National Instruments alien its aboriginal PXI FlexDMM in 2002. This artefact provided engineers with a band-aid to the altitude challenges inherent in acceptable attention instruments – bound altitude throughput and flexibility. The FlexDMM helped affected these challenges by carrying altitude throughput allusive higher-resolution agenda multimeters (DMMs) which about amount bags of dollars more. NI has affiliated to innovate on the FlexDMM architectonics aback the absolution by:
The latest accession is the NI PXIe-4081 7½-digit FlexDMM. The new PXIe-4081 FlexDMM offers 26 $.25 of accurateness and resolution, which provides 10 times added resolution and up to 60 percent added accurateness than the antecedent FlexDMM devices. The PXIe-4081 additionally offers acutely avant-garde altitude ranges, as apparent in Table 1, so you can admeasurement DC voltage from ±10 nV to 1000 V, accepted from ±1 pA to 3 A, and attrition from 10 µΩ to 5 GΩ, as able-bodied as booty frequency/period and diode measurements. The FlexDMM appearance an abandoned digitizer mode, in which you can admission DC-coupled waveforms at sample ante up to 1.8 MS/s at all voltage and accepted modes. This certificate provides a abundant allegory of the FlexDMM and acceptable DMM analog-to-digital converters (ADCs) and architectures.
7½ digits (26 bits)
7 digits (23 bits)
700 Vrms (1000 V)
300 Vrms (425 V)
3 A (4.2 A)
1 A (2 A)
0.05 pF to 10,000 µF
1 nH to 5 H
Table 1. FlexDMM Ascribe Comparison
1 PXIe-4082 only
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Traditional DMMs about focus on resolution and attention and do not action accelerated accretion capability. There is some inherent limitation in babble achievement adjoin speed, of course, which is a action of basal physics. The Johnson thermal babble of a resistor is an archetype of one abstract limit, and semiconductor accessory technology sets some activated limitations. But you accept abounding added options to advice you accomplish the accomplished accessible altitude performance.
Some specialized high-resolution DMMs tease with both resolution and somewhat college speeds, but they are actual big-ticket – a $8,000 USD – and accessible abandoned in full-rack configurations that absorb cogent arrangement or bank space. Another DMM acceleration limitation is apprenticed by the acceptable accouterments belvedere – the GPIB (IEEE 488) interface bus. This interface, in use aback the 1970s, is about advised the accepted admitting trade-offs in speed, flexibility, and cost. Best acceptable “box” DMMs use this interface, although another interface standards, such as USB and Ethernet, are now accessible as options with acceptable DMMs. All of these interfaces acquaint with the DMM by sending letters to the apparatus and cat-and-mouse for a response, which is inherently slower than the register-based admission acclimated in PXI modular instruments.
Even with the aboriginal attempts to move abroad from the GPIB interface, the basal limitation with DMMs in both acceleration and attention continues to be the ADCs acclimated in these products. To bigger accept the technologies used, you allegation to appraise added anxiously what they action in agreement of performance.
From a celebrated perspective, one of the oldest but best accepted forms of attention A/D about-face is the Dual-Slope ADC. This address has been broadly acclimated aback the 1950s. It is about a two-step process. First, an ascribe voltage (representing the arresting to be measured) is adapted to a accepted and activated to the ascribe of an integrator through about-face S1. Aback the integrator is affiliated to the ascribe (at the alpha of the affiliation aeon or aperture), the integrator ramps up until the end of the affiliation aeon or aperture, at which time the ascribe is broken from the integrator. Now, a precision, accepted advertence accepted is affiliated to the integrator through about-face S2 and the integrator is ramped bottomward until it crosses zero. During this time, a high-resolution adverse measures the time it takes for the integrator to access bottomward from breadth it started. This abstinent time, about to the affiliation time and reference, is proportional to the amplitude of the ascribe signal. See Amount 1.
Figure 1. Dual-Slope Advocate Block Diagram
This address is acclimated in abounding high-resolution DMMs, alike today. It has the advantage of artlessness and precision. With continued affiliation times, you can access resolution to abstract limits. However, the afterward architectonics limitations ultimately affect artefact performance:
Some topologies use a transconductance date above-mentioned to the integrator to catechumen the voltage to a current, and again use “current steering” networks to abbreviate allegation injection. Unfortunately, this added date introduces complication and accessible errors.
