Hybrid digital and analog signal generation systems and methods

An analog signal generating source comprising two or more digital-to-analog converters (DAC) combined to generate one or more frequency components. The analog signal source comprises a first path for generating substantially low frequency signals, the first path comprising a first one of the DACs; and a second path for generating substantially high frequency signals, the second path comprising a second one of the DACs. The analog signal source also comprises a data processor for processing an input signal and providing the processed input signal to the first and second paths; a combining circuit configured to combine outputs of the first and second paths into the source signal; a feedback portion configured to sense the source signal; and a servo loop configured to use the sensed source signal to adjust as need to maintain the source signal to substantially agree with the input signal.

FIELD

This disclosure relates to voltage and current analog source generation, including by hybrid Alternating Current (AC) and Direct Current (DC) systems and methods. More generally, it relates to electronics, analytical instrumentation, software, and infrastructure for signal sourcing and signal measuring. The disclosure also relates to systems that measure signals for materials and device characterization and other applications under challenging experimental conditions that can cause high levels of noise and interference. In these contexts, it relates to digital-to-analog conversion and vice versa.

BACKGROUND

Materials and device property measurements (e.g., electron transport properties such as Hall, mobility and carrier concentration, etc.) are often highly sensitive to noise, interference, and stray signals. For example, superconductive properties are typically measured at extremely low temperatures (e.g., lower than 4 K) necessary for observing those properties without excessive noise. These measurements may also require very high field strength (e.g., in excess of 5 T), which can complicate experimental setups. Handling noise, interference, and stray signals under these compromising conditions is critical for obtaining reliable, accurate data.

Any noise or irregularities introduced via an input or source signal can manifest in the sample and throughout the entire measurement system. Because of this, input or source signal problems are pervasive. They degrade the measurement itself. They can also adversely affect, and be made worse by, any electronics and equipment they traverse. The best way to handle this is to generate source signals that introduce as little noise or ambiguity as possible.

Techniques such as dithering, subranging, pulse width modulation, least significant bit/most significant bit (LSB/MSB) dual digital-to-analog converter (DAC) architectures, etc. all can improve resolution and reliability. Combining stable components with low drift references, temperature-controlled elements, chopper amplifier methods, Sigma-Delta blocks, successive approximation register analog-to-digital converter (ADC) techniques, etc. can improve accuracy.

While these techniques are useful, they can be inadequate for extremely sensitive material measurements performed under compromising conditions. In particular, they have limited flexibility in addressing noise, glitches, and other ambiguities introduced into a measurement system via a source signal. Material measurement systems are often dynamic. They regularly require re-configuration and re-configuring to accommodate a variety of different kinds of measurements. Their source signal generators need to have built-in flexibility. Current systems often apply source processing (e.g., signal gain) uniformly to different and incompatible aspects of the source. Many fail to treat the DC and AC source components differently and independently. This creates problems and limits flexibly since DC and AC components often scale and range very differently.

Therefore, there is a critical need for new and improved solutions for providing robust, high quality, low noise source or output signals that can apply signal processing and gain in a flexible manner. There is a need for systems that will do this for source signals built from DC and AC components that differ substantially in range.

SUMMARY

An analog signal generating source comprising two or more digital-to-analog converters (DAC) combined to generate one or more frequency components. The analog signal source comprises a first path for generating substantially low frequency signals, the first path comprising a first one of the DACs. The analog signal source comprises a second path for generating substantially high frequency signals, the second path comprising a second one of the DACs. The analog signal source also comprises a data processor for processing an input signal and providing the processed input signal to the first and second paths. The analog signal source comprises a combining circuit configured to combine outputs of the first and second paths into the source signal. The analog signal source comprises a feedback portion configured to sense the source signal. The analog signal source comprises a servo loop configured to use the sensed source signal to adjust as need to maintain the source signal to substantially agree with the input signal.

In some embodiments, the data processor providing the processed input signal to the first and second paths comprises at least one of: feeding a DC input to the first path; feeding a low frequency input to at least one of the first path and the second path; and feeding a high frequency input to at least one of the first path and the second path. The servo loop maintaining the source signal to substantially agree with the input signal may comprise at least one of: removing DC errors; removing low frequency errors; and maintaining an integrity of the high frequency signals. A bandwidth of the feedback portion may be substantially higher than a bandwidth of the first path. The feedback portion sensing the source signal may comprise comparing the input signal to the output signal. The data processor processing an input signal may comprise at least one of: removing high frequency signals that are higher than a bandwidth of the first path but within a bandwidth of the feedback portion; and removing high frequency signals that are higher than a bandwidth of the feedback portion and higher than a bandwidth of the first path.

The first path may be configured for direct current (DC). The feedback portion may comprise at least one analog-to-digital converter (ADC). The data processor may comprise ADC feedback and the data processor processing an input signal may comprise: comparing the input signal to the source signal; and removing from the input signal high frequency signals that are higher than a bandwidth of the first path but within a bandwidth of the feedback portion. The second path may be configured to accommodate at least one of: more than one frequency and complex waveforms characterized by a plurality of frequency components. The first path may comprise an integrater. The first path may comprise a first gain configured to range the first path prior to combining with the second path. The second path may comprise a second gain configured to range the second path prior to combining with the first path. The first and second gains may be configured to operate independently of one another. The feedback portion may comprise signals from outside the signal source. The first path may comprise at least one of a dithering function, a sub-ranging function, and a pulse width modulation in series with the output of the second path. The second path may be configured to at least one of: change a DC offset of the low frequency signals faster than a low frequency response time of the servo loop; and change a DC offset of low frequency transient information faster than a low frequency response time of the servo loop.

