NI evaluated these and other ADC alternatives for the NI 4070 6 1/2-digit FlexDMM (PCI and PXI). However, no existing ADC provided the noise, linearity, speed, and flexibility required to achieve the goal of high-speed, high-precision measurements. With a growing demand for fast-isolated measurements, having a built-in high-speed digitizing is essential. To meet these demands, NI developed an ADC using a combination of off-the-shelf high-speed ADC technology and a custom-designed sigma-delta ADC, shown in Figure 4. This combination optimizes linearity and noise for 7-digit precision and stability and offers digitizer sampling rates of up to 1.8 MS/s.
Figure 4. FlexADC Converter
Figure 4 shows a simplified model of how the FlexADC operates. At low speeds, the circuit exploits the advantages of the sigma-delta converter. The feedback DAC is designed for extremely low noise and exceptional linearity. The lowpass filter provides the noise shaping necessary for good performance across all resolutions. No ramp down is necessary, because the ultra-high precision 1.8 MS/s modulator provides extremely high-resolution conversion without it. At high speeds, the 1.8 MS/s modulator combines with the fast-sampling ADC to provide continuous-sample digitizing. The DSP provides real-time sequencing, calibration, linearization, AC true-rms computing, and decimation, as well as the weighted noise filtering used for the DC functions.
The FlexADC has several advantages, as follow:
- The unique architecture of the FlexDMM offers a continuously variable reading rate from <5 S/s at 7 digits to 5 kS/s at 4 1/2 digits, as shown in Figure 4a
- The FlexADC can be operated as a digitizer with a sampling rate of up to 1.8 MS/s, as shown in Figure 4b
- Due to the custom sigma-delta modulator, noise shaping and digital filtering has been optimized for use in DMM and digitizer applications
- Unlike other ADC conversion techniques, it is not necessary to turn the input signal on and off; therefore, continuous contiguous signal acquisition is possible
- Direct AC voltage conversion and frequency response calibration are possible without the use of a conventional analog AC TRMS converter and analog "trimmers" for flatness correction
- Input signal noise can be dramatically reduced on all functions with appropriate noise-shaping algorithms (refer to the DC Noise Rejection section)
- Advanced host-based functions can be implemented with LabVIEW, once signals are digitized, leading to an almost endless list of signal characterization options (FFT, calculating impedances, AC crest factor, peak, AC average, and the like)
Figure 4a. FlexDMM DC Reading Rates
Figure 4b. FlexDMM Isolated Digitizer Sample Rates
Table 1 compares all four of these ADC architectures:
Table 1. ADC Architecture Comparison
Alternating current (AC) signals are typically characterized by rms amplitude, which is a measure of their total energy. Rms stands for root-mean-square; to compute the rms value of a waveform, you must take the square root of the mean value of the square of the signal level. Although most DMMs do this nonlinear signal processing in the analog domain, the FlexDMM uses an onboard digital signal processor (DSP) to compute the rms value from digitized samples of the AC waveform. The result is quiet, accurate, and fast-settling AC readings.
The rms algorithm used by the FlexDMM requires only four periods (cycles) of the waveform to obtain a quiet reading. For example, it requires a measurement aperture of 4 ms to accurately measure a 1 kHz sine wave. The advantage brought about by this technique extends to system performance. With traditional DMMs, it is necessary to wait for an analog TRMS converter to settle before a measurement can be made. With the FlexDMM, there is no TRMS converter to settle. The result is faster AC reading rates, and this advantage is realized in systems with switching, as Figure 5 shows:
Figure 5. FlexDMM AC Reading Rates
Virtual AC Coupling
The rms algorithm employed by the FlexDMM is largely insensitive to any DC component of the signal being measured. Thus, the AC coupling capacitor typically found on traditional DMMs that blocks the DC signal component is not always necessary using the FlexDMM. A coupling capacitor is available for situations where a very large DC offset must be blocked before digitization, but for applications without large DC components, such as AC power line and audio signals, the capacitor can be bypassed by using DC-coupled AC. No long time constant is associated with an input coupling capacitor, and so the AC settles very quickly. The importance of settling time is apparent in automated systems that scan through multiple devices under test (DUTs) or in multiple AC signal levels in a component such as a power supply.
