 600, more non-linear devices D 2 314 can be provided in the feedback path than the number of non-linear devices D s 304. In the example shown in Figure 3 two non-linear devices D s 304 are provided and one non-linear device D 2314. This makes the input impedance R of the circuit in and R 0 The relationship between them is a ratio. For example, in circuit 600, using the diode models shown in equations (1) and (2) above, R in is equal to half of R 0 , as shown in the following equations (8)-(11):

[0045] (8)

[0046] (9)

[0047] (10)

[0048] (11).

[0049] However, the embodiments of the present disclosure are not limited to Figure 6 the embodiments shown. Different numbers of nonlinear devices D s 304 and nonlinear device D 2 314 can be provided to change the ratio of R in and R 0 . In addition, the nonlinear device D s 304 does not have to be in parallel, but can be provided in series.

[0050] Figures 7 - 10 Embodiments of other active shunt ammeters 700, 800, 900, and 1000 according to certain embodiments of the present disclosure are respectively illustrated. The components of the active shunt ammeters 700, 800, 900, and 1000 are similar to those discussed above with respect to Figure 3 , but are arranged in different circuits. Therefore, the components are given the same reference numerals.

[0051] Figure 7 The active shunt ammeter 700 for 0 can include: a first feedback path to the inverting input of op-amp 302; and a second feedback path including resistor R s 316 and amplifier 318 and a nonlinear device D s 314, the nonlinear device D s 314 is electrically coupled to resistor 316 and nonlinear device D s 304 and capacitor 306. The nonlinear device D Measure 304 and capacitor 306 are electrically coupled to the input of op-amp 302. In an embodiment, the voltage can be measured at V sThe current of 304. Similar to Figure 3 , if R 0 C s is set or selected to 1 on the gain bandwidth, the input impedance R in is approximately equal to R 0 .

[0052] Similar to Figure 7 the active shunt ammeter, in Figure 8 the active shunt ammeter 800, op-amp302 is electrically coupled to resistor 316, and resistor 316 is in turn electrically coupled to the non-linear device D 2 314 and amplifier 318. Amplifier 318 includes a negative feedback path. The non-linear device D s 304 is connected to the input of op-amp302 and is in parallel with capacitor C s 306. In this embodiment, the voltage can be measured at V Measure 802 and this voltage is used to determine the current flowing through the connected device under test. Similar to Figure 3 , if R 0 C s is set or selected to 1 on the gain bandwidth, the input impedance R in is approximately equal to R 0 .

[0053] The active shunt ammeter 900 similar to the active shunt ammeter 800 may include a non-linear device D s 306 in parallel with capacitor C s 304, which are electrically coupled to the input of op-amp 302. Also electrically coupled to the input of op-amp 302 are the output of resistor 316 and amplifier 318. The input of amplifier 318 is electrically coupled to resistor 306 and the non-linear device D 2 314, and the non-linear device D 2 314 is also electrically coupled to ground. The other input of amplifier 318 is connected to the output of op-amp302.

[0054] In the active shunt ammeter 900, a voltage measurement can be taken at V Measure 902 to determine the current. Similar to other active shunt ammeters, if R 0 C s is set or selected to 1 on the gain bandwidth, the input impedance R in is approximately equal to R 0 .

[0055] When Figure 3 the pair of diodes D s 304 and D 2When 314 is changed to transistors 1002 and 1003, the active shunt ammeter 1000 is illustrated as Figure 3 the embodiment shown in Figure 10 nor does it include the optional capacitor C 2 312. The operation of the active shunt ammeter 1000 is substantially equivalent to Figure 3 the operation of the embodiment shown in , and thus will not be described in further detail. Such transistors can be used as non-linear devices in any of the embodiments discussed above.