Every device in the signal source may share the same clock. At least one of the first and second paths and the feedback portion may comprise at least one of a phase shift and a group delay. The signal source may comprise a third path for generating substantially high frequency signals, wherein the combining circuit may be configured to combine outputs of the first, second, and third paths into the source signal. The outputs of the first path, second path, and feedback portion may be accessible to a user via the data processor. The source signal may be fed to at least one of a voltage source, a current source, a power supply, a source measure unit, a temperature controller, a measurement system, and a radio frequency (RF) source. The signal source may be configured to provide an additional source signal independent of the source signal provided by the combining circuit. The source signal may control a source measure unit. The first path may be configured to add a small AC signal to at least one of a large DC signal and a low frequency ramping signal to produce the first path output. The second path may be configured to accept high frequency signals; and the first path may be configured to accept frequency information relating to the high frequency signals. The first path may comprise filtering. The second path may comprise filtering. The signal source may comprise a third path that comprises filtering. The filtering may be implemented in at least one of the first, second, and third paths after the two or more DACs. The first path may be configured to generate harmonic signals. The second path may be configured to generate harmonic signals. The signal source may comprise a third path configured to generate harmonic signals.

The signal source may be used in conjunction with at least one of a voltage source, a current source, a power supply, a source measure unit, a temperature controller, a materials parameter measurement system, and a radio frequency (RF) source. The combining circuit may be configured to combine outputs of the first and second paths into more than one source signal. The signal source may be configured to operate as a controller for a source measure unit. The combining circuit may be configured to combine outputs of the first and second paths into at least one harmonic signal.

Aspects of the present disclosure also include a method of sourcing a signal, the method comprising providing an input signal to a data processor and processing the input signal via the data processor. The method also comprises sending the processed input signal via the data processor to a first and second path, the first and second paths each comprising two or more DACs to generate two or more frequency components. The method also comprises generating, via the first path, a first path output of substantially low frequency signals; generating, via the second path, a second path output of substantially high frequency signals; combining the first and second path outputs into a source signal via a combining circuit; and sensing the source signal via a feedback portion. The method further comprises providing, via the feedback portion, the sensed source signal to a servo loop; and using the sensed source signal to maintain, via the servo loop, the source signal to substantially agree with the input signal.

DETAILED DESCRIPTION

Hybrid Sourcing

Overview

This disclosure addresses the problem of improving source signal quality with a methodology called “hybrid sourcing.” Hybrid sourcing creates high quality analog source signals from both AC and DC components. It tailors gain paths for AC and DC differently to leverage the different advantages of AC and DC sourcing electronics. Its source signals have extremely low levels of noise and glitching. They have greater flexibility and range than conventional, single range sourcing. Variations of the present disclosure include hybrid sourcing using DACs, generally referred to herein as “hybrid DAC systems.” These solutions are particularly useful for applications requiring low noise signals with a high degree of reproducibility and reliability. U.S. Provisional Patent Application No. 63/057,745, discusses material measurement applications of hybrid sourcing in more detail.

Hybrid DAC systems may include DACs, ADCs, filtering elements, combining means (e.g., summing elements and integrators), feedback elements, etc., among other components. A hybrid DAC may include a software-executing processor or processing means. It may also include analog hardware and/or digital hardware. Processing algorithms can precisely control operation to deliver extremely accurate output signals.

Hybrid DAC systems offer flexibility in configuring source signals for different requirements. They may control a number of properties through fine tuning components, gain, and other variables and/or selecting specific components. Configurable properties include: resolution, quantization, update rates, offset errors, gain errors, differential non-linearity errors, integral non-linearity errors, calibration errors, output noise, dynamic range, output bandwidth, source impedance, output drive capabilities, switching noise, phase errors, drifts vs. time and temperature, etc. These and other concepts are explored below.

Exemplary converter systems that may be used within the context of the present disclosure include any suitable DAC or ADC. Specific examples include DACs using an ADC (e.g., the Linear Technology 24-bit LTC2400) as a feedback element in a digitally corrected loop to realize 20-bit performance. In this variation, a DC DAC can be a slave to ADC feedback. A comparator can determine a difference between the intended and actual output of the system. A corrected code can be generated and presented to, for example, the DAC. This may correct the DAC's drifts and nonlinearity to a desired accuracy. The ADC may, in some variations, set accuracy. The may digitally sense feedback by placing the ADC at the load. These and many other ways to use the DAC (e.g., LSB/MSB sub-ranging, pulse width modulation (PWM), and high-speed integrating) are within the scope of the present disclosure.