The FlexDMM uses a digital filter to ensure AC accuracy for all frequencies up to the specified limits. This filter is factory calibrated for every AC range. Traditional precision DMMs use variable capacitors or DAC-driven RC feedback circuits to calibrate AC frequency response. The FlexDMM, because of the DSP-based algorithm, eliminates variable capacitors and other analog components, which could drift out of calibration. Eliminating these traditional "analog drift components" gives the FlexDMM exceptional temperature specifications and 2-year accuracies.
While the array of AC and high-speed capability available with the FlexDMM is impressive, no compromise was made in offering high-stability, metrology-class DC voltage and resistance functions. Several factors contribute to the FlexDMM achieving this performance:
- The availability and quality of miniature surface-mount high-performance, precision components has improved dramatically over the past 10 years
- Smaller, tightly laid out electronic packaging actually improves performance, especially thermal tracking between precision components
- The use of the FlexADC and DSP for AC voltage computation and frequency response calibration simplifies input signal conditioning into a common path, reducing components, complexity, and switching
- The lack of "Front-Rear" switch (common in box DMMs) simplifies the input layout, reduces crucial circuit signal path resistance and improves signal integrity
- The power supply consumes no space on the measurement module because it is built into the PXI chassis
Input Signal Conditioning
A major source of measurement error in most traditional DMMs is electromechanical relay switching. Contact-induced thermal voltage offsets can cause instability and drift. The FlexDMM eliminates all but one relay in the DC volts, AC volts, and resistance path. A special relay-contact configuration cancels the thermal errors in this single relay. This relay is switched only during self-calibration. All measurement-related switching for function and range changing is done with low-thermal-offset, highly reliable solid-state switching. Thus, electromechanical relay wear-out failures are all but eliminated. Figure 6 shows an overnight drift performance of the most sensitive range, the 100 mV range. Each division is 500 nV. For comparison, the same measurement made under identical conditions with a traditional 6 1/2-digit multimeter and a full-rack 8 1/2-digit multimeter is also shown in Figure 6:
Figure 6. FlexDMM 100 mV Range Stability with Shorted Input, Compared to a Traditional 6 1/2-Digit Multimeter and a Full-Rack 8 1/2-Digit Multimeter
Onboard Precision References
The FlexDMM employs some of the most stable onboard references available. As a voltage reference, the FlexDMM uses a well-known thermally stabilized reference that provides unmatched performance. This voltage reference is thermally shielded for optimum performance. The result is a maximum reference temperature coefficient of less than 0.3 ppm/ºC. Time stability of this device is on the order of 8 ppm/year. No other DMM in this price range offers this reference source and the accompanying stability. The FlexDMM offers a 2-year guarantee of accuracy.
Resistance functions are referenced to a single 10 k Ω highly stabilized metal-foil resistor originally designed for demanding aerospace applications. This component has a guaranteed temperature coefficient of less than 0.8 ppm/ºC and a time stability of less than 25 ppm/year.
Linearity is a measure of the "quality" of a DMM transfer function. It is important in conversion-component characterization applications to offer DNL and INL (integral nonlinearity) performance substantially better than that available in off-the-shelf ADCs. The Flex ADC is designed for excellent linearity, both DNL and INL. Linearity is also important because it determines the repeatability of the self-calibration function.
Figure 7 shows a typical NI 4070 linearity plot measured on the 10 V range from -10 to +10 V:
Figure 7. 10 VDC Range Linearity
Traditional DMMs are calibrated at a particular temperature, and this calibration is characterized and specified over a limited temperature range, usually ±5 ºC (or even ±1 ºC in some cases). Thus, whenever the DMM is used outside of this temperature range, its accuracy specifications must be derated by a temperature coefficient, usually on the order of 10 % of the accuracy specification/ºC. So 10 ºC outside of this specified range, you may have twice the specified measurement error, which can be a serious concern when absolute accuracy is important.
