Abstract:
A source-measure unit (SMU) may be implemented with a control loop configured in the digital domain. The output voltage and output current may be measured with dedicated ADCs (analog-to-digital converters). The readings obtained by the ADCs may be compared to a setpoint, which may be set in an FPGA (field programmable gate array) or DSP (digital signal processing) chip. The FPGA or DSP chip may then be used to produce an output to drive a DAC (digital-to-analog converter) until the output voltage and/or output current reach the respective desired levels. The readback values may be obtained by averaging the voltage and/or current readings provided by the ADCs. The averaging may be weighted to improve noise rejection. The digital control loop provides added flexibility to the SMU and a decrease in the accuracy requirements on the DAC, while also for solving potential range-switching issues that may arise within the SMU.

Description:
PRIORITY CLAIM 
     This application claims benefit of priority of Provisional Application Ser. No. 60/986,380 titled “Source-Measure Unit Based on Digital Control Loop” and filed on Nov. 8, 2007, whose inventor is Chris Regier, which is hereby incorporated by reference in its entirety as though fully and completely set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to measurement and data acquisition systems and, more particularly, to the design of source-measure units. 
     2. Description of the Related Art 
     Scientists and engineers often use measurement systems to perform a variety of functions, including measurement of a physical phenomena a unit under test (UUT) or device under test (DUT), test and analysis of physical phenomena, process monitoring and control, control of mechanical or electrical machinery, data logging, laboratory research, and analytical chemistry, to name a few examples. 
     A typical measurement system comprises a computer system, which commonly features a measurement device, or measurement hardware. The measurement device may be a computer-based instrument, a data acquisition device or board, a programmable logic device (PLD), an actuator, or other type of device for acquiring or generating data. The measurement device may be a card or board plugged into one of the I/O slots of the computer system, or a card or board plugged into a chassis, or an external device. For example, in a common measurement system configuration, the measurement hardware is coupled to the computer system through a PCI bus, PXI (PCI extensions for Instrumentation) bus, a GPIB (General-Purpose Interface Bus), a VXI (VME extensions for Instrumentation) bus, a serial port, parallel port, or Ethernet port of the computer system. Optionally, the measurement system includes signal-conditioning devices, which receive field signals and condition the signals to be acquired. 
     A measurement system may typically include transducers, sensors, or other detecting means for providing “field” electrical signals representing a process, physical phenomena, equipment being monitored or measured, etc. The field signals are provided to the measurement hardware. In addition, a measurement system may also typically include actuators for generating output signals for stimulating a DUT. 
     Measurement systems, which may also be generally referred to as data acquisition systems, may include the process of converting a physical phenomenon (such as temperature or pressure) into an electrical signal and measuring the signal in order to extract information. PC-based measurement and data acquisition (DAQ) systems and plug-in boards are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor, among others. Typically, in a measurement or data acquisition process, analog signals are received by a digitizer, which may reside in a DAQ device or instrumentation device. The analog signals may be received from a sensor, converted to digital data (possibly after being conditioned) by an Analog-to-Digital Converter (ADC), and transmitted to a computer system for storage and/or analysis. Then, the computer system may generate digital signals that are provided to one or more digital to analog converters (DACs) in the DAQ device. The DACs may convert the digital signal to an output analog signal that is used, e.g., to stimulate a DUT. 
     Multifunction DAQ devices typically include digital I/O capabilities in addition to the analog capabilities described above. Digital I/O applications may include monitoring and control applications, video testing, chip verification, and pattern recognition, among others. DAQ devices may include one or more general-purpose, bidirectional digital I/O lines to transmit and received digital signals to implement one or more digital I/O applications. DAQ devices may also include a Source-Measure Unit (SMU), which may apply a voltage to a DUT and measure the resulting current, or may apply a current to the DUT and measure the resulting voltage. SMUs are typically configured to operate according to what is commonly referred to as “compliance limits”, to limit the output current when sourcing voltage, and limit the output voltage when sourcing current. In other words, a compliance limit on the measured signal may determine the (maximum) value of the sourced signal. For example, when applying a source voltage to a DUT and measuring current, a given current value (e.g. 1 A) specified as the compliance limit would determine the (maximum) input (source) voltage that might be provided to the DUT. In most cases compliance limits may depend and/or may be determined based on the DUTs, e.g. the maximum (absolute) value of the current that may flow into the DUT, or the maximum (absolute) value of the voltage that may be applied across the terminals of the DUT. 