Comparison with Non Hybrid Sourcing

For comparison,FIG.1Aillustrates a non-hybrid source signal chain100that applies the same gain chain to both AC and DC components. Herein, when describing signal sourcing, the words “chain” and “path” will be used interchangeably. Use of chain100is typically limited to situations where AC102and DC104inputs are generally in the same or similar range. Here, small AC signals relative to the DC signal have only a few bits of resolution. In this narrow set of circumstances, both AC and DC signals can be amplified together. However, such a single gain chain does not have flexibility. The only parameter in chain100with any variability is the gain of variable amplifier120, which is applied to both AC and DC components simultaneously. Moreover, since all of the components carry both signal types, this topology prevents independent configuration of AC and DC portions of the source signal.

In chain100, element106adds the AC102and DC104signals together before applying any gain. DAC108converts the AC/DC combined signal to analog. It then provides the combined signal to variable amplifier120. The gain of amplifier120may be selected by the user. It may also be ranged automatically, and/or according to feedback from the “Analog output” signal at sample110(e.g., via a feedback loop (not shown)). From variable amplifier120, the combined signal is sent to the sample110via second amplifier112.

Herein the term “sample” will be used interchangeably with the phrase “device under test” (DUT).” It is to be understood that either a DUT or a “sample” may be a device or a sample of material. Often, in the context of the material measurements disclosed herein, devices (e.g., transistors) are created for the express purpose of testing a material in the created device (e.g., semiconducting materials).

FIGS.1B-1Gshow the effect of a small AC signal superimposed on a larger DC signal. They show that, although the AC signal is smaller than the noise floor in these cases, it can be detectable (e.g., with a lock-in amplifier). The user can set an amplitude of the AC signal with higher resolution (e.g., sixteen bits, as opposed to two or three).

FIGS.1B and1Cshow the highest AC amplitude case of the three cases, a 100 mV AC sine wave140superimposed on a 5V DC offset130.FIG.1Bshows several cycles of the wave140as measured by a digital multimeter.FIG.1Cis a blowup of a few of those cycles.

One easily sees fromFIGS.1B and1Cthat the wave140is barely visible when superimposed on the appropriate scale for viewing the DC signal130. Yet a large gain suitable for configuring the small AC signal140will also magnify errors and noise in the larger DC signal130. At the same time, a smaller gain more appropriate for the large DC signal130will not adequately amplify the AC signal140. Therefore, configuring a single gain chain (e.g.,100) to simultaneously accommodate both DC130and AC140signals will be difficult and prone to error.

FIGS.1D and1Efurther illustrate the point for an AC signal150with an even smaller amplitude, 1 mV. AC signal150is superimposed on the same 5V DC offset130. As best shown by insetFIG.1E, the amplitude of wave150is imperceptible when viewed on the appropriate scale for the DC offset130.FIGS.1F and1Gmake the same point with a still smaller 100 μV signal, a source amplitude typical in materials characterization experiments. As shown in insetFIG.1G, the 100 μV amplitude of wave160is buried within and dominated by the noise of the DC offset130and of the instrumentation (oscilloscope). In this situation, common for material applications, ranging a single gain chain100to configure both signals is impractical or impossible in practice due to the resolution of practical converters.

Hybrid Source Variations

The variations that follow contrast the features and operation of exemplary hybrid signal generation systems/sources with those of non-hybrid chain100. Hybrid systems separate low-frequency and high-frequency generation paths, allowing separate and independent configuration of each over a range of frequencies. Separate configuration simultaneously generates high-quality AC signals and highly accurate high-resolution DC signals. Combining the separately configured signals provides an analog output signal with a superior range of frequencies and reliability to those achieved using a single path (e.g., chain100).

FIG.2illustrates a relatively simple hybrid system in the form of an exemplary source signal chain200. Chain200dedicates separate DACs206and208and amplifiers220aand220bfor separately configuring AC and DC paths. The separate configuration paths are labeled inFIG.2as “AC/High Frequency Path” and “DC/Low Frequency Path,” respectively. Element210sums the outputs of both paths and sends them to amplifier212. After amplification, the Analog Output signal is routed to sample110.

More specifically, chain200feeds AC input202to the “AC/High Frequency Path.” That path first converts the signal using DACs206, then amplifies it using variable gain220a. Separately, DC input204is fed to the “DC/Low Frequency Path.” That path converts the signal using DAC208, then amplifies it using variable gain220b. Both paths are summed at210, then provided to sample110via amplifier212. Variable gains220aand220bmay be set by ranges and other settings, or by user preference, protocol, or may be pre-set. Separate ranging is particularly important for hybrid generation techniques. The high frequency AC generation containing separate ranging allows a hybrid DAC to create small AC signals riding on large DC outputs (see, e.g.,FIGS.1B-1G).

FIG.3illustrates another exemplary source signal chain300where AC and DC configurations are separate and parallel. Chain300prevents the AC path (“AC/High Frequency Path”) from affecting the accuracy of the DC path (“DC/Low Frequency Path”) by including the AC input302in the configuration of the DC contribution to the Analog Output signal. Chain300can accomplish reasonable accuracy while using less circuitry (i.e., fewer components than more complicated variations) than many hybrid systems.

More specifically, as shown inFIG.3, chain300sends a summation of the AC302and DC304inputs at306to the DAC308in the DC/Low Frequency Path. It concurrently routes the AC302input to the DAC310in the AC/High Frequency Path. Chain300then amplifies both the AC signal and a composite DC/AC signal by a variable amplifier (220aand220b, respectively). Gains220aand220bfor the DC and AC circuits are different and can be configured for each. They may be set in the same manner described above for gains220aand220bin the context ofFIG.2.