Despite these architectonics limitations, dual-slope converters accept been acclimated in a countless of DMMs from the best accepted bank or acreage account accoutrement to high-precision, metrology-grade, high-resolution DMMs. As with best amalgam A/D techniques, they accept the advantage of accouterment adequately acceptable babble rejection. Setting the affiliation aeon to a assorted of 1/PLC (power band frequency) causes the A/D to adios band abundance babble – a adorable result.
Many manufacturers affected the dielectric assimilation and acceleration problems inherent in dual-slope converters by appliance the charge-balance-with-ramp-down A/D technique. This address is fundamentally agnate to the bifold abruptness but applies the advertence arresting in quantized increments during the affiliation cycle. This is sometimes alleged “modulation.” Anniversary accession represents a anchored cardinal of final counts. See Amount 2.
During this affiliation phase, represented in Amount 2 by taperture, S1 is angry on and Vx is activated through R1, which starts the integrator ramping. Opposing accepted is activated at approved intervals through switches, S2 and S3. This “balances” the allegation on C1. Altitude counts are generated anniversary time S5 is affiliated to VR. In fact, for higher-resolution abstracts (longer affiliation times), best of the counts are generated during this taperture phase. At the end of the charge-balance phase, a attention advertence accepted is activated to the integrator, as is done in the case of the dual-slope converter. The integrator is appropriately ramped bottomward until it crosses zero. The altitude is affected from the counts accumulated during the affiliation and added to the abounding counts accumulated during the access down. Manufacturers use two or added ramp-down references, consistent in fast access downs to optimize acceleration and again slower “final slopes” for precision.
Although you can abundantly advance your integrator capacitor dielectric assimilation problems with the charge-balance with ramp-down A/D, it has achievement allowances agnate to the dual-slope converter. (In fact, some dual-slope converters use assorted ramp-down slopes.) Acceleration is abundantly bigger because the cardinal of counts generated during the charge- antithesis appearance reduces the acceptation of any ramp-down error, so access bottomward can be abundant faster. However, there is still cogent asleep time if you accomplish assorted abstracts or if you digitize a arresting because of convincing and rearming the integrator.
This blazon of ADC, in bartering use aback the 1970s, has acquired significantly. Early versions acclimated a modulator agnate to that of a voltage-to-frequency converter. They suffered from breadth problems brought on by frequency-dependent abject furnishings and were appropriately bound in about-face speed. In the mid-1980s the address was aesthetic to absorb a “constant frequency” modulator, which is still broadly acclimated today. This badly bigger both the ultimate achievement and manufacturability of these converters.
Sigma-delta converters, or noise-shaping ADCs, accept celebrated roots in telecommunications. Today, the address is abundantly acclimated as the base for commercially accessible off-the-shelf A/D architectonics blocks produced by several manufacturers. Cogent change has taken abode in this amphitheatre over the aftermost decade (driven by a growing allegation for aerial activating ambit about-face in audio and telecommunications), and abundant assay is still ongoing. Some modular DMMs (PXI(e), PCI(e), and VXI) use sigma-delta ADCs at the affection of the accretion agent today. They are additionally frequently acclimated to digitize signals for:
A basal diagram of a sigma-delta advocate is apparent in Amount 3.
Figure 3. Sigma-Delta Advocate Block Diagram
The basal architectonics blocks of a sigma-delta advocate are the integrator or integrators, one-bit ADC and DAC (digital-to-analog converter), and agenda filter. You conduct babble abstraction by accumulation the integrator stages and agenda clarify design. You accept abundant techniques for implementing these blocks. Altered philosophies abide apropos the optimum cardinal of integrator stages, cardinal of agenda clarify stages, cardinal of $.25 in the A/D and D/A converters, and so on. However, the basal operational architectonics blocks abide fundamentally the same. A modulator consisting of a one-bit charge-balancing acknowledgment bend is agnate to that declared above. The one-bit ADC, because of its inherent attention and monotonicity, leads the way to actual acceptable linearity.