     In the case of most SMUs, the setpoint (the desired output voltage when sourcing and regulating voltage, or the desired current value when sourcing and regulating current) and the compliance limits are typically programmable. SMUs are available to cover a variety of signal levels, from the microvolt (μV) range to the kilovolt (kV) range, and from the femtoampere (fA) range to the ampere (A) range. Some SMUs can deliver or dissipate significant power, while other SMUs may be operated at low power. The accuracy of SMUs is typically less than the accuracy of high-quality calibrators and/or digital multi meters (DMMs). 
       FIG. 1  shows a block diagram of a typical prior art SMU. SMUs are normally implemented with precision digital-to-analog converters (Voltage DAC  102  and Current DAC  104 ) to program the setpoint and compliance limits. The output voltage (across output terminals  120  and  122 ) or output current (flowing into output terminal  120 ) is typically set using analog control loops ( 108 ) by comparing the outputs to the levels set by DACs  102  and  104 , respectively. Each output voltage or output current may be controlled separately, with only one of the analog control loops closed at any given time. An output stage  112  may provide current to shunt resistor  118 , with current sense element  114  coupled across the terminals of resistor  118  to provide the current for measurement to the measurement multiplexer  110 , from which the signal can be provided to measurement ADC  106 . A voltage sense element  116  may be coupled across the output terminals  120  and  122  to provide the voltage for measurement to the measurement multiplexer  110 , from which the voltage signal can be provided to measurement ADC  106 . In some SMUs, separate ADCs (instead of single ADC  106 ) may be used to read the analog output voltage or the analog output current. The architecture exemplified in  FIG. 1  is however generally limited in flexibility and is high in complexity resulting from requirements to minimize glitches during range switching. In order for the SMU to operate accurately, a high level of accuracy is required for the DACs (e.g.  102  and  104 ) and ADCs (e.g.  106 ) configured in the SMU. 
       FIG. 2  shows a block diagram of one prior art example of a digital power supply. In some systems, power supplies may be configured to provide some SMU functionality. For example, while most power supplies are designed to provide a constant voltage to a load, in many cases the voltage level is programmable, and in some cases the current provided to the load can be read by the power supply. Consequently, interest in what are called “digital power supplies” has increased in recent years. Digital power supplies are generally switched-mode power supplies (SMPSs) in which the analog control loop has been replaced by one or more ADCs ( 204 ) configured to measure the output voltage and possibly other parameters, along with a microcontroller ( 202 ) that controls the power switching elements to set the output voltage. Microcontroller  202  may be configured to perform the digital control and PWM (pulse width modulation) signal generation to control output transistors  212  and  214  via respective gate drive circuits  206  and  208 , generating a load current in inductor  226 . Resulting current flowing through resistor  216  may be provided to multiplexer  210 , to be multiplexed into ADC  204  when measuring current. The input voltage may be sensed from a common node between resistors  218  and  220  coupled to input voltage V t . The output voltage may be sensed from a common node between resistors  222  and  224 , which may be collectively coupled across load capacitor  228 . While digital control provides these devices with some degree of flexibility, they lack the full programmability and 4-quadrant operation of a true SMU. Furthermore, their dynamic range and accuracy doesn&#39;t reach the level of true SMUs. 
     Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein. 
     SUMMARY OF THE INVENTION 
     In one set of embodiments, an SMU (source-measure unit) may be implemented with a control loop configured in the digital domain. The output voltage and output current may be measured with dedicated ADCs (analog-to-digital converters). When sourcing current, the current readings obtained by the ADCs may be compared to a Current Setpoint, and when sourcing voltage, the voltage readings may be compared to a Voltage Setpoint, to regulate the current and voltage outputs, respectively. The setpoints may be set in an FPGA (field programmable gate array) or DSP (digital signal processing) chip. The FPGA or DSP chip may be used accordingly to produce an output to drive a DAC (digital-to-analog converter) until the output voltage and/or output current reach the respective desired levels. The SMU may be configured to source one type of signal while measuring another type of signal. For example, the SMU may be configured to measure the voltage across the terminals of a device under test (DUT), when sourcing (and regulating) a current to the DUT, and similarly, the SMU may be configured to measure the current flowing into the DUT, when sourcing (and regulating) the voltage applied across the terminals of the DUT. 
