Multi-stage current measurement architecture

Abstract

A test and measurement instrument includes a current measurement device having an input to accept a current for measurement, an output to deliver an output current from the current measurement device, a sense path through which current is measured, and an active bypass device to deliver an amount of current from the input of the current measurement device to the output of the current measurement device without passing through the sense path. The active bypass device can be adjusted to allow the current sense device to operate without bypass for a particular range of target current values. Some current measurement devices can include more than one active bypass circuit, each active bypass circuit adjusted for a different range of input current values.

Description

[0001] Priority

[0002] This disclosure claims the benefit of U.S. Provisional Application No. 62 / 757,979, filed November 9, 2018, entitled “MULTI-STAGE CURRENT PROBE ARCHITECTURE FOR HIGH-SPEED HIGH-PRECISION AND HIGH-DYNAMIC-RANGE CURRENT MEASUREMENT,” which is incorporated by reference herein in its entirety. TECHNICAL FIELD

[0003] The subject matter relates to apparatuses and methods for high-speed current measurement for electronic devices. BACKGROUND

[0004] In many industries, measuring, characterizing, and understanding the current consumption of devices is an important design step. For example, measuring current in a power delivery network (PDN) requires high-bandwidth current measurement. Some devices, such as Internet of Things (IOT) devices, cycle rapidly between different operating states that differ by many orders of magnitude in current draw. The current draw of these devices is marked by periods of low current draw punctuated by activity pulses where the current draw can increase by a factor of about 1,000,000 for a brief period. For example, an active listening device draws a relatively low current while it is waiting for a wake-up word. Once the wake-up word is detected, the device captures ambient sound signals that follow the wake-up word, converts them to electrical signals, and then transmits the signals to a control device on a communication network, such as a cloud network. After the listening device receives the appropriate command through the cloud network, the listening device performs the action directed at it by the control device. Although the wake-up word detection occurs at the listening device with a relatively low current draw, after being woken up, the listening device rapidly switches to a high current mode to perform the rest of the action. After performing the last action, the listening device reenters the low current mode while it is waiting for another wake-up word. Other devices have multiple current modes. It is important for manufacturers, repair shops, and other institutions to be able to measure the current drawn by various devices in all modes, i.e., while the device rapidly switches its power requirements.

[0005] If a large value shunt resistor is used as the current measurement device when measuring low level currents, a large load voltage can occur during the high current pulse time. This load voltage occurs because a portion of the high current is drawn through the large shunt resistor during the high current pulse. As a result, unless the power supply is a very large power supply that can change output levels very quickly, the voltage supplied to the attached device will quickly drop through the large shunt resistor. This voltage drop can be so severe that the supplied voltage drops below the minimum operating voltage of the connected device, which can cause the device to shut down or go off line. If a small value shunt resistor is used as the current measurement device, the load voltage can be kept small, however, the details of the low current draw period can be lost in the noise floor of the measurement system.

[0006] Clamp-on test and measurement probes can allow very high bandwidth measurements with little load on the circuit under test. However, the noise floor and DC accuracy of clamp-on probes can be limited. For example, the smallest resolvable current can not be sufficient to fully characterize many designs today.

[0007] Embodiments of the disclosed technology address the shortcomings in the prior art. BRIEF DESCRIPTION OF DRAWINGS

[0008] Figure 1A is a circuit block diagram illustrating an in-line current measurement in conjunction with a typical connected device according to an embodiment.

[0009] Figure 1B is a circuit block diagram illustrating an in-line current measurement in conjunction with other types of typical connected devices according to an embodiment.

[0010] Figure 2 is a circuit diagram of a current measurement device using a sensing element such as a shunt resistor.

[0011] Figure 3 is a circuit diagram of a current measurement device using a sensing element and an active clamp according to an embodiment.

[0012] Figure 4 is a circuit diagram of a current measurement device using a sensing element, an active clamp, and a bypass capacitor according to an embodiment.

[0013] Figure 5 is a circuit diagram of a current measurement device using multiple sensing elements, an active clamp, and a bypass capacitor according to an embodiment.