Next, the AC Configuration signal is high pass filtered312to remove low frequency components. The DC Configuration signal is low pass filtered314to remove high frequency components. The filtered AC and DC signals are then summed at316. The summed signal is amplified at318and sent to the sample110.

Chain300allows parallel configuration of the AC DAC310and the DC DAC308, their associated path components, along with facilitating the separate ranging capability in each path (e.g., to address ranging issues shown inFIGS.1B-1G). It removes DC errors from the AC generation path and vice versa, via filters312and314. Specifically, high pass filter/AC312removes DC/low frequencies from the AC/High Frequency Path. In other words, DC errors that may be generated by the AC DAC310and amplifier220aare eliminated by filter312so they do not affect the DC output performance in the Analog Output signal. Low pas filter/DC314similarly removes AC errors from the DC/Low Frequency path.

Chain300's addition of the AC (high frequencies) to the DC/Low Frequency Path at306allows the separate AC and DC paths to be configured and then combined at the output.

However, frequency limitations of AC/High Frequency Path and DC/Low Frequency Path of chain300can influence overall performance. Variability and/or drift in the crossover frequencies of filters312and314can cause frequency errors due to initial component tolerances, temperature drifts, and drifts that occur over time. These effects may cause the bandwidths of AC and DC paths to differ substantially from filter transition frequencies providing a flat frequency response at sample110. Output can lower significantly when parasitic or inherent frequency performance of the paths approaches the transition frequencies of the filters312and314.

Selecting filter312and314(primary) poles and their secondary characteristics can create a relatively flat output spectrum from DC to high frequencies. Filters312and314, and more particularly their respective transition frequencies and poles, should be chosen with these considerations in mind.

FIGS.4-12help explore these filter considerations.FIG.4shows an exemplary circuit400that illustrates filter considerations for chain300. High frequency signals traverse the upper path402. Low frequencies traverse the lower path404. High and low frequency signals are combined by the series RC circuit406. Note that other circuits than the RC circuit406may also provide recombination. Element408is an AC signal source.FIG.5is a signal flow graph500that represents circuit400. To simply presentation, the values inFIG.5are normalized.

FIGS.6-12explore effects of the transition frequencies of the high pass filter312and low pass filter314in400changes the frequency response of the circuit. They show how a flat frequency response in system300may not be achieved for some practical values of these frequencies. In particular, different variations600-1200based on different transition frequencies of filters312and314exhibit dramatically different frequency performance. They show how changes in filters312and314can tailor the frequency performance of chain300.

FIG.6shows an extreme frequency response600of circuit400in which ω1and ω2are so far away from a normalized frequency that there is no perceptible dip in the frequency response. In variation600, the transition frequency ω1of the high pass filter312is small (1×10−18in relative units) and the transition frequency ω2of the low pass filter314is large (1×1018in relative units). These conditions effectively remove filters312and314from circuit400. They set a low enough transition for the high pass filter312that it passes essentially all frequencies. They also set a high enough transition for the low pass filter314so that it also passes essentially all frequencies. As shown inFIG.6, the response600of circuit400under these conditions does not vary with frequency.

FIG.7shows another extreme case700. In700, the transition frequency of the high pass filter312and the transition frequency of the low pass filter314are both equal to one. In this configuration, the secondary and primary filter poles of both filters312and314are coincident. The arrangement is often called a buffered twin-tee filter. The overall response is a “notch”702at 1 Hz.

FIGS.8and9show intermediate cases.FIG.8shows the response800of circuit400when the transition frequency ω1of the high pass filter312is 0.01 and the transition frequency ω2of the low pass filter314is 100. In this configuration, the secondary poles of each of the two filters312and314are displaced from their primary poles by a factor of 10. This turns the narrow notch702of700into a broader “valley”802that extends over two decades in frequency. The depth is approximately 1% of maximum.FIG.9, in contrast, shows the response900when the secondary poles of filters312and314are each a factor of 100 displaced from the primary poles. This happens with the transition frequency ω1of filter312is 0.1 and that ω2of filter314is 10. As shown inFIG.9, the response800exhibits a valley902with depth approximately 10% and width approximately 4 decades in frequency. In each case inFIGS.7-9, the frequency response is symmetrical about 1 Hz.

FIGS.10and11show cases where the response is offset from 1 Hz centering. Specifically,FIG.10shows the response of circuit400when the low-frequency secondary pole is displaced by a factor of 100, and the high-frequency secondary pole is essentially omitted. This happens with the transition frequency ω1of filter312is 1×10−18and that ω2of filter314is 100. The response1000exhibits a valley1002centered at 10 Hz, with depth approximately 1%. The width of valley1002spans approximately 2 decades in frequency.FIG.11shows the response1100of circuit400when the high-frequency secondary pole is displaced by a factor of 100 and the low-frequency secondary pole is omitted. This happens with the transition frequency ω1of filter312is 0.01 and that ω2of filter314is 1×10−18. The response1100exhibits a valley1102centered at 0.1 Hz, with depth approximately 1% and width approximately 2 decades in frequency.