Unfortunately, keeping the environmental temperature of a precision instrument within ±5 ºC can be challenging in a production environment, or in a test system composed of multiple instruments, sources, and the like. Instruments in a system are subject to temperature rise caused by inherent compromises in air circulation and other factors.
If the excursions in temperature exceed these limits and tight specifications are required, then recalibration is also required at the new temperature. Take, for example, the 10 VDC range on traditional DMMs. A DMM may have an accuracy of:
1-year accuracy: (35 ppm of reading + 5 ppm of range) for T = 23±5 ºC
In this specification, if you apply 5 V to the input, the error is:
35 ppm of 5 V + 5 ppm of 10 V = 225 µV, for the temperature range 18 to 28 ºC
This is the traditional method of specifying accuracy. If the ambient temperature is outside of the 18 to 28 C range, the user needs to "derate" the accuracy using the temperature coefficient (tempco). With the traditional method, the only way to achieve the specified accuracy outside of the 18 to 28 ºC range is to fully recalibrate the system at the desired temperature. Of course, this process is often impractical and expensive. In the example above, if the DMM ambient temperature is 50 ºC, perhaps due to stacking of many instruments in the rack with limited airflow, and the tempco is specified as:
tempco = (5 ppm of reading +1 ppm of range)/ºC, then the additional error is:
22 ºC x tempco = (120 ppm of reading + 22 ppm of range) or 1045 µV total uncertainty. This error at 50 ºC ambient temperature is nearly five times worse than the specified 1-year accuracy.
Assuring PPM-Level Precision
To eliminate errors caused by these effects, the FlexDMM incorporates a proprietary self-calibration function for DC volts, resistance, and digitizer mode. This function is significant for the following reasons:
- The self-calibration function corrects for all signal-path gain and offset errors within the DMM back to the precision, high-stability internal voltage reference previously described.
- Self-calibration accounts for all resistance current source, gain, and offset errors. In resistance mode, all errors are corrected back to the single internal 10 kΩ precision resistor.
- Self-calibration takes one minute and fully recalibrates all ranges of voltage, resistance, and digitizer functions. In traditional DMMs, more than 10 minutes are required to perform this function.
The result is a highly accurate, ultra-stable DMM at any operating temperature, well outside of the traditional 18 to 28 ºC, with the use of self-calibration. For the example above, the additional error introduced by temperature coefficient using self-calibration would be fully covered in the 90-day and 2-year specifications and would be:
tempco with self-cal: < (0.3 ppm of reading + 0.3 ppm of range)/ºC
(already accounted for in the specification)
This error represents an enormous improvement in accuracy over the full operating temperature range of the DMM. Figure 8 summarizes these results:
Traditional 6 1/2 (1-Year)
NI PXI-4070 (2-Year)
|Measurement within 18-28 ºC
|Measurement at 50 ºC
|Measurement at 50 ºC
1045 µV (no self-cal available)
Figure 8. Example Summary - Uncertainty Analysis, Measuring 5 V on 10 V Range
Using the FlexDMM with Self-Cal provides accuracy at 50 ºC that is eight times better than traditional methods.
||Recalibrate time drift of
Corrects for AC flatness drift
|Every 2 years
||To full specifications
||6 1/2-digit precision
Recalibrates Measurement path and ADC
For VDC, resistance, digitizer
|90 days or for temperature change <5 ºC
||To specifications on VDC, resistance, and digitizer functions over FULL operating temperature range
Figure 9. Calibration Comparison
The FlexDMM has a full suite of resistance measurement features and offers both 2- and 4-wire resistance measurement capability. The 4-wire technique is used when long test cables and switching result in "test lead" resistance offsets that make measurements of low resistance difficult. However, there are situations when offset voltages introduce significant errors. For these situations, the FlexDMM offers offset-compensated resistance measurements, which are insensitive to offset voltages found in many resistance measurement applications, such as the following:
- Switching systems using uncompensated reed relays (uncompensated reed relays can have offset voltages greater than 10 µV caused by the Kovar lead material used at the device glass seal)
- In-circuit resistance measurements (for example, power supply conductors being measured for resistance, while the circuit under test has power applied)
- Measuring the source resistance of batteries, dynamic resistance of forward-biased diodes, and the like
In the first situation above, a test system is built with switching optimized for things other than resistance measurements. For example, reed relays are common in RF test systems because of their predictable impedance characteristics and high reliability. In such a system, it may be desirable also to measure resistances of units under test (UUTs), and the reed relays may already exist in the system.