[0056] Figure 11 An example of a simplified circuit 1100 based on the embodiments discussed above is illustrated. For example, as Figure 11 seen in , the non-linear device D s 1102 is connected to the input 1104 of the ammeter 1100. The capacitor C s 1106 is illustrated in Figure 11 . The capacitor C s 1106 may represent the parasitic capacitance of the non-linear device D 2 1102. In some embodiments, a capacitor may be added for the capacitor C s 1106. It is known that the capacitance value of C s 1106 can allow the ammeter 1100 to be designed to have an input impedance that is constant with respect to frequency. The input 1104 is also connected to the control circuit 1110. The control circuit 1110 may include a resistor R 2 1112 in series with the non-linear device D 0 1114. As discussed in detail above, other components may be provided in the ammeter to provide different circuit functions and variations.

[0057] In Figure 11 , the input current I in flows through the non-linear device D s 1106. The control circuit 1110 forces the voltage across the non-linear device D s 1106 to be approximately equal to the V 2 across the non-linear device D x -V in across the non-linear device D 2 1114. The current flowing through the non-linear device D 0 1114 also flows through the resistor R x 1112. The control circuit 1100 controls the voltages V lo and V in such that the difference between the voltage V lo and the voltage V o is equal to the voltage across the resistor R b -V a). This results in the input resistance R in being proportional to the resistor R o 1112.

[0058] Figure 12 FIG. illustrates a block diagram of a test and measurement instrument in accordance with some embodiments of the present disclosure. As will be readily understood by those skilled in the art, the test and measurement instrument 1200 may include Figure 12 additional components not shown in

[0059] The test and measurement instrument 1200 includes one or more ports 1202, and the ports 1202 may be any electrical signal transmission medium. The test and measurement instrument 1200 may be any device that can measure current, such as but not limited to a source measurement unit or a digital multimeter, as described above. The ports 1202 may include a receiver, a transmitter, and / or a transceiver. If the test and measurement instrument 1200 is a source measurement unit, the port 1202 may be coupled to an ammeter 1204 or a source, and the ammeter 1204 may be any one of the ammeters 300, 400, 500, or 600. Although for ease of illustration, only one processor 1206 is shown in Figure 12 , as will be understood by those skilled in the art, multiple different types of processors 1206 may be used in combination instead of a single processor 1206. The processor 1206 may be electrically coupled to the ammeter 1204 to assist in measuring, displaying, and / or storing the current of the connected device under test.

[0060] One or more processors 1206 may be configured to execute instructions from the memory 1208 and may perform any method and / or associated steps indicated by such instructions. The memory 1208 may be implemented as a processor cache, random access memory (RAM), read only memory (ROM), solid state memory, (one or more) hard disk drives, or any other memory type. The memory 1208 serves as a medium for storing data, computer program products, and other instructions.

[0061] The user input 1210 is coupled to the one or more processors 1206. The user input 1210 may include a keyboard, a mouse, a trackball, a touch screen, and / or any other controls that the user may employ with the GUI on the display 1212. The display 1212 may be a digital screen, a cathode ray tube-based display, or any other monitor for displaying waveforms, measurements, and other data to the user. Although the components of the test and measurement instrument 1200 are depicted as integrated within the test and measurement instrument 1200, those of ordinary skill in the art will understand that any of these components may be external to the test instrument 1200 and may be coupled to the test instrument 1200 in any conventional manner (e.g., wired and / or wireless communication media and / or mechanisms). For example, in some embodiments, the display 1212 may be remote from the test and measurement instrument 1200.

[0062] The embodiments discussed herein allow for a wide dynamic range of currents with accuracy at all current levels. The various embodiments may allow the user to accurately measure a device at both low and high currents without having to change the ammeter. For example, if a device suddenly receives a large current after previously receiving a low current, the ammeters of the embodiments disclosed herein are capable of accurately measuring both the low and high currents.

[0063] In this disclosure, the singular forms "a," "an," and "the" include plural referents unless the context otherwise indicates. The term "or" is intended to be inclusive and means any one, any, several, or all of the listed items. The terms "comprises," "comprising," or other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or product that comprises a list of elements does not necessarily include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Relative terms such as "about," "approximately," "substantially," and "generally" are used to indicate a possible variation of ±10% of the stated or understood value.