The variations in circuit400represented by600-1100show that the secondary poles of filters312and314can be substantially displaced to minimize the valley at the crossover. For example, displacing the secondary poles by a factor of 1×10−4(corresponding to a transition frequency ω1of filter312of 0.0001 and a transition frequency ω2of filter314of 10,000) results in a valley1202of depth 1×10−4and width 1×108, as shown inFIG.12. In applications where the dual-path approach includes a negative feedback loop (e.g., a chopper-stabilized operational amplifier), valley1202may be of little consequence and displacements of a factor of ten, or a hundred, etc. can suffice. In variations, valley1202may be eliminated using secondary transfer functions that are more complex than the single pole assumed in600-1200.

FIG.13illustrates another exemplary hybrid DAC source signal chain1300having a DC feedback loop. The DC feedback loop can correct for errors in the Analog Output signal. It can maintain separate the AC and DC paths while allowing for a flat response from high frequencies response down to DC.

In chain1300, AC and DC inputs1302and1304, respectively, are summed1306and sent to the DC Configuration path via DAC1308. The AC input1302is fed to the AC Configuration path via DAC1310. The AC Configuration path then passes through a variable amplifier220abefore it is summed1312with the signal from the DC Configuration path, after it has been amplified by220b. Gains220aand220bmay be set, as discussed above, in the context ofFIGS.9and10. The sum1312is then fed via amplifier1314and to sample130.

DC feedback is accomplished as follows. The DC Configuration path from DAC1308is summed1316with the DC input signal after DAC1308processing, then to variable amplifier220cvia1318. Gain220cmay be set as discussed above for220aand220b. Subsequently, the DC Configuration signal is summed at1312with the AC Configuration signal. This feedback loop essentially treats the AC path as a disturbance to the DC path, allowing for a flat frequency output to sample130.

In chain1300, the AC/high frequency path and the DC/low frequency path are separate and allow for component configuration and separate path ranging. The DC/low frequency path of1300can be configured (e.g., via DAC1308and/or gain220b) for DC. In particular, DAC1308can be configured with low DC offsets. In addition, output summing element1312can be chosen to reduce DC errors in the output to amplifier1314and the ultimate Analog Output to sample110. Also, the feedback elements (i.e., amplifier220and sum1316) can be chosen with low DC offsets and low drifts with temperature and time.

These adjustments can remove the non-linear aspects of the frequency response shown inFIGS.7-11. The Feedback Path in1300can remove the valley1201in response1200(FIG.12). The Feedback Path can also compensate for negative effects on DC performance caused by other portions of the chain1300. For example, it can compensate for DC offset errors, such as those emanating from components such as amplifier1314, AC path DAC1310, and amplifier220a, among other components. Combining the AC and DC paths with a Feedback Path where the AC path is a disturbance for the DC path (see addition of AC and DC signals at1306, which is sent to the DC path) can allow for nearly seamless transitions between ranging and other variations in the measurement system. Additionally, it can introduce feedback lines that only respond to low frequency (e.g., DC configured).

DC/low frequency path components are generally less configured for bandwidth performance as compared to components designed for AC. This encompasses most of the components in the DC/low frequency “servo loop”1330, which refers to the DC DAC and feedback mechanism (i.e., the loop1330encompassed by elements1304,1306,1308,1316,1318,220b,1312,1314,220c, and1316in chain1300). This typically means that the DC/low frequency servo loop1330can effectively respond to frequencies only up to a specified frequency limit. Above that limit, spurious offsets, lags, errors, and other problems can manifest. Below it, the DC/Low Frequency path can handle the signal with relatively little problem. Chain1300sends all frequencies to the DC/Low Frequency path via1306, which adds together the AC1302and DC1304inputs. The Feedback Path can subtract output frequencies from the DC/low frequency path after DAC1308. The DC/low frequency servo loop1330can adjust the Analog Output signal until the Feedback Path and the DC DAC1308output are equal. Changes in the Analog Output can be continuously compensated, configuring DC/low frequency output.

In chain1300, the servo loop1330may have a limited bandwidth. The loop1330will not affect frequencies outside of that bandwidth. In this case, the bandwidth of the Feedback Path and the DC/low frequency DAC1308output response may need to be significantly higher than that of the servo loop1330. The feedback signal and DC/low frequency DAC can be designed to remove high frequencies before being sent to the rest of the DC/low frequency path. This can prevent the servo loop1330from removing small amounts of the high frequency signals, which could cause errors. Frequencies fed to the AC/High Frequency Path can be summed directly at1312into the output after the servo loop1330. Frequencies higher than the Feedback Path's bandwidth are not fed back to the input through the feedback path and are not affected by the servo loop1330.

The DC and low frequency performance of chain1300can be defined by the DC/low frequency DAC1308, the Feedback Loop's subtracting element1316, feedback gain element220c, along with DC parasitic errors. These DC parasitic errors can include thermoelectric errors, low frequency crosstalk, loop parasitic capacitance, among others. Proper layout of the circuit board where1300is mounted (e.g., printed circuit board (PCB)) layout can reduce such DC parasitic errors.