     In one set of embodiment, the SMU may be configured to check the measured entity (current or voltage) against specified compliance limits, which may effectively limit the magnitude of the sourced signal. The SMU may be configured to vary (lower) the regulated current or voltage from its respective setpoint, when the measured voltage or current, respectively, exceeds the value specified by the compliance limit. For example, if the setpoint for sourcing and regulating current is 1 A, and the voltage compliance limit is 5V, if sourcing a 1 A current in the DUT results in a measured voltage that exceeds 5V, the SMU may lower the current to below 1 A until the measured voltage no longer exceeds the allowed 5V. Similarly, if the setpoint for sourcing and regulating voltage is 5V, and the current compliance limit is 1 A, if sourcing a 5V voltage across the terminals of the DUT results in a measured current that exceeds 1 A, the SMU may lower the voltage to below 5V until the measured current no longer exceeds the allowed 1 A. 
     The readback values (for the measured current and/or voltage) may be obtained by averaging the current and/or voltage values received from the ADCs. The averaging may be weighted to provide noise rejection advantages. Placing the control loop in the digital domain may result in added flexibility of the SMU, and a decrease in the accuracy requirements on the DAC. The digital control loop may also offer the possibility of novel approaches for solving potential range-switching issues that may arise within the SMUs. 
     In one set of embodiments, an SMU may comprise output terminals configured to couple the SMU to a DUT, and further configured to convey an analog output signal to the DUT to effect an output current flowing into the DUT and an output voltage in the DUT. The SMU may include a first converter configured to generate a first digital value representative of the output current, a second converter configured to generate a second digital value representative of the output voltage, and a digital control loop configured to receive the first digital value and the second digital value, and generate a digital control signal based on the first digital value and the second digital value to regulate a specified function of the output current and the output voltage to remain at a value corresponding to a setpoint. The specified function may be the output current, the output voltage, power, or resistance, to name a few. The first digital value, being representative of the output current, may correspond to a current measurement, and the second digital value being representative of the output voltage may correspond to a voltage measurement. The first digital value and second digital value may therefore effectively be used in measuring and/or controlling any function, which may be defined and/or processed in the digital control loop. For example, multiplying the first digital value and the second digital value may provide a measurement of power. Accordingly, the digital control value may be generated by the digital control loop to effect desired changes in the output current and/or the output voltage depending on what the selected function is. For example, if the selected function is power, then both the output current and output voltage may be regulated, or only one of the output current and output voltage may be regulated based on the measured output current and output voltage. 
     Overall, embodiments of an SMU comprising a digital control loop may offer a number of advantages. For example, DAC errors may be corrected by the digital loop, reducing the accuracy requirements on the DAC. The control algorithm may be as simple or as complex as desired, and may be configured ranging from a simple integrator to a nonlinear adaptive system, thereby offering the potential to enhance stability and speed. It may also be possible to generate functions beyond the standard current-limited voltage source and voltage-limited current source. For example, it may be possible to generate constant power or constant resistance functions. In addition, voltage range-switching may be performed transparently, and current range-switching, potentially requiring shunt switching, may be performed more accurately. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing, as well as other objects, features, and advantages of this invention may be more completely understood by reference to the following detailed description when read together with the accompanying drawings in which: 
         FIG. 1  is shows a block diagram of a typical prior art SMU (source-measure unit); 
         FIG. 2  shows a block diagram of one prior art example of a digital power supply; 
         FIG. 3  shows the basic architecture of one embodiment of a novel SMU; 
         FIG. 4  shows one embodiment of a novel SMU in which anti-alias filtering has been added to the ADCs (analog-to-digital converters); 