[0064] Aspects of this disclosure are susceptible to various modifications and alternative forms. Specific aspects have been shown by way of example in the drawings and are described in detail herein. However, it should be noted that the examples disclosed herein are presented for purposes of clear discussion and are not intended to limit the scope of the general concepts disclosed to the specific aspects described herein, unless otherwise expressly defined. Accordingly, this disclosure is intended to cover all modifications, equivalents, and alternatives of the aspects described in the drawings and claims.

[0065] References in the specification to aspects, examples, etc. indicate that the described items may include particular features, structures, or characteristics. However, each disclosed aspect may or may not include the particular feature, structure, or characteristic. Additionally, unless otherwise specified, such phrases do not necessarily refer to the same aspect. Further, when a particular feature, structure, or characteristic is described in connection with a particular aspect, such a feature, structure, or characteristic may be employed in connection with another disclosed aspect, whether or not such another disclosed aspect explicitly describes such a feature.

[0066] Aspects of the present disclosure may operate on specifically created hardware, in firmware, on a digital signal processor, or on a specially programmed general-purpose computer including a processor operating according to programming instructions. The terms controller or processor as used herein are intended to include a microprocessor, a microcomputer, an application specific integrated circuit (ASIC), and a dedicated hardware controller. One or more aspects of the present disclosure may be embodied in computer-usable data and computer-executable instructions, such as embodied in one or more program modules executed by one or more computers (including a monitoring module) or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types when executed by a processor in a computer or other device. The computer-executable instructions may be stored on a non-transitory computer-readable medium (such as a hard disk, an optical disk, a removable storage medium, a solid-state memory, a random access memory (RAM), etc.). As will be understood by those skilled in the art, the functions of the program modules may be combined or distributed as needed in various aspects. Additionally, such functions may be embodied in whole or in part in firmware or hardware equivalents (such as integrated circuits, FPGAs, etc.). Particular data structures may be used to more effectively implement one or more aspects of the present disclosure, and such data structures are contemplated within the scope of the computer-executable instructions and computer-usable data described herein.

[0067] In some cases, the disclosed aspects may be implemented in hardware, firmware, software, or any combination thereof. The disclosed aspects may also be implemented as instructions carried or stored on one or more non-transitory computer-readable media that may be read and executed by one or more processors. Such instructions may be referred to as a computer program product. As discussed herein, computer-readable media means any medium that can be accessed by a computing device. By way of example and not limitation, computer-readable media may include computer storage media and communication media.

[0068] A computer storage medium means any medium that can be used to store computer-readable information. By way of example, and not limitation, computer storage media may include RAM, ROM, electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, compact disc read-only memory (CD-ROM), digital video disc (DVD) or other optical disc storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, and any other volatile or non-volatile, removable or non-removable media implemented in any technology. A computer storage medium does not include the signal itself and transient forms of signal transmission.

[0069] A communication medium means any medium that can be used for communication of computer-readable information. By way of example, and not limitation, communication media may include coaxial cable, fiber optic cable, air, or any other medium suitable for communication of electrical, optical, radio frequency (RF), infrared, acoustic, or other types of signals.

[0070] Examples

[0071] Illustrative examples of the techniques disclosed herein are provided below. Embodiments of these techniques may include any one or more of the examples described below, as well as any combination.

[0072] Example 1 is an ammeter, comprising: a resistor; an input having an input resistance approximately equal to the resistance of the resistor; a first non-linear device in series with the resistor; and a second non-linear device electrically coupled to the input.

[0073] Example 2 is the ammeter of Example 1, wherein each of the first non-linear device and the second non-linear device is a pair of diodes in parallel.

[0074] Example 3 is the ammeter of either Example 1 or 2, wherein each of the first non-linear device and the second non-linear device is a clamping circuit.

[0075] Example 4 is the ammeter of Example 3, wherein the clamping circuit is a switch or a pair of diodes in parallel.