[0014] Figure 6 is a circuit diagram of a current measurement device using multiple sensing elements, multiple active clamps, and multiple bypass capacitors according to an embodiment.

[0015] Figure 7 This is a circuit diagram of a current measurement device according to an embodiment, which uses multiple sensing elements, multiple active clamps, and multiple bypass capacitors.

[0016] Figure 8 This is a circuit diagram of a current measurement device using a sensing element, an active clamp, and an active shunt according to an embodiment.

[0017] Figure 9 This is a circuit diagram of a current measurement device using a sensing element, multiple active clamps, and multiple active shunts according to an embodiment.

[0018] Figure 10 This is a graph of the output of a current measuring device using a sensing element according to an embodiment, with various operating areas marked on it. Detailed Implementation

[0019] As described herein, the embodiments relate to circuits and methods for high-speed current measurement in electronic devices. Embodiments of the disclosed technology can provide current measurement capabilities over a wider range and at a higher resolution than existing solutions, without sacrificing load voltage or bandwidth.

[0020] Figure 1A This is a circuit block diagram illustrating an in-line current measuring device 50. The current measuring device 50 receives input current from a device such as a power supply 46. The current measuring device 50 also supplies current to devices such as the device under test or the device under test (DUT) 48. Because the current measuring device 50 is part of the current supply path from the power supply 46 to the DUT 48, devices such as the current measuring device 50 are sometimes referred to as in-line current measuring devices; however, not all embodiments of the invention are limited to in-line devices.

[0021] In operation, the current measuring device 50 measures the amount of current supplied to the DUT 48 by the power supply 46. The current measurement result can be provided as an analog voltage, illustrated as Vo in Figure 1. The current measurement result can also be digitized by a digitizer 52 (such as an analog-to-digital converter (ADC) or other suitable device). In such an embodiment, the output is a digital output whose value corresponds to the current measured by the current measuring device 50.

[0022] Figure 1B Similar to Figure 1A, except that the connected power supply is a remotely sensed power supply 47, such as a 2280S series power supply available from Keithley Instruments. In this example, the power supply 47 attempts to control the voltage measured between the Sense HI and Sense LO leads by varying the voltage between the Force HI and Force LO (i.e., ground) leads. Remotely sensed power supplies typically limit the user to a 1 volt drop between the Force HI and Force LO lines to maintain system operation, otherwise the connected DUT 49 can implement an input voltage drop and shut down. In many cases, previous inline current measurement devices exceed the 1 volt drop due to the relatively high voltage drop across the Force HI lead from the load voltage. In contrast, embodiments of the present invention can be configured to limit the load voltage, as described below. Limiting the load voltage helps prevent the voltage drop in the power supplied to the DUT 49 from being so large, and thus ensures the continued operation of the DUT during testing.

[0023] Figure 2 is a circuit diagram illustrating a method for current measurement using a sensing element 101. The simple current measurement method depicted in FIG. 1 uses a sensing element 101 to produce a voltage (Vo) from an input current (I). The sensing element 101 can be a resistive element with a size selected for a particular range for current measurement. The load voltage is given as I*R, which means that a large increase in current can cause a failure condition for a system with a fixed power supply due to a large voltage drop during switching (as described above).

[0024] Figure 3 is a circuit block diagram illustrating a method and apparatus for current measurement using a sensing element 101 and an active clamp 102, according to an embodiment. The combination of the sensing element 101 and the active clamp 102 is an example of circuit components that can be present in the current measurement device 50 of FIG. 1. The active clamp 102 shunts current away from the sensing element 101 based on a signal from the sensing element 101, thereby limiting the voltage. This active clamp 102 can be used to limit the power dissipation in the sensing element 101 and / or limit the load voltage generated by the system.

