Isolated Digitizer Measurements

Publish Date: Feb 03, 2017 | 3 Ratings | 5.00 out of 5 | Print | Submit your review

Overview

In this tutorial, you learn how the NI 4070 can operate as both a 6½ digit digital multimeter and a fully isolated, high-voltage digitizer, capable of acquiring waveforms at sample rates up to 1.8 MS/s at ±300 V input.

For more information return to the Digital Multimeters.

Table of Contents

  1. Waveform Acquisition Defaults
  2. NI 4070 Waveform Acquisition Measurement Cycle
  3. Analog Bandwidth
  4. Nyquist Theorem
  5. Overranging
  6. Input Coupling
  7. Sample Rate
  8. DC Noise Rejection for Waveforms
  9. Transient Measurement Considerations
  10. Current Waveforms

1. Waveform Acquisition Defaults

The following table lists the default waveform acquisition settings for the NI 4070 Digital Multimeter.

Waveform Function
AutoZero
ADC Calibration
DC Noise Rejection
Voltage OFF OFF Normal
Current OFF OFF Normal


Waveform Default Settle Times

Function
Settle Time
V (100 mV - 10 V) 1 ms
V (100 V, 300 V) 2 ms
I 100 µs

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2. NI 4070 Waveform Acquisition Measurement Cycle


The NI 4070 Digital Multimeter waveform measurement cycle is made up of several phases: Settle time, waveform acquisition phase, AutoZero, and ADC Cal. Refer to the figure below for relative timing of these phases. The length of the waveform measurement phase is determined by the selected sample rate and number of points. Generally the settling time is selected by the device driver based on the specified function and range.

An internal switch time is required to configure the analog circuitry of the NI 4070 Digital Multimeter for the next measurement. There is also default settle times preceding both ADC calibration and AutoZero . These settle times are not user programmable and are optimized in the design.

When you enable ADC calibration, gain drift errors are removed from the waveform data. At higher speeds, the lower resolution measurements make ADC calibration unnecessary.

AutoZero removes offset from the waveform data. An AutoZero measurement is taken and stored. This offset is subtracted from each point.

Waveform Acquisition Time
The minimum and maximum times for waveform acquisitions are 8.89 µs and 149 s respectively. To determine if you settings meet the required time, use the following formula:

8.89 µs < n/r < 149 s

where
n equals the number of samples

r equals the rate

For example, if you attempt to acquire two samples at a rate of 1.8 MS/s, the result is:

2/1,800,000 S/s = 1.11 µs

1.11 µs is less than the minimum aperture time, and the driver returns an error. Therefore, you must acquire a minimum of 16 samples at the rate of 1.8 MS/s.

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3. Analog Bandwidth


Analog bandwidth describes the frequency range (in Hz) in which a signal can be acquired accurately. This limitation is determined by the inherent frequency response of the input path, which causes loss of amplitude and phase information. Analog bandwidth is the frequency at which the measured amplitude is 3 dB below the actual amplitude of the signal. This amplitude loss occurs at very low frequencies if the signal is AC coupled and at very high frequencies regardless of coupling. When the signal is DC coupled, the bandwidth of the amplifier extends all the way to the DC voltage. The following figure illustrates the effect of analog bandwidth on a high-frequency signal.



As indicated in the specifications, the NI 4070 Digital Multimeter has a voltage measurement bandwidth of 300 kHz. This specification means that only signals 300 kHz can be accurately acquired. Note that this number is different than the maximum sampling rate of the NI 4070 Digital Multimeter (1.8 MS/s). The Nyquist Theorem implies that the maximum signal frequency that can be effectively acquired is 900 kHz. However, the bandwidth actually limits the maximum frequency to 300 kHz. Refer to the section on  Sample Rate for more information.

Bandwidth Considerations
The resistance (ohms) and the capacitance present in the signal path can affect high frequency signals. The larger the capacitance and the resistance (ohms), the lower the bandwidth of the signal path. The NI 4070 Digital Multimeter input capacitance can be found in the specification. Note that additional capacitance can be introduced in the cabling that is used, and any other front-end accessories such as switches. This capacitance not only impacts the performance of the waveform acquisition, but can also impact the source of the signal by loading it. Having a low impedance source and minimal added capacitance is best for optimal operation when acquiring high frequency signals. Use short cable lengths of a low-capacitance, low resistance (ohms), and low dielectric absorption for best results.

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4. Nyquist Theorem


The Nyquist theorem states that a signal must be sampled at least twice as fast as the bandwidth of the signal; otherwise, aliased products in the band of interest can distort the signal, causing loss of information. An alias is a false lower frequency component that appears in sampled data acquired at too low a sampling rate. The following figure shows an 800 kHz sine wave digitized by a 1 MS/s ADC. The dotted line indicates the aliased signal recorded by the ADC at that sample rate.



The 800 kHz frequency aliases back in the passband, falsely appearing as a 200 kHz sine wave. To prevent aliasing in the passband, a lowpass filter limits the frequency content of the input signal above the Nyquist rate. The analog bandwidth of the NI 4070 Digital Multimeter is 300 kHz and you can avoid aliased signals in that passband by sampling at the full rate of 1.8 MS/s.