         FIG. 5  shows one embodiment of a novel SMU in which shunt switching has been added in the feedback loop between the output and the current ADC; 
         FIG. 6  shows one embodiment of a novel SMU configured to minimize glitches when switching shunt resistors; 
         FIG. 7  shows one embodiment of a novel SMU configured with gain-ranging on the voltage ADC; 
         FIG. 8  shows one embodiment of a novel SMU configured with multiplexers at the ADC inputs; 
         FIG. 9  shows one embodiment of a novel SMU configured with gain-ranging on the DAC (digital-to-analog converter); 
         FIG. 10  shows one embodiment of a novel SMU configured with multiple DACs; 
         FIG. 11  shows one possible transfer function with hysteresis for the compound DAC; 
         FIG. 12  shows one embodiment of a novel SMU configured to operate with a 4-wire connection to the DUT (device under test); and 
         FIG. 13  shows diagrams illustrating Voltage Mode and Current Mode compliance limits and respective setpoints, according to one embodiment. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Furthermore, note that the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not a mandatory sense (i.e., must).” The term “include”, and derivations thereof, mean “including, but not limited to”. The term “coupled” means “directly or indirectly connected”. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 3  shows the basic architecture of one embodiment of a novel SMU (source-measure unit) in which the entire control loop has been configured in the digital domain. A DUT (device under test), not shown, may be coupled between output terminals  320  and  322 . Setpoints and compliance limits may be provided (programmed) to Digital Control Loop (DCL)  302 , which may provide a control output through DAC (digital-to-analog converter)  304  to Output Stage  310 . Feedback from Output Stage  310  may be provided to Current ADC (analog-to-digital converter)  306  and Voltage ADC  308  via respective Current Sense element  312  and Voltage Sense element  314 . The current feedback may be taken from the current flowing through current shunt resistor  316 , and the feedback voltage may be taken from across output terminals  320  and  322 . Current ADC  306  and Voltage ADC  308  may then provide the readback current and voltage values into DCL  302 . 
     DCL  302  may be configured to check the measured current (from Current ADC  306 ) resulting from a sourced voltage, against the specified current compliance limit provided (or programmed) into DCL  302 . DCL  302  may similarly be configured to check the measured voltage (from Voltage ADC  308 ) resulting from a sourced current, against the specified voltage compliance limit provided (or programmed) into DCL  302 . To regulate the output, DCL  302  may be configured to check the measured current (from Current ADC  306 ) resulting from a sourced current, against the specified current setpoint provided (or programmed) into DCL  302 . DCL  302  may similarly be configured to check the measured voltage (from Voltage ADC  308 ) resulting from a sourced voltage, against the specified voltage setpoint provided (or programmed) into DCL  302 . 
     The compliance limits may effectively limit the magnitude of the sourced signals. DCL  302  may be configured to vary (lower) the regulated current or voltage from its respective setpoint, when the measured voltage or current, respectively, exceeds the value specified by the compliance limit. For example, if the setpoint for sourcing and regulating current is 1 A, and the voltage compliance limit corresponding to a given DUT is 5V, when sourcing a 1 A current in the DUT results in a voltage measurement that exceeds 5V, DCL  302  may operate to lower the value of the sourced current below 1 A, until a sourced current value is reached for which the measured voltage no longer exceeds the allowed 5V. Similarly, if the setpoint for sourcing and regulating voltage is 5V, and the current compliance limit corresponding to a given DUT is 1 A, when sourcing a 5V voltage across the terminals of the DUT results in a current measurement that exceeds 1 A, DCL  302  may operate to lower the value of the sourced voltage below 5V, until a sourced voltage value is reached for which the measured current no longer exceeds the allowed 1 A. 
       FIG. 4  shows a second embodiment of a novel SMU in which anti-alias filtering has been added to the ADCs. As shown in  FIG. 4 , anti-alias filter  412  may be coupled between Current Sense element  312  and Current ADC  306 , and anti-alias filter  414  may be coupled between Voltage Sense element  314  and Voltage ADC  308 . Anti-alias filters  412  and  414  may operate to ensure that DCL  320  does not respond to out-of-band disturbances. 