[0076] Example 5 is the ammeter of any one of Examples 1-4, further comprising: an operational amplifier having an input and an output, the resistor being electrically coupled to the input of the operational amplifier, and the second non-linear device being electrically coupled to the output of the operational amplifier; and a feedback path electrically coupled between the output and the input of the operational amplifier, the feedback path including the first non-linear device.

[0077] Example 6 is the ammeter of Example 5, further comprising: a first capacitor in the feedback path and a second capacitor electrically coupled to the output of the amplifier, the first capacitor and the second capacitor having approximately equal capacitances.

[0078] Example 7 is an ammeter of either Example 5 or 6, wherein the feedback path includes a third non-linear device.

[0079] Example 8 is the ammeter of Example 7, wherein the third non-linear device is in series with the first non-linear device.

[0080] Example 9 is an ammeter of any one of Examples 5 - 8, wherein the resistor is a first resistor, and the feedback path further includes: a second resistor; and a third resistor, the third resistor being in series with the second resistor and in parallel with the first non-linear device.

[0081] Example 10 is the ammeter of Example 9, further comprising: a fourth resistor, electrically coupled to the output of the amplifier and in parallel with the first non-linear device; and a fifth resistor, electrically coupled to the first resistor and the fourth resistor and in series with the fourth resistor.

[0082] Example 11 is an ammeter of any one of Examples 1 - 10, wherein during operation of the ammeter, the input resistance of the ammeter is approximately equal to the resistor resistance at all frequencies.

[0083] Example 12 is a test and measurement device, comprising: an input port configured to receive current from a device under test; and the ammeter of claim 1, configured to measure the current from the device under test.

[0084] Example 13 is an ammeter, comprising: an operational amplifier having a first input, a second input, and an output, the operational amplifier having an input impedance at the first input and the second input; a feedback path electrically coupled between the output of the operational amplifier and the first input, the feedback path including a first non-linear device; an amplifier electrically coupled to the input of the operational amplifier and the output of the operational amplifier; a second non-linear device electrically coupled to the output of the amplifier; and a resistor electrically coupled between a second capacitor and the second input of the operational amplifier, the resistance of the resistor being approximately equal to the input impedance or a multiple of the input impedance.

[0085] Example 14 is the ammeter of Example 13, wherein the feedback path includes a third non-linear device.

[0086] Example 15 is the ammeter of Example 14, wherein the third non-linear device is in series with the first non-linear device.

[0087] Example 16 is an ammeter of any one of Examples 13 - 15, wherein the resistor is a first resistor, and the feedback path further includes a second resistor; and a third resistor, the third resistor being in series with the second resistor and in parallel with the first non-linear device.

[0088] Example 17 is the ammeter of Example 16, further comprising: a fourth resistor electrically coupled to the output of the amplifier and in parallel with the first nonlinear device; and a fifth resistor electrically coupled to the first resistor and the fourth resistor and in series with the fourth resistor.

[0089] Example 18 is a method of measuring the current flowing through a device under test, the method comprising: receiving an input signal representing the current flowing through the device under test; generating an output voltage across a first nonlinear device; and providing a control circuit having a second nonlinear device in series with a resistor, the control circuit being configured to force the voltage across the second nonlinear device to be equal to the output voltage and to control the input resistance to be proportional to the resistance of the resistor.

[0090] Example 19 is the method of Example 18, wherein the output voltage is generated across the first nonlinear device in the feedback path of an operational amplifier.

[0091] Example 20 is the method of either Example 18 or 19, wherein the current flowing through the first nonlinear device also flows through the resistor.

[0092] Previous versions of the disclosed subject matter have many advantages, which have either been described or will be apparent to those of ordinary skill in the art. Even so, not all of these advantages or features are required in all versions of the disclosed devices, systems, or methods.

[0093] In addition, the written description mentions specific features. It is to be understood that the disclosure in this specification includes all possible combinations of those specific features. Where a specific feature is disclosed in the context of a particular aspect or example, that feature may also be used, to the extent possible, in the context of other aspects and examples.