The improved DC/low frequency performance of chain1300can be achieved for low frequency signals generated by either the AC/high frequency DAC1310or the DC/low frequency DAC1308. The signals can be summed into the servo loop's1330input at1306and fed back through the Feedback Path. This can allow the servo loop1330to configure these signals. Because frequencies generated above the servo loop's1330frequency response are not affected, this maintains the AC/high frequency's performance characteristics.

There are advantages to producing low frequency signals in either the AC and DC path. Although frequencies above the servo loop's1330bandwidth need to be sourced through the AC/High Frequency Path, low frequency signals can be sourced through either path (or both simultaneously). Generating all frequencies in the AC/high frequency path may be advantageous, especially for signals with a consistent amplitude. In this case, AC gain performance can be held constant across the system's bandwidth. Ranging in this mode also can allow small amplitude AC signals to be generated in either path. This allows generation superposition of small AC signals on DC (or low frequency AC) offsets. This technique can create accurate, high resolution ramps with signals having very small AC frequencies riding on the ramps.

Separately ranging AC and low frequency signals, as in chain1300, can be helpful in other scenarios. For example, separate ranging can assist in generating source signals for harmonic measurements with dual frequencies. Accurately generating signals with harmonic frequencies can greatly improve accuracy, reliability, and reproducibility of materials characterization measurements.

Generating low frequency signals via the DC/Low Frequency Path in chain1300can also be advantageous. The DC/Low Frequency Path can generate very fine resolution signals without ranging because the servo loop's1330feedback path dominates its performance characteristics. Doing so in combination with loop1330integration can produce accurate high-resolution signals. Ranging can provide additional improvements in resolution or noise at the potential expense of range changing errors. Chain1300's architecture also facilitates increasing the DC/low frequency path's resolution. Chain1300uses an integrator1318, in part, for this purpose. Variations include other methods such as dithering DACs, pulsed width modulation, etc.

FIG.14shows another exemplary source signal chain1400. In1400, the DC feedback is digitized using an ADC. This brings feedback configuration to the above-described hybrid techniques. It introduces less DC inaccuracy and enhances the DC DAC resolution. It leverages the fact that ADCs are typically more accurate and offer better control than DACs.

The AC/High Frequency Path in chain1400is identical to that in chain1300inFIG.13. The DC/Low Frequency Path of chain1400differs from that of chain1300primarily by including ADC1402in DC feedback, as part of its servo loop1430. However, there are some other subtle differences. Specifically, DC feedback from the sample110is fed via variable amplifier720cto ADC1402where it is converted to an analog signal. That signal is then summed1404with DC the combined DC input/AC input signal from1306. Subsequently, the combined signal is fed to DAC1308, via1406, amplifier720band then summed with the AC Configured signal at1312. Gains720a,720b, and720ccan all be set as described above with respect toFIG.13.

The ADC1402feedback in chain1400can provide better accuracy and resolution because the best performing low frequency ADCs can have better performance than DACs. The Feedback Path in1400is also combined in the digital realm (i.e., with1404). This can reduce errors from analog combination. Digitizing feedback can allow creation of complex waveforms that would be extremely difficult to generate with analog feedback. The digital feedback can also enhance the DC DAC's1308resolution. Since the accuracy is determined by the ADC feedback path, this allows use of a low-quality, inexpensive DAC1308. The DC DAC's1308accuracy does not affect the Analog Output signal accuracy. The integrator1406in the DC/Low Frequency Path also enhances the resolution of the path's resolution. Since the resolution and accuracy are primarily achieved through the Feedback Path, averaging a high bit ADC1402with good linearity and noise can readily increase the overall resolution to well below the system's noise specifications.

The improved accuracy of chain1400also allows feeding the Analog Output signal back from remote locations. The Feedback Path in chain1400can remove many errors and offsets that occur in cabling and interconnections.

Chains1300and1400can improve transient response. Quickly changing DC signals can be processed in the digital domain. Sharp changes in the DC signal can be sent to the AC/High Frequency Path. Since the AC/High Frequency Path can change its output quickly, a large spike in the DC signal can be anticipated. The signal can be processed to make a large, fast transient occur in the AC path. Once the DC signal has reached its new level, the servo loops1330/1430can servo the DC signal again. This technique allows for fast changes, quick settling time, keeps the full resolution of the DC integrator, and keeps the accuracy of the DC loop.

Chains200,300,1300and1400all are shown with two generating paths, DC/Low Frequency and AC/High Frequency. However, it should be understood that more than two generating paths are possible and may be advantageous. Three or more generating paths may, for example, handle increased signaling bandwidth. They can add more DACs to the signal processing to improve accuracy. Generating very high frequencies and combining the outputs can provide high quality outputs with wide bandwidths.

Digital Source Synthesis

Variations of the disclosed systems can create source signals using direct digital synthesis. Direct digital signal gives greater consistency and control over the source signal. A digital signal also tends to have less interference and noise. Since these issues ultimately result in noise or ambiguity in the output signal, using direct digital synthesis can improve the accuracy and reproducibility of measurements. Although certain specific examples are described below, it is to be understood that any suitable mechanism for providing a digital source signal may be used in conjunction with any of the variations described herein.

FIG.15illustrates one exemplary variation of a digitally synthesized source channel1500. Digitally synthesized source channel1500may provide analog signal inputs102,202,302,1302, and1402above. Source Processing1502shown inFIG.15may include any of the signal chains disclosed herein, including chains200,300,1300and1400.