[0025] The active clamp 102 may include sensing circuitry for providing bipolar voltage clamping at any desired voltage level. This means that the active clamp 102 performs a clamping action when the sensing circuitry detects a positive or negative voltage of sufficient magnitude to turn on the active clamp, as described below. When the voltage is within the desired limits, the active clamp 102 can be disabled and therefore does not shunt any current. Furthermore, the clamp can be configured to have limited gain in its active circuitry. This provides a “softer” clamp that can operate linearly at a much higher speed and avoids errors caused by non-linear device switching in the circuitry.

[0026] In one embodiment, the active clamp 102 includes a pair of n-type metal-oxide-semiconductor (NMOS) transistors coupled in parallel, as well as a positive sensing element and a negative sensing element. When the voltage sensed by the positive sensing element increases toward a positive voltage limit, the positive sensing circuit causes the voltage on the gate of the NMOS transistor coupled thereto to increase. This voltage initiates the transistor's turn-on and shunts current to bypass the sensing element 101. During this period, the negative sensing element does not sense a negative voltage, therefore the voltage it generates for the NMOS transistor connected thereto is zero, or below the NMOS transistor's turn-on voltage, and thus its NMOS transistor remains off.

[0027] Similarly, as the voltage sensed by the negative sensing element increases toward the negative voltage limit, the negative sensing circuit increases the voltage on the gate of the NMOS transistor coupled thereto. This voltage begins to turn on the transistor and shunt current so that it bypasses the sensing element 101. During this period, the positive sensing element does not sense a positive voltage, and therefore the voltage it generates for the NMOS transistor connected thereto is zero or lower than the turn-on voltage of the NMOS transistor, and therefore its NMOS transistor remains off.

[0028] Figure 4 This is a circuit diagram of a current measurement device using a sensing element 101 and an active clamp 102 according to an embodiment. Figure 4 As shown, the configuration can be modified by adding a capacitor 103 connected in parallel with the active clamp 102 and the sensing element 101. Figure 3 An example is the implementation of capacitor 103. For instance, the size of capacitor 103 can be set to match the cutoff frequency of active clamp 102. Capacitor 103 can then shunt sudden current spikes that exceed the bandwidth of active clamp 102. This may cause the circuit to exhibit capacitive behavior rather than inductive behavior at high frequencies, which may be desirable for a shunt device.

[0029] Figure 5 This is a circuit diagram illustrating a current measurement method using multiple sensing elements 104, 105 and an active clamp 102 according to an embodiment.Figure 5 The embodiments of FIG. 1 can be altered by including additional sensing elements 104 and amplifiers 106 in the circuit, as shown in FIG. 2. Figure 4 The embodiments of FIG. 1 can be altered by including additional sensing elements 104 and amplifiers 106 in the circuit, as shown in FIG. 2. Figure 5 In the configuration shown in FIG. 2, the voltage across the first sensing element 104 (Vo1) will always be proportional to the current I, while the voltage across the second sensing element 105 (Vo2) will be proportional to I as long as the clamp 102 is not shunting current.

[0030] In embodiments, resistors can be used for the sensing elements 104, 105. If resistors are used for the sensing elements 104, 105, and the second sensing element 105 is chosen to be much larger than the first sensing element 104, then Figure 5 The circuit shown in FIG. 2 can be used as a seamless dual range ammeter. For smaller currents, the larger second sensing element 105 is used to measure the current with higher fidelity. As the current increases, the clamp 102 activates and limits the load voltage of the current measurement circuit. When the clamp 102 activates, Vo2 can no longer be an accurate representation of the current I. But Vo1 is still valid, and the load voltage remains low, allowing the current measurement to continue. As the magnitude of the signal being measured increases, the effect of the loss of fidelity of the smaller first sensing element 104 is less.

[0031] Figure 6 is a circuit diagram showing a current measurement method using multiple sensing elements 104, 105, 107 and multiple active clamps 102, 108, according to an embodiment. As Figure 6 The embodiments of FIG. 1 can be altered by including additional sensing elements 104 and amplifiers 106 in the circuit, as shown in FIG. 2. Figure 5 The embodiments of FIG. 1 can be altered by including additional sensing elements 104 and amplifiers 106 in the circuit, as shown in FIG. 2. Figure 6 In the configuration shown in FIG. 2, the voltage across the first sensing element 104 (Vo1) will always be proportional to the current I, while the voltage across the second sensing element 105 (Vo2) will be proportional to I as long as the clamp 102 is not shunting current.