[0094] Moreover, when a method having two or more defined steps or operations is recited in this application, the defined steps or operations may be performed in any order or simultaneously, unless the context excludes those possibilities.

[0095] Although specific embodiments of the invention have been illustrated and described for purposes of illustration, it will be understood that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.

Claims

1. An ammeter, comprising: a resistor; an ammeter input having an input resistance approximately equal to the resistance of the resistor; a first non-linear device in series with the resistor; a second non-linear device electrically coupled to the ammeter input; an operational amplifier having an input and an output, the resistor being electrically coupled to the input of the operational amplifier, and the second non-linear device being electrically coupled to the output of the operational amplifier; and a feedback path electrically coupled between the output and the input of the operational amplifier, the feedback path including the first non-linear device.

2. The ammeter according to claim 1, wherein each of the first non-linear device and the second non-linear device is a pair of parallel diodes.

3. The ammeter according to claim 1, wherein each of the first non-linear device and the second non-linear device is a clamping circuit.

4. The ammeter according to claim 3, wherein the clamping circuit is a switch or a pair of parallel diodes.

5. The ammeter according to claim 1, further comprising: a first capacitor in the feedback path and a second capacitor electrically coupled to the output of the operational amplifier, the first capacitor and the second capacitor having approximately equal capacitances.

6. The ammeter according to claim 1, wherein the feedback path includes a third non-linear device.

7. The ammeter according to claim 6, wherein the third non-linear device is in series with the first non-linear device.

8. The ammeter according to claim 1, wherein the resistor is a first resistor, and the feedback path further comprising: a second resistor; and a third resistor in series with the second resistor and in parallel with the first non-linear device.

9. The ammeter according to claim 8, further comprising: a fourth resistor electrically coupled to the output of the operational amplifier and in parallel with the first non-linear device, a fifth resistor electrically coupled to the first resistor and the fourth resistor and in series with the fourth resistor.

10. The ammeter according to claim 1, wherein during operation of the ammeter, the input resistance of the ammeter is approximately equal to the resistance of the resistor at all frequencies.

11. A test and measurement device, comprising: an input port configured to receive current from a device under test; and the ammeter according to claim 1, configured to measure the current from the device under test.

12. An ammeter, comprising: an operational amplifier having a first input, a second input and an output, the operational amplifier having an input impedance at the first input and the second input; a feedback path electrically coupled between the output and the first input of the operational amplifier, the feedback path including a first non-linear device; an amplifier electrically coupled to the first input of the operational amplifier and the output of the operational amplifier; a second non-linear device electrically coupled to the amplifier output of the amplifier; and a resistor electrically coupled between the amplifier output and the second input of the operational amplifier, the resistance of the resistor being approximately equal to or a multiple of the input impedance.

13. The ammeter according to claim 12, wherein the feedback path includes a third non-linear device.

14. The ammeter according to claim 13, wherein, the third non-linear device is connected in series with the first non-linear device.

15. The ammeter according to claim 12, wherein the resistor is a first resistor, and the feedback path further comprises: a second resistor; and a third resistor, connected in series with the second resistor and in parallel with the first non-linear device.

16. The ammeter according to claim 15, further comprises: a fourth resistor, electrically coupled to the output of the amplifier and in parallel with the first non-linear device, a fifth resistor, electrically coupled to the first resistor and the fourth resistor and in series with the fourth resistor.

17. A method of measuring current flowing through a device under test, the method comprises: receiving an input signal, the input signal representing the current flowing through the device under test; generating an output voltage across a first non-linear device; and providing a control circuit, the control circuit having a second non-linear device connected in series with a resistor, the control circuit being configured to force the voltage across the second non-linear device to be equal to the output voltage and control the input resistance to be proportional to the resistance of the resistor.

18. The method according to claim 17, wherein the first non-linear device is provided in the feedback path of an operational amplifier.

19. The method according to claim 17, wherein the current flowing through the first non-linear device also flows through the resistor.

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