The source may be principally derived from a waveform table1502. Table1502can be an algorithm (software or firmware) that generates the waveform based on inputs1504. Inputs1504may direct the table1502to select the particular wave form to source. Inputs1504may select frequency, phase shift, and lag, among other things. Each of inputs1504is not necessarily used in every variation. They may be stored locally, may be input directly by the user, may be generated by other software and/or according to measurement or diagnostic protocols.

Reference signals1506may also be input to table1502. References1506include source references from lock-in amplifiers (e.g., source lock-in references from channels 1-3) and phase-locked loop (PLL) reference. References1506may be selected by mux1508and sent to multiplexer (mux)1510where they are combined with wave form settings1504and additional references1516. References1506may be chosen by the user, other software and/or according to measurement or diagnostic protocols. They are then sent to the table1502for selection of the specific waveform to output as a source signal. An output waveform from the table1502may then be further processed1502by any signal processing method described herein and provided to the source pod104. Channel1500can also use a lock-in reference with optional phase shift1504, rather than be chosen directly via inputs1504. In this case, the source's frequency and phase can be determined by a lock-in reference signal (e.g., reference1512). Optional phase shift1504can set the phase relationship with the reference1512. The external phase relationship can be configured differently for each channel.

FIGS.16and17show an exemplary variation of a source wave table1600that can be provided by element1502of digital source1500. Waveform1700inFIG.17is generated by plotting the data in table1600.FIG.17plots a single period of waveform1700in relative units.

In one variation, a source signal supply algorithm can repetitively increment through the table1600representing one or more periods of a waveform. The table1600provides waveform amplitude (Output) vs. time (Position), both in normalized units. Using normalized units is not a requirement. It is convenient for scaling either the voltage or time dependence of the waveform based on inputs1304. In this way, the table1600determines the waveform1700's shape. The rate at which the algorithm cycles through the table1600, called the phase increment (element1504,FIG.15), determines the waveform1700's frequency.

The “Position” of the table1600need not change by an integer. In certain variations, for example, a higher resolution phase accumulator (element1504,FIG.15) can be used to keep track of waveform1700's phase. The phase accumulator1504can increment by non-integer amounts to translate this phase into a Position in the table1600.

The waveform1700inFIG.17can be smoothed and/or made continuous, either by the table1502itself or in source processing1510using a low pass filter. For low-pass smoothing, discrete output values with a non-zero width in time twcan replace the values in the table ofFIG.16. Note that the table1600can vary substantially in length and complexity. In some variations, it can have thousands (e.g., 4000 or more) entries. As shown inFIG.18A, this creates a “stepped” output waveform1802. Applying an analog low pass filter to1802(e.g., AC DAC output) creates a smooth waveform1804shown inFIG.18B. AC waveform1804can be combined with a DC offset setting (not shown) and fed into a closed loop DC sourcing system in Source Processing1510. This can be part of the hybrid sourcing variations discussed above in the context ofFIGS.2-14.

Transfer Function

FIG.18Cshows a generalized form of an exemplary transfer function1850that may be used in conjunction with chain1400of the present disclosure. The intent of the analysis performed by1850and the associated transfer function is to demonstrate the flat frequency response of the system of1400.FIG.18reduces the circuit to more generic signal elements for analysis of the transfer function.

Transfer function1850governs which source, AC (V2) or DC (V1), dominates its output Vout.

Vout1852of the generalized hybrid DAC transfer function is given by:

It can be assumed that the inner loop is faster than the outer loop. The gain k establishes the transition frequency. Below this frequency V1(s) dominates. Above this frequency V1(s)+ dominates. If A1V1(s)=A2V2(s), then the outer loop is transparent, leaving the inner loop's transfer function. In the case of a voltage source, G(s) is the open loop transfer function of an op amp, and H(s) is unity. In the case of a current source, G(s) is the open loop transfer function of an op amp, and H(s) is a sense resistor and amplifier.

In either case, the gain A1should exceed A2. That is, the DC range should be equal to or greater than the AC range. For a current source, the sense resistor is selected for the higher range.

Measurement System Integration

Overview of Measurement System

FIG.19shows an exemplary source1900configuration with a “Source Pod”1950configuration. The source configuration1900connects to and controls source pod1950by a “head” or control unit1960. Both source pods1950and head units1960of system1900are described in more detail in U.S. Pat. No. 11,762,050, to Fortney, “INTEGRATED MEASUREMENT SYSTEMS AND METHODS FOR SYNCHRONOUS, ACCURATE MATERIALS PROPERTY MEASUREMENT.” It should be understood that, whileFIG.19shows system1900with a particular hybrid DAC, system1900may accommodate any such system disclosed herein. In particular, system1900may accommodate chains100,200,300,1300, and1400.

The Sample source signal (i.e., the signal sent by the chain1900to sample110) is a combination of an AC signal (“AC configured signal”) and a DC signal (“DC configured signal”). These signals are combined to create the Sample source signal via a variable gain220that can be dynamically ranged to avoid glitching in the signal. The DC configured signal is generated based on DC feedback from the combination of the AC configured signal and the DC configured signal.