[0032] Figure 7 is a circuit diagram showing a current measurement method using multiple sensing elements 104, 105, 107 and multiple active clamps 102, 108, according to an embodiment. As Figure 7As shown, by defining the output Vo as the difference between the voltage of the sensing element and the input voltage, rather than differentially monitoring each sensing element, the process can be simplified. Figure 6 The design complexity of this embodiment is reduced. The multiplexer 109 can then be controlled to switch sensing element signals based on which of the active clamps 102, 108 is active. While this arrangement may lose the benefits of multiple simultaneous measurements, this simplified arrangement allows operation using only a single ADC and amplifier 106. The single ADC can, for example, operate to digitize the amplifier's output, in... Figure 7 The Chinese character is marked as "Vo".

[0033] For purely resistive sensing elements, there may be a trade-off between the magnitude of the voltage signal and its bandwidth. Increasing the resistor value can produce a larger output voltage and improve the signal-to-noise ratio. However, the measurement bandwidth is set by the product of the bypass capacitor 103 and the sensing resistor, meaning that a smaller resistor value may be desired for higher bandwidth operation. Active shunt circuitry can allow for increased resistor bandwidth and load voltage by making the surrounding circuitry "see" a much smaller resistor.

[0034] Figure 8 This is a circuit block diagram illustrating a possible implementation of a current measurement device according to an embodiment, using a sensing element 101, an active clamp 102, and an active shunt 110. (See diagram for details.) Figure 8 As shown, due to the structure of the active shunt 110, the resistance sensed by the active clamp 102, bypass capacitor 103, and current I is (Rs / n), where "n" can be any positive number. For example, the bottom resistor 111 can be R = 100 ohms, the second resistor 112 can be (100-1)*R = 9.9k ohms, and the sensing resistor 113 can be Rs = 100 ohms. In this example, the circuit will have an apparent input resistance Rs / n, thus providing the circuit with high bandwidth and low load voltage, as if Rs113 were actually 1 ohm. Meanwhile, all current still flows through Rs113. Therefore, Figure 8 The example shown in the diagram achieves the following advantages: the signal is in, for example... Figure 2 In the conventional configuration shown, a 1-ohm resistor will produce a signal 100 times stronger. (Using...) Figure 8 In the configuration shown, the positive and negative sensors in the active clamp 102 can be coupled to the Vo2 signal, thus allowing the clamp 102 to also benefit from the larger signal.

[0035] Operational amplifiers come in various arrangements that can create active shunt circuits, such as Figure 8 The active shunt 110. U.S. Patent No. 9,274,145 provides additional details on some example configurations of active shunt circuits.

[0036] For convenience and with reference to the views provided in the accompanying drawings, the term "bottom" is used (as used above to refer to the bottom resistor 111). In practical applications, the circuit can have many orientations and configurations.

[0037] Figure 9 This is a circuit block diagram illustrating a current measurement method according to an embodiment, using a sensing element 101, multiple active clamps 102, and multiple active shunts 110, 114. Figure 9 As shown, the two active shunts 110 and 114 can be used together. In the configuration shown, the current return path may not be grounded, but it can instead go to a floating connection driven by a buffer. Depending on the desired bandwidth, load voltage, and output semaphore value, the "n" ratio of the two stages can be the same or different.

[0038] Therefore, embodiments of the disclosed technology can provide any or all of the following advantages. Instead of simply reducing the value of the sensing resistor, the embodiments can take an effective approach to improve the load voltage of the ammeter or current measuring device. The use of active clamping and bypass capacitors with finite gain stages at the terminals allows for wide-bandwidth clamping operation that does not exhibit inductive behavior at high frequencies. Furthermore, the use of multiple synchronized current ranges can meet the needs of, for example, IoT developers who want to observe the operation of their devices in two rapidly switching modes (wake-up / sleep, transmit / standby, etc.). Additionally, the simultaneous use of two different current shunts can provide a greater dynamic range without suffering from spurious range changes or missed measurements due to sudden transients.