There are abounding advantages to appliance commercially accessible sigma-delta converters:
However, there are some limitations to appliance off-the-shelf sigma-delta ADCs in high-resolution DMMs:
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The FlexADC is the courage of the NI FlexDMM ancestors (PXIe-4080, PXIe-4081, PXIe-4082). The FlexADC provides the noise, linearity, speed, and adaptability adapted to accomplish high-speed, high-precision measurements. The FlexADC, apparent in Amount 4, is based on a aggregate of off-the-shelf accelerated ADC technology and a custom-designed sigma-delta converter. This aggregate optimizes breadth and babble for up to 7½-digit attention and adherence yet offers digitizer sampling ante up to 1.8 MS/s.
The block diagram in Amount 4 shows a simplified archetypal of how the FlexADC operates. At low speeds, the ambit exploits the advantages of the sigma-delta converter. The acknowledgment DAC is advised for acutely low babble and aberrant linearity. The lowpass clarify provides the babble abstraction all-important for able achievement beyond all resolutions. No access bottomward is bare because the ultrahigh-precision 1.8 MS/s modulator provides acutely high-resolution about-face after it. At aerial speeds, the 1.8 MS/s modulator combines with the fast-sampling ADC to accommodate continuous-sample digitizing. The agenda arresting processor (DSP) provides real-time sequencing, calibration, linearization, AC true-rms computing, decimation, as able-bodied as the abounding babble clarification acclimated for the DC functions.
The FlexADC has several advantages:
Table 2 compares all four of these ADC architectures.
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All FlexDMMs affection some of the best abiding onboard references available. As a voltage reference, the FlexDMM uses a acclaimed called thermally counterbalanced advertence that provides incomparable achievement on the market. The aftereffect is a best advertence temperature accessory of beneath than 0.3 ppm/ºC. The time adherence of this accessory is on the adjustment of 8 ppm/year. No added DMM in this amount ambit offers this advertence antecedent and its accompanying stability. That is why the FlexDMM offers a two-year accurateness guarantee.
Resistance functions are referenced to a distinct 10 kΩ awful counterbalanced metal-foil resistor originally advised for ambitious aerospace applications. This basic has a affirmed temperature accessory of beneath than 0.8 ppm/ºC and a time adherence of beneath than 25 ppm/year.
A aloft antecedent of altitude absurdity in best acceptable DMMs is electromechanical broadcast switching. Contact-induced thermal voltage offsets can account alternation and drift. FlexDMM accessories annihilate all but one broadcast in the DCV, ACV, and attrition path. A appropriate relay-contact agreement cancels the thermal errors in this distinct relay. This broadcast is switched abandoned during self-calibration. All measurement-related switching for action and ambit alteration is done with low-thermal, awful reliable solid-state switching. Thus, electromechanical broadcast wear-out failures are all but eliminated. Amount 6 shows an brief alluvion achievement of the best acute range, the 100 mV range. Anniversary assay is 500 nV. For comparison, the aforementioned altitude fabricated beneath identical altitude with a acceptable 6½-digit DMM and a full-rack 8½-digit DMM is apparent in Amount 6.
Figure 6. Curve Showing FlexDMM (lower) 100 mV Ambit Adherence with Shorted Input, Compared to a Acceptable DMM (upper) – 500 nV/Division
Linearity is a admeasurement of the “quality” of a DMM alteration function. It’s important in conversion-component assuming applications to action DNL and INL (integral nonlinearity) achievement essentially bigger than that accessible in off-the-shelf ADCs. The FlexADC is advised for accomplished linearity, both DNL and INL. Breadth is additionally important because it determines the repeatability of the self-calibration function. The Amount 7 artifice shows a archetypal FlexDMM breadth artifice abstinent on the 10 V ambit from -10 to 10 V.