       FIG. 5  shows a third embodiment of a novel SMU in which shunt switching has been added in the feedback loop between the output (output terminal  320 ) and Current ADC  306 . As shown in  FIG. 5 , different current shunt resistors  516  may be switched into the feedback loop between the output of Output Stage  310  and the inputs of Current Sense element  312 , using a multiplexer  524  and a set of switches  518 . While  FIG. 5  shows three switches ( 518 ) and three current shunt resistors ( 516 ), alternate embodiments may be configured with a greater or lesser number of switches and/or resistors, as desired. Shunt switching may provide the SMU with the capability to cover a wider dynamic range of current. Any glitches that may result from switching between the various current shunt resistors may be minimized by adjusting the settings of DAC  304  simultaneously with the shunt-switching operation. Since the current is being measured and the values of the current shunt resistors ( 516 ) are known, it is possible to calculate the value to which DAC  304  may be set to minimize potential glitches. Any errors in the calculations may eventually be corrected by DCL  302 . 
       FIG. 6  shows a fourth embodiment of a novel SMU configured to further minimize glitches that may occur when switching between current shunt resistors  516  that have been added in the feedback loop between the output and Current ADC  306 . In this case, if switches  518  happen to be slow switches, they may be operated gradually to transition between the shunt resistors ( 516 ). Current shunt resistors  516  may also be switched into the feedback loop between the output of Output Stage  310  and the inputs of a second Current Sense element  612 , using additional multiplexer  624 , with the output of additional Current ADC  606  be coupled back to DCL  302 . Additional Current ADC  606  and additional Current Sense element  612  may be provided to make it possible to simultaneously measure the current conducted by two different current shunt resistors. Thus a reading of the total current may be obtained during the shunt-switching operation, while DCL  302  remains stable and predictable. One possible way to reduce the number of switches and multiplexers in this arrangement may be to impose a specified or predetermined switching sequence on the switches. For example, Second Current ADC  606  may be configured to always measure the current through shunt # 1 . If, in conjunction, the switching sequence for switching from shunt # 2  to shunt # 3  follows the sequence shunt # 2 /shunt # 1 /shunt # 3 , the current may be monitored continuously while a multiplexer would only be required in front of first Current Sense element  312 . 
       FIG. 7  shows one embodiment of a novel SMU configured with gain-ranging on voltage ADC  308 . As shown in  FIG. 7 , a programmable-gain amplifier (PGA)  716  may be coupled between Voltage Sense element  314  and the input of Voltage ADC  308  to help maximize dynamic range of the SMU. In one embodiment, PGA  716  may be configured with two or more resistors ( 724 - 728 ) and a multiplexer  718  to switch between the resistors to change the gain of PGA  716 . The inclusion of PGA  716  and its corresponding circuitry may not result in additional potential glitches, since the control system (in the form of DCL  302 ) offers the capability of immediately compensating for changes in the gain of PGA  716 . 
       FIG. 8  shows one embodiment of a novel SMU configured with multiplexers  816  and  818  coupled to the inputs of Current ADC  306  and Voltage ADC  308 , respectively, to allow for self-calibration. As shown in the embodiment of  FIG. 8 , first multiplexer  816  may be configured to selectively provide to Current Sense element  312  the calibration signals CAL+/CAL−, or the voltage values taken from the terminals of resistor  316 , representing the current flowing in resistor  316 . Similarly, second multiplexer  818  may be configured to selectively provide to Voltage Sense element  314  the calibration signals CAL+/CAL−, or the voltage values taken from output terminals  320  and  322 , representing the feedback/output voltage. A Calibration Signal Generator  826 , which may be comprised in the SMU or may be configured separate from the SMU, may be used to generate calibration signals CAL+ and CAL−, which may are provided to multiplexers  816  and  818  to select in lieu of the measured entities during calibration. 
       FIG. 9  shows one embodiment of a novel SMU that is similar in concept to the embodiment shown in  FIG. 7 . In the embodiment shown in  FIG. 9 , the SMU is configured with gain-ranging on DAC  304  to increase the dynamic range of the SMU (as opposed to providing gain ranging on Voltage ADC  308 , as shown in  FIG. 7 ). As shown in  FIG. 9 , a programmable-gain amplifier (PGA)  910  may be coupled between the output of DAC  304  and the input of Output Stage  310  to help maximize dynamic range of the SMU. As in the embodiment shown in  FIG. 7 , the PGA may again include two or more resistors ( 924 - 928 ) and a multiplexer  918  to switch between the resistors to change the gain of the amplifier (PGA  910 ). Any glitches that may occur when switching between the resistors may be minimized by adjusting the setting for DAC  304  simultaneously with the gain-switching operation. 