[0039] Figure 10 This is a graph of the load voltage of a current measuring device using a sensing element according to an embodiment, with various operating areas marked on it according to device operation. The graph illustrates a current measuring device (such as...) with a single measurement range. Figure 3 and Figure 4 The load voltage of the device shown in the diagram is such that the active clamp 102 is inactive within a single measurement range. In multi-stage current measurement devices (such as...) Figures 5-9 In the device shown, there will be at least two regions in which at least one active clamp 102 of the multi-stage current measurement device is inactive. (Reference) Figure 10 The example clamped load voltage Vo was plotted, and the graph was divided into three distinct operating regions, A, B, and C. Operating region A comprises a region with a gradually decreasing slope on the left side of the curve, operating region B is located in the middle with a steep slope, and operating region C is located on the right side of the curve with a gradually decreasing slope. (Return to Reference) Figure 3The active clamp 102 is active in operating regions A and C, but not active in operating region B, which is a central portion with a steep slope. Figure 10 The shape of the clamped load voltage Vo in operating region A of the graph is caused by the action of the negative side clamp. This means that the negative sense element of the active clamp 102 has detected a negative voltage with a large enough output to turn on the active clamp 102, for example an NMOS transistor within the active clamp. Similarly, operating region C is caused by the action of the positive side clamp, i.e. the combination of the positive sense element and the NMOS transistor coupled thereto. In contrast, operating region B is the load voltage when the current measurement device is in its target region, which for this embodiment is the region with a load voltage of + / - 50mV. In other embodiments, the values of the sense elements 101 and the turn on voltage of the active clamp 102 can be chosen so as to select different operating regions. Thus, operating region B is the region when the active clamp 102 is completely turned off and does not pass any current, which means that all the current through the current measurement device is measured by the sense elements 101.

[0040] As mentioned above, although Figure 10 the graph shown in Fig. 1 includes a single region in which the active clamp is not active and has a steeper, more precise slope, the graph of the load voltage of a current measurement device such as the one shown in Fig. 2, which includes multiple regions, will include at least two regions in which at least one of the active clamps 102 in the multi-stage current measurement device is not active, and thus there will be at least two regions with a relatively steeper slope for the clamped load voltage. Figures 5-9

[0041] Thus, embodiments of the disclosed technology can provide current measurement capability over a greater range and with higher resolution than existing solutions, without sacrificing load voltage or bandwidth.

[0042] As one example, at least some implementations with multiple active shunts in series and load voltage clamping technology can provide the following characteristics:

[0043] High range bandwidth: approximately 10 MHz; low range bandwidth: approximately 1 MHz; noise floor: 0.4 µA RMS ; basic accuracy: 0.05% + 0.1 µA (calibrated); measurement range: 3 A - 100 nA; load voltage: 230 mV @ 2.2 A, which looks capacitive at high frequencies.

[0044] ​The embodiments can operate on specially created hardware, in firmware, digital signal processors, or general purpose computers programmed with software in accordance with the instructions provided herein. The term "controller" or "processor" as used herein is intended to include microprocessors, microcomputers, ASICs, and specially programmed hardware controllers. One or more aspects can be embodied in computer-usable data and computer-executable instructions, such as in one or more program modules, executed by one or more computers (including the 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 can be stored on a non-transitory computer readable medium such as a hard disk, optical disk, removable memory, solid state memory, RAM, etc. The functions of the various elements can be combined or distributed as desired in various embodiments. In addition, the functionality can be embodied in whole or in part in firmware or hardware equivalents such as integrated circuits, field programmable gate arrays (FPGA), and the like. Particular data structures can be used to more effectively implement one or more aspects of the disclosed systems and methods and are contemplated herein as being within the scope of computer- executable instructions and computer-usable data described herein.