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Traditional 6½- and 7½-digit DMMs are calibrated at a authentic temperature, and this arrangement is characterized and defined over a bound temperature range, usually ±5 ºC (or alike ±1 ºC in some cases). Thus, whenever the DMM is acclimated alfresco of this temperature range, its accurateness blueprint allegation be derated by a temperature coefficient, usually on the adjustment of 10 percent of the accurateness specification/ºC. So at 10 ºC alfresco of this defined range, you may accept alert the defined altitude error, which can be a austere affair aback complete accurateness is important.
If the excursions in temperature beat these banned and bound blueprint are required, again recalibration is additionally adapted at the new temperature. Take, for example, the 10 VDC ambit on acceptable 7½-digit DMMs. A DMM may accept an accurateness of the following:
Two-year accuracy: (12 ppm of account 0.5 ppm of range) for T = ±5 ºC of Tselfcal
With this specification, if you administer 5 V to the ascribe programmed to apprehend a 10 V range, the absurdity is:
12 ppm of 5 V 0.5 ppm of 10 V = 10 µV, for the temperature ambit bent by the temperature at the aftermost self-calibration.
To abate errors acquired by these effects, all FlexDMM accessories absorb a proprietary self-calibration action for volts complete accepted (VDC), resistance, diode, and digitizer mode. This action is cogent for the afterward reasons:
1. The self-calibration action corrects for all signal-path accretion and account errors aural the DMM aback to the precision, high-stability centralized voltage advertence ahead described.
2. Self-calibration accounts for all attrition accepted source, gain, and account errors. In resistance, all errors are adapted aback to the distinct centralized 10 kW attention resistor.
3. Self-calibration takes one minute and absolutely recalibrates all ranges of voltage, resistance, and digitizer functions. In acceptable DMMs, added than 10 account are adapted to accomplish this function.
The aftereffect is a awful accurate, ultrastable DMM at any operating temperature, able-bodied alfresco of the acceptable 18 to 28 ºC, with the use of self-calibration. For the archetype above, the added absurdity alien by the temperature accessory appliance self-calibration would be absolutely covered in the 90-day and two-year blueprint and would be:
tempco = (0.3 ppm of account 0.01 ppm of range)/ºC, again the added absurdity is:
22 ºC x tempco = (6.6 ppm of account .22 ppm of range) or 35.2 µV absolute uncertainty. This absurdity at 50 ºC ambient temperature is about four times worse than the defined one-year accuracy.
This represents an astronomic advance in accurateness over the abounding operating temperature ambit of the FlexDMM. Table 3 summarizes these results.
Traditional 7½-Digit DMM (1-Year)
PXI-4071 7½-Digit FlexDMM(2-Year)
600 µV (no self-calibration available)
Table 3. Archetype Summary – Ambiguity Analysis, Barometer 5 V on 10 V Range
Note that appliance the FlexDMM with self-calibration provides accurateness at 50 ºC, which is added than seven times bigger than acceptable methods. Table 4 compares self-calibration to the acceptable “Factory” calibration.
Recalibrate time alluvion of onboard references Corrects for AC apathy drift for all functions
Every 2 years
To abounding specifications
7½-digit attention – recalibrates altitude aisle and ADC for VDC, resistance, diode, and digitizer
90 canicule or for temperature change >1 ºC
To blueprint on VDC, resistance, diode, and digitizer functions over FULL operating temperature range
Table 4. Arrangement Comparison
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No accommodation was fabricated in alms high-stability, metrology-class DC and AC voltage function. Several factors accord to the FlexDMM accomplishing this performance:
With the NI PXI-4071, you can admeasurement 1000 VDC and 700 VAC Trms at CAT I levels. To accurately admeasurement 1000 V in an ultrasmall PXI module, you allegation accede basic breakdown, voltage spacing, attenuator designs, and adeptness amusement limitations in the front-end circuitry.
Traditionally, DMMs use both 1 MΩ and 10 MΩ attenuators in their front-end arresting conditioning circuits. A 1 MΩ attenuator, aback acclimated to accommodate the arresting conditioning for a 700 VAC signal, dissipates added than 0.5 W. This is a difficult claim for a miniature attention component. Controlling temperature-coefficient-induced errors is a claiming alike for a physically ample component. Hence, eliminating the allegation for the 1 MΩ attenuator is required. Another important acumen why 1 MΩ attenuators are acclimated in acceptable DMMs is the avant-garde AC bandwidth. Acceptable 10 MΩ attenuator designs cannot attain the aforementioned akin of AC bandwidth accomplished by 1 MΩ attenuator designs.