In the second situation, an example would be measuring the resistance of a power supply bus wire with the power on (you should exercise great care when performing this type of test). With resistance in the range of 10 m, if there is 100 mA flowing through this resistance, the voltage drop is:
V = 100 mA x 10 m = 1 mV
A DMM without offset compensation in the 100 Ω range would interpret this as 1 Ω, because the DMM will think that this voltage is being generated by its internal 1 mA current source passing through the wire being measured. With Offset-Compensated Ohms enabled on the FlexDMM, the 1 mV offset is distinguished and rejected, and the correct value of resistance is returned.
Figure 10. First Cycle with Current Source ON
Figure 11. Second Cycle with Current Source OFF
This measurement involves two cycles. The first cycle is measured with the current source on, and the second cycle is measured with the current source off. The net result is the difference between the two measurements. Because the offset voltage is present in both cycles, it is subtracted out and does not enter into the resistance calculation, as shown below:
DC Noise Rejection
DC Noise Rejection is an exclusive NI feature available for DC measurements on the FlexDMM. Each DC reading returned by the FlexDMM is actually the mathematical result of multiple high-speed samples. By adjusting the relative weighting of those samples, the sensitivity to different interfering frequencies can be adjusted. Three different weightings are available - normal, second-order, and high-order.
When Normal DC noise rejection is selected, all samples are weighted equally. This process emulates the behavior of most traditional DMMs, providing good rejection of frequencies at multiples of f0 (where f0 = 1/taperture , the aperture time selected for the measurement). Figure 11 illustrates normal weighting and the resulting noise rejection as a function of frequency. Note that good noise rejection is obtained only very near multiples of f0.
Figure 11. Normal DC Noise Rejection
Second-order DC noise rejection applies a triangular weighting to the measurement samples, as shown in Figure 12. Note that very good rejection is obtained near even multiples of f0, and that rejection increases more rapidly with frequency than with normal sample weighting. Also note that the response notches are wider than they are with normal weighting, resulting in less sensitivity to slight variations in noise frequency. You can use second-order DC noise rejection if you need better power-line noise rejection than you can get with normal DC noise rejection but if you cannot afford to sample slowly enough to take advantage of high-order noise rejection. For example, you could set the aperture to 33.333 ms for a 60 Hz power-line frequency.
Figure 12. Second-Order DC Noise Rejection
Figure 13 illustrates high-order sample weighting and its resulting noise rejection as a function of frequency. Notice that noise rejection is good starting around 4 f0 and is excellent above 4.5 times f0. Using high-order rejection, there is almost no sensitivity to noise at any frequency above 4.6 times f0. A FlexDMM using high-order DC noise rejection with a 100 ms aperture (10 readings/s) can deliver full 6 1/2-digit accuracy at any frequency above 46 Hz with more than 1 V of interfering power-line noise on the 10 V range. This performance is equivalent to >110 dB normal-mode rejection, insensitive to variations in power line frequency.
Figure 13. High -Order DC Noise Rejection
Table 2 summarizes the differences between the three DC noise rejection settings, as follow:
Table 2. DC Noise Rejection Settings
|DC Noise Rejection Setting
||Lowest Frequency for Noise Rejection
||High-Frequency Noise Rejection
||Best >110 dB rejection
Traditional DMMs use multiple shunts, switching, and the like to achieve dynamic range on the current function. The FlexDMM uses a remarkable high-power precision shunt resistor coupled with a high-gain, ultra low-noise, signal-conditioning amplifier. The result is an uninterrupted measurement path for testing currents drawn by electronic devices at full load and during device standby - offering a dynamic range from 1 A to 10 nA, with virtually no settling tails due to self-heating or thermals.