More specifically, the AC Configured DAC1914provides the AC configured source signal to amplifier1916in source pod1950where it is combined with a DC configured source signal by1918and provided to ranged amplifier220, then onto sample110. The waveform shape, amplitude, frequency, and phase of the source provided to the AC Configured DAC1914may be pre-programmed, selected by the user, and/or selected among options by the head1960according to user preference and/or protocol (e.g., measurement or diagnostic). The output of1918is also provide as DC feedback via amplifier1924to DC Configured ADC1926of the head102. The DC feedback signal is then sent to a DC Configured DAC1930via offset1928, then routed via amplifier1932to1918.

As shown inFIG.19, the range of ranged amplifier220, along with other settings, can be selected by via a “ranges & other settings” signal sent via ranges & other settings element1922of head102. Ranges and other settings may be pre-programmed, selected by the user, and/or selected among options by the head1960according to user preference and/or protocol (e.g., measurement or diagnostic).

Source pod1950may further include digital (non-analog) circuitry capable of performing various functions, including analysis, communication of data, command information, power regulation, timing, and communication with external devices. In variations, source pod1950has the capability to de-activate this non-analog circuitry while providing its source signal or performing a measurement. Doing so decreases the amount of interference and noise in the signal or measurement. For the same reason, digital signals in the source pod1950may be isolated from a measurement pod1950and the head102.

Other variations of system1900include any suitable number of heads1960, source pods and measure pods1950. For example,FIG.20shows another exemplary variation2000, where a head unit1960can have six channels that can support three measure type pods1950aand three source type pods1950b. In this variation, the head1960is also shown connected to an optional computer2002and three exemplary sampled or devices under test (DUTs)110. Again, this configuration is merely exemplary. There is no requirement for equal numbers of measure1950aand source1950bpods. One source1950acould provide the excitation signal for all three DUTs110, for example.

Balanced Current Sourcing

As shown inFIG.19, source pod1950may further include balanced current sourcing (BCS) capabilities1932. BCS1932is explained in more detail in U.S. Pat. No. 6,501,255 (the '255 patent) to Pomeroy entitled, “DIFFERENTIAL CURRENT SOURCE WITH ACTIVE COMMON MODE REDUCTION,” and filed on Oct. 4, 2001, the entirety of which is herein incorporated by reference.

Briefly, source systems (e.g.,1900) can be vulnerable to inconsistent loading causing current spikes and/or asymmetries between input/out. These spikes may harm components of those systems. There is a need for current balancing in the materials measurement context where both floating and grounded loads can be addressed without substantially altering or rewiring circuitry. BCS1932addresses this need.

As discussed in the '255 patent, BCS1932drives the load with two modified Howland current sources that are out of phase with one another. In the context of system1900, BCS1932uses a sensing resistor to measure a source current associated with a source signal sent to the sample from the source pod1950(“Sample source signal” inFIG.19). It then varies a resistance range of the sensing resistor according to the magnitude of the measured source current. BCS1932can also balance load by altering resistance of either (or both) of the source and measurement pods1950based on this source signal measure. For example, when the measured source current exceeds a threshold, the BCS1932can increase or decrease the resistance of one or both pods1950to lower the current below the threshold. The threshold current may represent, for example, a current above which damage would be done to one or more of the components of system1900.

Variations inclusive of system1900and others may employ chains100,200,300,1300, and1400for additional purposes than those described above. For example, chains100,200,300,1300, and1400may be employed in conjunction, or as part of, current or voltage sources (e.g., to regulate voltage or current output). They may be employed in a temperature controller, other parameter controller, power supply, or a source measure unit (e.g., a device that measures the output of a current or voltage source). They may, for example, operate in conjunction with or as a controller for a source measure unit. They may be employed in other measurement systems (e.g., systems that measure materials parameters: systems that measure current/voltage (I-V) characteristics, resistivity, superconducting transport, force current, voltage, current, transconductance, breakdown/leakage, etc.). Chains100,200,300,1300, and1400may also be incorporated into devices that transmit, receive, and/or supply radio frequency (RF) energy and/or communications and/or other communications.

Although chains200,300,1300, and1400have been discussed above as providing only a single analog output, it is to be understood that they may provide multiple analog outputs. For example, the summation316of chain300may be altered such that it produces two or more combinations of the signals from the AC/High Frequency Path and the DC/Low Frequency path. Similar alterations can be made to210and1312. As discussed above, they can generate one or more harmonic signals.

While various inventive aspects, concepts and features of the inventions may be described and illustrated herein as embodied in combination in the exemplary embodiments, these various aspects, concepts and features may be used in many alternative embodiments, either individually or in various combinations and sub-combinations thereof. Unless expressly excluded herein all such combinations and sub-combinations are intended to be within the scope of the present inventions. Still further, while various alternative embodiments as to the various aspects, concepts and features of the inventions—such as alternative materials, structures, configurations, methods, circuits, devices and components, software, hardware, control logic, alternatives as to form, fit and function, and so on—may be described herein, such descriptions are not intended to be a complete or exhaustive list of available alternative embodiments, whether presently known or later developed. Those skilled in the art may readily adopt one or more of the inventive aspects, concepts or features into additional embodiments and uses within the scope of the present inventions even if such embodiments are not expressly disclosed herein.