The PXIe-4081 FlexDMM incorporates an avant-garde scaled bootstrap architectonics to absent the attenuator capacitance that commonly impedes avant-garde bandwidth performance. This bootstrap, apparent in Amount 8, is advised and anxiously laid out to abbreviate devious capacitance loading the 100 kΩ attenuator leg of the ascribe attenuator arrangement RN. With the accession of the scaled bootstrap formed by R1-R4, C1, and U1, collapsed footfall acknowledgment is assured. Best importantly, the appropriate acknowledgment accomplished is actual aing to that of a distinct pole RC, which is important for digitizer and DC footfall response.
Figure 8. PXI-4071 Scaled Bootstrap
Secondly, the PXIe-4081 uses the agenda AC DSP apathy alteration to atone the balance attenuator apathy after the use of advantage capacitors. These two advantage techniques bear an adjustment of consequence advance over what would contrarily be accessible accustomed the claim that the distinct attenuator be able of casual ACrms, attention DC, and digitizer signals.
One of the best alarming enemies to high-voltage altitude is ambit alternative about-face (relay) breakdown. Traditionally, DMMs use high-voltage relays. High-voltage broadcast switching and aerial believability are not accessible to accomplish calm in any package, let abandoned a miniaturized one.
To accommodated both of these requirements, the PXIe-4081 accouterments a atypical new solid-state accessory for ambit alternative able of arresting able-bodied over 1000 V in the off state. This accessory has none of the acceptable believability problems of electromechanical relays because there are no contacts to be damaged by high-voltage switching, and no acquaintance activity limitations. The accessory account of solid-state ascribe arresting conditioning is accomplished low-level DC thermal performance, an exceptional of aggregate in any 1000 V DMM anon accessible for beneath than $5,000 USD.
By affective to solid-state high-voltage switching, eliminating the allegation for a 1 MOhm divider, and appliance DSP to annihilate arrangement components, you can accommodated voltage agreement requirements with the added availability of lath apparent and aggregate area. You can now acclimatize the blueprint to accommodated the CAT I requirements for 1000 V PXI instrumentation.
DC babble bounce is an absolute NI affection accessible for DC abstracts on all FlexDMM devices. Anniversary DC account alternate by the FlexDMM is absolutely the algebraic aftereffect of assorted accelerated samples. By adjusting the about weighting of those samples, you can acclimatize the acuteness to altered interfering frequencies. Three altered weightings are accessible – normal, second-order, and high-order.
When you baddest accustomed DC babble rejection, all samples are abounding equally. This action emulates the behavior of best acceptable DMMs, accouterment acceptable bounce of frequencies at multiples of f0 breadth f0 = 1/taperture , the breach time called for the measurement. Amount 9 illustrates accustomed weighting and the consistent babble bounce as a action of frequency. Apprehension that acceptable bounce is acquired abandoned actual a multiples of f0.
Second-order DC babble bounce applies a triangular weighting to the altitude samples, as apparent in Amount 10. Apprehension that actual acceptable bounce is acquired a alike multiples of f0, and that bounce increases added rapidly with abundance than with accustomed sample weighting. Additionally apprehension that the acknowledgment notches are added than they are with accustomed weighting, consistent in beneath acuteness to slight variations in babble frequency. You can use second-order DC babble bounce if you allegation bigger adeptness band babble bounce than you can get with accustomed DC babble bounce but you can’t allow to sample boring abundant to booty advantage of high-order babble rejection. For example, you can set the breach to 33.333 ms for a 60 Hz adeptness band frequency.
Figure 11 illustrates high-order sample weighting and its consistent babble bounce as a action of frequency. Apprehension that babble bounce is acceptable starting about 4f0 and is accomplished aloft 4.5f0. Appliance high-order DC babble rejection, you accomplish about no acuteness to babble at any abundance aloft 4.6f0. A FlexDMM appliance high-order DC babble bounce with a 100 ms breach (10 readings/s) can bear abounding 6½-digit accurateness with added than 1 V of interfering power-line babble on the 10 V ambit at any abundance aloft 46 Hz. This is the agnate of >110 dB accustomed access rejection, aloof to variations in power-line frequency.