       FIG. 10  shows one embodiment of a novel SMU, in which increased dynamic range may be achieved by configuring the SMU with multiple DACs ( 304   a  and  304   b ) in lieu of the single DAC  304  shown in other embodiments. As previously explained, various embodiments do not impose strict accuracy requirements on DAC  304  due to the control system (in the form of DCL  302 ) having the capability of compensating for any inaccuracies that may result from operation of DAC  304 . Therefore, multiple DACs  304   a  and  304   b  may be configured to provide a coarse/fine arrangement, respectively. As shown in  FIG. 10 , the output of a first DAC  304   a  and the output of a second DAC  304   b  may be coupled to the input of Output Stage  310 , with DAC  304   b  configured to have a “finer” (i.e. more subtle) effect on the output of Output stage  310  than DAC  304   a . This may be accomplished by performing weighted summing via respective resistors R 1   1002  and R 2   1004 , where R 2  may be a multiple (N) of R 2 , to provide the differing resolutions. For example, resistor  1004  may be configured to have a value of 100 times that of resistor  1002 . In one set of embodiments, DACs  304   a  and  304   b  may be may be identical or of similar type, and may be configured with a bit-overlap to ensure there are no missing codes. In addition, the control logic in DCL  302  may be configured to provide hysteresis functionality to avoid unnecessary switching of coarse DAC  304   a.    
       FIG. 11  shows a possible transfer function  1102  with hysteresis for a compound DAC (e.g. comprising first DAC  304   a  configured to provide a coarse resolution and second DAC  304   b  configured to provide a fine resolution) of the embodiment of  FIG. 10 . The dashed lines of the transfer function curve represent the hysteresis. The input codes for DAC  304   a  are indicated by the step function  1104 , while the input codes for DAC  304   b  are indicated by the linear functions  1106 . As shown, a coarse setting may provide a base output, which may then be fine tuned by applying a higher resolution code to fine-tune the output residing between respective outputs corresponding to consecutive coarse input codes. 
     In addition to the 2-wire DUT connection method shown in the previous figures, various embodiments of the novel SMU may be configured to operate with 4-wire connections in addition to 2-wire connections, as shown in  FIG. 12 . In these embodiments, Current Sense element  312  may be coupled to terminal  320 , and signal ground may be coupled to terminal  322 , as shown, where terminals  320  and  322  may be coupled to a pair of nodes within the DUT via connections (e.g. leads and/or wires) that carry the DUT current. In contrast, one input of Voltage Sense element  314  may be coupled to terminal  1220 , while the other input of Voltage Sense element  314  may be coupled to terminal  1222 , where terminals  1220  and  1222  may be coupled to the pair of nodes within the DUT through connections (e.g. wires and/or leads) that carry negligible or no current. In this case, negligible (or no) current may refer to any current level that will not affect the desired accuracy of the voltage measurement obtained through Voltage Sense element  314  and ADC  308 , allowing terminals  1220  and  1222  to be configured for sensing voltage at the DUT without concern for lead/wire resistance. In other words, a 4-wire configuration, such as the one shown in  FIG. 12 , may allow sensing the voltage at the DUT through wires that carry negligible current, eliminating any impact that lead resistance may have on measurements performed using only a 2-wire connection. Thus, terminals  1220  and  1222  may be configured along with voltage sense element  314  to sense the voltage remotely at the DUT rather than locally at the output terminals (which may be terminals  320  and  322  in the embodiment shown in  FIG. 12 ), in order to obtain a more accurate voltage measurement of the DUT, or to more accurately regulate the voltage in the DUT. 