[0045] Examples

[0046] Illustrative examples of the technologies disclosed herein are provided below. An embodiment of each of the various technologies can include any one or more, and any combination of, the examples described below.

[0047] Example 1 : A test and measurement instrument comprising a current measurement device, the current measurement device comprising: an input configured to accept a current for measurement; an output configured to deliver an output current output from the current measurement device; a sensing path for the current having a sensing element configured to sense an amount of the current received at the input; and an active bypass device configured to deliver an amount of the current from the input of the current measurement device to the output of the current measurement device without passing through the sensing path.

[0048] Example 2 is the test and measurement instrument of Example 1, wherein the active bypass device is a clamp circuit.

[0049] Example 3 is the test and measurement instrument of Example 2, wherein the active bypass device comprises a controllable shunt device having a control input coupled to an output of the sensing element.

[0050] Example 4 is the test and measurement instrument of Example 3, wherein the controllable shunt device comprises one or more MOS transistors.

[0051] Example 5 is the test and measurement instrument of any of the preceding Examples 1-4, wherein the active bypass device includes a voltage sensing device configured to sense a voltage at an output of the sense element and generate an output voltage therefrom.

[0052] Example 6 is the test and measurement instrument of Example 5, wherein the voltage sensing device includes a positive voltage sensing device and a negative voltage sensing device, and wherein the output voltage of the voltage sensing device is configured to generate only a positive voltage based on a magnitude of the voltage sensed at the output of the sense element.

[0053] Example 7 is the test and measurement instrument of any of the preceding Examples 1-6, further comprising a capacitor coupled in parallel with the active bypass device, and wherein a value of the capacitor is selected based on electrical characteristics of the sense element and active bypass device.

[0054] Example 8 is the test and measurement instrument of Example 7, wherein the sense element, the active bypass device, and the capacitor are configured as a first sense system tuned to a first range of input current values, and further comprising a second sense system tuned to a second range of input current values, the first range being different than the second range.

[0055] Example 9 is the test and measurement instrument of Example 8, wherein the second sense system includes a second current sense element, a second active bypass device, and a second capacitor.

[0056] Example 10 is the test and measurement instrument of Example 8, further comprising a multiplexer coupled to outputs of the first sense system and the second sense system, and wherein the multiplexer is selectable to pass an output from the first sense system or the second sense system as an output of the current measurement device.

[0057] Example 11 is the test and measurement instrument of any of Examples 1-10, wherein the sense element includes an active shunt.

[0058] Example 12 is a method for measuring current by an inline current measurement device, the method comprising: accepting an input current to be measured; measuring a value of the input current by a current sense device; and actively bypassing current through the current sense device when the input current is not within a first range of values.

[0059] Example 13 is the method of Example 12, wherein actively bypassing current through the current sense device includes controlling a shunt device.

[0060] Example 14 is the method of Example 13, wherein controlling the shunt device includes driving the shunt device with an input derived from an output of the current sense device.

[0061] Example 15 is the method of example 13, wherein the shunt device is closed when the input current is within a threshold amount of a target current in a first range of values.

[0062] Example 16 is the method of example 15, wherein the target current is a positive current or a negative current.

[0063] Example 17 is the method of any of the preceding examples 12-16, further comprising measuring, by a second current sense device, a value of the input current, and actively bypassing current through the second current sense device using a second shunt device when the input current is not in a second range of values different from the first range of values.

[0064] Example 18 is the method of example 17, wherein the second shunt device is closed when the input current is within a threshold amount of a target current in the second range of values.

[0065] Example 19 is the method of example 18, wherein the target current is a positive current or a negative current.

[0066] Example 20 is the method of any of the preceding examples 12-19, wherein measuring, by the current sense device, a value of the input current comprises measuring the value of the input current with an active shunt.

[0067] The previously described versions of the disclosed subject matter have numerous advantages, either as described or as will be apparent to the ordinarily skilled artisan. Even so, not all of these advantages or features need be realized in a given implementation of the disclosed devices, systems or methods.