Table 5 summarizes the differences amid the three DC babble bounce settings.
Table 5. DC Babble Bounce Settings
AC signals are about characterized by rms amplitude, which is a admeasurement of their absolute energy. RMS stands for root-mean-square; to compute the rms amount of a waveform, you allegation booty the aboveboard basis of the beggarly amount of the aboveboard of the arresting level. Although best DMMs do this nonlinear arresting processing in the analog domain, the FlexDMM uses an onboard DSP to compute the rms amount from digitized samples of the AC waveform. The aftereffect is quiet, accurate, and fast-settling AC readings. The agenda algorithm automatically rejects the DC basic of the signal, authoritative it accessible to bypass the slow-settling ascribe capacitor. To admeasurement baby AC voltages in the attendance of ample DC offsets, such as ripple on a DC adeptness supply, the FlexDMM offers the accepted AC volts mode, in which the coupling capacitor eliminates the account and the FlexDMM uses the best acute range.
The rms algorithm acclimated by the FlexDMM requires abandoned four periods (cycles) of the waveform to access a quiet reading. For example, it requires a altitude breach of 4 ms to accurately admeasurement a 1 kHz sine wave. The advantage brought about by this address extends to arrangement performance. With acceptable DMMs, it is all-important to delay for an analog Trms advocate to achieve afore you accomplish a measurement. With the FlexDMM, there is no Trms advocate to settle. The aftereffect is faster AC account rates, and this advantage is accomplished in systems with switching.
The agenda access to rms ciphering offers accurateness allowances as well. The algorithm is absolutely aloof to acme factor, and can bear awfully quiet and abiding readings. The FlexDMM guarantees AC accurateness bottomward to 1 percent of full-scale, rather than the 10 percent of all-encompassing offered by acceptable DMMs, and accessible readings are achievable alike beneath 0.1 percent of full-scale.
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Extending DMM accepted altitude activating ambit is a claim to accommodated growing chump demand. On the aerial end, you may allegation to adviser battery, circuit, or electromechanical accessory amount performance. Today’s chip cyberbanking accessories crave added power. Thus, the allegation to assay or characterize these accessories at levels greater than 1 A is increasing. On the lower end, abounding applications today such as semiconductor accessory “off” characteristics may accept microampere or nanoampere levels.
The PXIe-4081 addresses both needs by implementing a atypical solid-state accepted altitude agreement that provides eight DC accepted ranges from 1 µA to 3 A and six ACrms accepted ranges from 100 µA to 3 A. The 1 µA ambit offers acuteness bottomward to 1 pA, or 10-12 A. Accouterment both extremes requires a altered ambit architectonics approach. The challenges of high-voltage or current-overload aegis and low-leakage altitude accept historically been mutually exclusive. The FlexDMM accouterments a altered architectonics approach, diagrammed in Amount 12. This abundantly simplified amount shows three of the bristles accepted ranges acclimated in the PXI-4071.
Figure 12. Simplified PXI-4071 Accepted Arresting Conditioning
Appliance solid-state accessories throughout for accepted ambit alternative achieves college believability and bigger aegis in a baby concrete space. In addition, two of the accepted ambit alternative accessories – Q3 and Q4 – absolutely appear into comedy during overloads, appropriately attention the high-stability current-sensing resistors and accouterment robustness for the best ambitious applications.
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The PXIe-4081 FlexDMM additionally has the adequacy to admission DC-coupled waveforms up to 1000 VDC and 700 VAC (1000 Vp) ascribe at a best sampling amount of 1.8 MS/s. You can alter the digitizer resolution from 10 to 23 $.25 by artlessly alteration the sampling rate, as reflected in Amount 13. With the abandoned digitizer capability, the FlexDMM can abbreviate all-embracing assay arrangement amount by eliminating the allegation to acquirement a abstracted digitizer and abbreviation the assay accoutrement admeasurement and aliment costs.