     The Current ADCs (e.g.  306 , and/or  606 ) and Voltage ADCs (e.g.  308 ) may be implemented as noise-shaping ADCs. In embodiments featuring noise-shaping ADCs, the noise shaping of the ADCs may operate to provide noise shaping to the DAC (e.g. DAC  304 ), which may be beneficial for reducing low-frequency noise. For example, the ADCs may be implemented using continuous-time sigma-delta modulators, which may obviate the need for anti-alias filters (such as the filters shown in  FIG. 4 ), resulting in reduced complexity and faster loop response (better stability). In case noise-shaping is not employed, it may still be preferable to provide sufficient dither for the ADCs and the DAC to de-correlate quantization noise from the signal. 
     In one set of embodiments, DCL  302  may be implemented with an ASIC, a DSP, an FPGA, or any other suitable digital circuitry configured to perform the designated functions of DCL  302 . An FPGA may be preferable for implementations utilizing National Instruments&#39; LabVIEW graphical programming interface to write and simulate the control code. When using an FPGA with LabVIEW, the control code may be deployed and tested through LV-FPGA (LabVIEW FPGA). The control system itself may be designed to emulate a traditional SMU, while allowing users to implement more advanced features by writing their own control algorithm (for example in LV-FPGA when using LabVIEW and an FPGA). Some embodiments may also implement more advanced features such as constant power delivery or sinking, or constant resistance generation. For safety considerations, especially in user-configured situations, the output of DAC  304  may be limited to a safe level, regardless of feedback. 
     In another set of embodiments, the control algorithm (which may be implemented in DCL  302 ) may be a PID (proportional integral derivative) controller or a variant thereof. In yet other embodiments the control algorithm may be based on fuzzy logic, or it may be nonlinear. The control algorithm may additionally be devised as an adaptive algorithm. In certain embodiments it may be configured to include programmable speed/stability tradeoff. For example, as a simple substitute for an adaptive control algorithm, the speed/stability tradeoff may be exposed to the users, who may be able to choose stable/normal/fast with any degree of resolution to match their expected test setup. The control system may also be designed to compensate for the load presented by an attenuator that may be required to measure high voltages. 
     The Digital Control Loop (e.g. DCL  302 ) may provide the added flexibility of being able to operate the SMU for different compliance limits, (and being able to program multiple setpoints), without additional components, which could not be achieved in prior art systems configured with analog control loops. In addition, the loop bandwidth may easily be adjusted by changing the controller coefficients, and control loop adjustments may be made through measurable and controllable settling times of the signal. The DCL may also be reconfigurable to the desired mode, controlling/generating current, voltage, power, resistance, or voltage with series impedance, which may be valuable in battery simulation applications. 
     Prior art systems configured with analog control loops required a different control loop for each control mode. A scanlist comprising a sequence of setpoints may be used to operate the SMU without requiring additional components, while also retaining the ability to operate within given compliance limits.  FIG. 13  shows example diagrams illustrating the Voltage setpoint  1306  when operating the SMU in Voltage Mode (which may refer to sourcing and regulating voltage to measure current), and the Current setpoint when operating in Current Mode (which may refer to sourcing and regulating current to measure voltage). Example values are shown for the Voltage mode operation, with the low current compliance limit being −1 A, the high current compliance limit being 1 A, with a Voltage setpoint of 5V. 
     It should also be noted that when trying to regulate power, for example, two ADCs may be required for obtaining the requisite measurements for the power regulation to be performed. One ADC may be required to measure current (e.g. ADC  306 ) and another ADC may be required to measure voltage (e.g. ADC  308 ), with the multiplication (to obtain the power value that may be compared to a setpoint) performed digitally, in DCL  302 , for example. Thus, certain embodiments, where the sourced and measured entity is the same (e.g. sourcing current and measuring current), may be configured with a single ADC. In one set of embodiments, when only always one entity (e.g. Current or Voltage) is to be measured, one of the ADCs may simply be removed. In another set of embodiments, the output from the Current Sense element and the output of the Voltage Sense element may be input to a multiplexer (not shown in any of the figures), which may be used to select whether sensed voltage or sensed current is to be converted to the digital value provided to the Digital Control Loop. These embodiments may be useful, for example, in certain applications where compliance limits were unnecessary, and/or the primary functionality was regulation of the output. 
     Although the embodiments above have been described in considerable detail, other versions are possible. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. Note the section headings used herein are for organizational purposes only and are not meant to limit the description provided herein or the claims attached hereto.