[0068] In addition, the written description refers to particular features. It is to be understood that the disclosure includes all possible combinations of those particular features. For example, if a particular feature is disclosed in the context of a particular aspect or embodiment, that feature can also be used, to the extent possible, in the context of other aspects and embodiments.

[0069] Also, when reference is made to a method having two or more defined steps or operations, the defined steps or operations can be carried out in any order or simultaneously, unless the context excludes those possibilities.

[0070] In addition, the use herein of the term “including”, as well as other forms such as “include”, “comprise” and “comprises”, is intended to be optionally inclusive, such that other components, features, steps, processes, operations, etc. can also be present. For example, the term “including” or “which includes” components A, B and C can mean that the only components A, B and C are present, or it can mean that components A, B and C are present, as well as one or more other components.

[0071] While particular embodiments have been illustrated and described, it will be understood that various modifications can be made without departing from the spirit and scope of the disclosure. Accordingly, the application should not be limited except by the appended claims.

Claims

1. A test and measurement instrument comprising a current measurement device, the current measurement device comprising: an input configured to accept a current for measurement; an output configured to deliver an output current output from the current measurement device; a sensing path for the current having a sensing element configured to sense an amount of the current received at the input; and a controllable shunt device configured to deliver an amount of the current from the input of the current measurement device to the output of the current measurement device without passing through the sensing path, the controllable shunt device having a control input coupled to an output of the sensing element, wherein the controllable shunt device comprises one or more MOS transistors and a voltage sensing device configured to sense a voltage at the output of the sensing element via the control input and generate an output voltage therefrom, the voltage sensing device comprising a positive voltage sensing device and a negative voltage sensing device, and the output voltage of the voltage sensing device is configured to generate only a positive voltage to control the MOS transistors based on a magnitude of the voltage sensed at the output of the sensing element.

2. The test and measurement instrument of claim 1, further comprising a capacitor coupled in parallel with the controllable shunt device, and wherein a value of the capacitor is selected based on electrical characteristics of the sensing element and the controllable shunt device.

3. The test and measurement instrument of Claim 2, wherein, the sensing element, the controllable shunt device, and the capacitor are configured for a first sensing system adjusted to a first range of input current values, and further comprising a second sensing system adjusted to a second range of input current values, the first range being different than the second range.

4. The test and measurement instrument of Claim 3, wherein, the second sensing system comprises: a second current sensing element; a second controllable shunt device; and a second capacitor.

5. The test and measurement instrument of claim 3, further comprising a multiplexer coupled to outputs of the first sensing system and the second sensing system, and wherein the multiplexer selects to deliver as the output of the current measurement device an output from either the first sensing system or the second sensing system.

6. A method for measuring a current by an online current measurement device, comprising: accepting an input current to be measured; measuring a value of the input current by a current sensing device; and actively shunting the input current when the input current is not within a first range of values by controlling an amount of the input current to be bypassed using a control signal generated by the current sensing device to control the controllable shunt device to deliver an amount of the current from the input of the current sensing device to the output of the current sensing device without passing through a sensing path of the current sensing device, the controllable shunt device receiving the control signal from the current sensing device, the controllable shunt device using a voltage sensing device to sense a voltage of the control signal output from the current sensing device; sensing voltage magnitudes of a positive voltage and a negative voltage at the controllable shunt device, the voltage sensing device comprising a positive voltage sensing device and a negative voltage sensing device; and generating only positive voltages based on a magnitude of a voltage sensed at an output of the current sense device to control one or more MOS transistors in the controllable shunt device.

7. The method of claim 6, wherein, the controllable shunt device is off when the input current is within a threshold amount of a target current in a first range of values.

8. The method of claim 6, further comprising: measuring a value of the input current by a second current sense device, and actively bypassing current through the second current sense device using a second controllable shunt device when the input current is not in a second range of values different from the first range of values.

9. The method of claim 8, wherein, the second controllable shunt device is off when the input current is within a threshold amount of a target current in the second range of values.

10. The method of claim 9, wherein, the target current is a positive current or a negative current.

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