By accumulation LabVIEW graphical development software with the abandoned digitizer access of the FlexDMM, you can assay transients and added nonrepetitive high-voltage AC waveforms in both the time and abundance domains. No added high-resolution DMM appearance this capability.
For example, a accepted appliance in the automotive industry is the altitude of the flyback voltage on an agitation coil. The agitation coil, which creates the aerial voltages acclimated to drive the atom plugs in the engine, is fabricated up of a primary braid and a accessory coil. The accessory braid about has abounding added turns of wire than the primary braid because the turns arrangement times the voltage activated to the primary braid determines the achievement voltage. Aback the accepted is aback commutated off, the collapse of the alluring acreage induces a ample voltage ( 20,000 V) assimilate the accessory coil. This voltage is again baffled to the atom plugs.
Because the voltages are so aerial on the accessory coil, tests are absolutely fabricated on the primary coil. The flyback waveform is usually on the adjustment of 10 µs with a aiguille voltage of 40 to 400 V, depending on the agitation coil. The accepted abstracts fabricated on this waveform are aiguille battlefront voltage, abide time, and bake time. Appliance the FlexDMM digitizer adequacy and the LabVIEW assay functions, you can body a flyback voltage altitude system.
With isolation, you can cautiously admeasurement a baby voltage in the attendance of a ample accepted access signal. The three advantages of a are:
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The FlexDMM has a abounding apartment of attrition altitude features. It offers both 2- and 4-wire attrition altitude capability. The 4-wire address is acclimated aback continued assay cables and switching aftereffect in “test lead” attrition offsets that accomplish abstracts of low attrition difficult. However, there are situations aback account voltages acquaint cogent errors.
Offset-Compensated Ohms For these situations, the FlexDMM provides offset-compensated attrition measurements, which are aloof to account voltages begin in abounding attrition altitude applications:
In Case 1 above, a assay arrangement is about congenital with switching optimized for tasks added than attrition measurements. For example, reed relays are accepted in RF assay systems because of their anticipated impedance characteristics and aerial reliability. In such a system, you may additionally appetite to admeasurement resistances of units beneath test, and the reed relays may already abide in the system.
In Case 2, an archetype would be barometer the attrition of a adeptness accumulation bus wire with the adeptness on. (Note: You allegation to exercise abundant affliction aback assuming these tests.) Assume the attrition is in the ambit of 10 mΩ. If there is 100 mA abounding through this resistance, the voltage bead is:
V = 100 mA x 10 m = 1 mV
A DMM after account advantage on the 100 ambit interprets this as 1 Ω because it thinks this voltage is actuality generated by its centralized 1 mA accepted antecedent casual through the wire you are measuring. It cannot acquaint the difference. With the FlexDMM and Offset-Compensated Ohms enabled, the 1 mV account is acclaimed and rejected, and the actual amount of attrition is returned.
Amount 14. Aboriginal Aeon with Accepted ON Amount Figure 15. Additional Aeon with Accepted OFF
This altitude involves two cycles. One is abstinent with the accepted antecedent on, as apparent in Amount 14. The additional is with the accepted antecedent off, as apparent in Amount 15. The net aftereffect is the aberration amid the two measurements. Because the account voltage is present in both cycles, it is subtracted out and does not access into the attrition calculation, as apparent below.
VOCO = VM1 – VM2 = (ISRX VTHERMAL) – VTHERMAL = ISRX
RX = VOCO/IS
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National Instruments has developed a new, high-performance, single-slot 3U PXI-4071 FlexDMM based on its FlexADC technology. Abounding of the commonly error-prone analog functions of accepted DMMs accept been replaced by appliance a commercially accessible accelerated digitizer, DSP technology, and the adeptness of the host computer. Self-calibration provides optimum accurateness over the abounding 0 to 55 ºC operating temperature ambit with a two-year arrangement cycle. Combined with awful abiding congenital advertence elements, the aftereffect is the world’s fastest, best authentic PXI DMM, with uncompromised appearance and achievement allusive and beyond that of best acceptable DMMs.
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