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5. Overranging


The NI 4070 Digital Multimeter sigma-delta modulator has a characteristic overload recovery behavior that you must be aware of when selecting the range of measurement for waveform acquisition. Select the input range carefully, so that the acquired signals, including instantaneous peaks, are within range at all times. For instance, when the NI 4070 Digital Multimeter is configured for the 10 V input range, the signal must remain within +/- 10.5 VDC (21 VP-P). Signals applied to the NI 4070 Digital Multimeter that are outside of the input range cause the sigma-delta modulator within the ADC to enter into an overrange condition. In this condition, the software returns the value of NaN (Not a Number) for all the measurements during the time that the signal is outside the range. During recovery from an overrange condition, the NI 4070 Digital Multimeter may return several invalid data points once the signal recovers to within the range limits. To prevent the ADC from being saturated, it is imperative that the signal being measured is always within the input range limits. When the input signal is outside the limits, the niDMM ReadWaveform and niDMM FetchWaveform functions return an overrange warning. If your application encounters this warning, you should consider your data to be corrupted, and you should acquire the data again with the appropriate input range selections.

We recommend starting with the highest available range first before settling on the range of measurement. By doing this, it is possible to get a "preview" of the measured waveform while avoiding transient overloads caused by slew rate limitations. This is also a good way to determine the signal peaks before zooming in with a more sensitive range.

For the NI 4070 Digital Multimeter, the frequency response is similar across all ranges, so there is no loss of signal integrity by using this approach. If the signal is small, some resolution is lost. However, once you identify this small signal and loss of resolution, you can carefully select a more sensitive range.

The following figure shows the NI 4070 Digital Multimeter waveform acquisition overrange behavior. The dotted line represents the actual voltage waveform signal at the input terminals, and the solid line represents the data returned by a waveform acquisition using an NI 4070 Digital Multimeter in 1 V range. The discontinuities in the solid line waveform represent NaN values in the data, and the spikes in the solid line represent the invalid data as the signal returns to the range limit.

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6. Input Coupling


The input channels can be configured for DC coupling or AC coupling. DC coupling allows DC and low-frequency components of a signal to pass through without attenuation. In contrast, AC coupling removes DC offsets and attenuates low frequency components of a signal. Use AC coupling to zoom in on AC signals with large DC offsets, such as switching noise on a 12 V power supply. Refer to the specifications for input limits that must be observed regardless of coupling.

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7. Sample Rate


Sample rate is the rate at which a signal is sampled by an analog-to-digital converter. A higher sample rate captures more waveform details while a sample rate that is too low can distort the waveform.

The figure below illustrates a 100 kHz sine wave sampled at 500 kS/s:



The figure below shows the same sine wave sampled at 1.8 MS/s:



The faster rate digitizes 18 points per cycle of the input signal compared with 5 points per cycle with the slower ADC. In this example, the higher sample rate more accurately captures the waveform shape as well as frequency. This process is called over-sampling and can greatly improve the quality of the waveform measurement. The NI 4070 Digital Multimeter allows for 6 times over-sampling when acquiring a voltage waveform at the limit of its analog bandwidth of 300 kHz.

When performing a waveform acquisition, select a sampling rate that observes the Nyquist Theorem . Excessive over-sampling has tradeoffs. First, excessive over-sampling gives you more data than you may need, unnecessarily consumes processor resources, and can slow the display. Second, additional noise becomes apparent since the noise performance of the NI 4070 Digital Multimeter improves as your sampling rate decreases.

Selecting a sampling rate is typically easy to determine. For example, 44.1 kHz and 48 kHz are standard sample rates for audio signals. The NI 4070 Digital Multimeter can sample these rates at 45 kS/s and 50 kS/s, respectively, while 24 kS/s is available for low-bandwidth audio applications. Most mid-bandwidth applications, such as audio, can benefit from the use of DC noise rejection to reduce noise and improve frequency response.

The NI 4070 Digital Multimeter can make very-high-resolution low-distortion audio measurements, but it does not have a low pass filter lower than the bandwidth limitation to provide the alias protection of most audio products. To obtain the flattest response and alias protection with the NI 4070 Digital Multimeter, sample at the maximum rate of 1.8 MS/s and post-process (filter and decimate) in software. You can use the Analysis functions in LabVIEW or CVI to filter the acquired signal.

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8. DC Noise Rejection for Waveforms


The DC Noise Rejection feature that can reduce noise in DC measurements in DMM mode can also be used to alter the noise and frequency response of waveform acquisitions. All three settings of the DC Noise Rejection parameter are available, and they may be selected by using a property node, just as is the case for DMM-mode readings.

Only Normal DC Noise Rejection is available for sampling rates above 200,000 S/s.

Normal
Normal DC Noise Rejection gives notches in the frequency response at multiples of the sampling rate, and the response is attenuated by 4 dB at half the sampling rate, in addition to any attenuation present at the maximum sampling rate of 1.8 MS/s. For example, with a selected sampling rate of 1 kS/s, response is down 4 dB at 500 Hz, shown in the first following graph, and is 0 at multiples of 1 kHz as shown in the second following graph:




2nd Order
2nd order DC Noise Rejection gives notches in the frequency response at even multiples of the sampling rate, and the response is attenuated by 1.9 dB at half the sampling rate, in addition to any attenuation present at the maximum sampling rate of 1.8 MS/s. For example, with a selected sampling rate of 1 kS/s, response is down 1.9 dB at 500 Hz, shown in the first following graph, and is 0 at multiples of 2 kHz as shown in the second following graph:




High Order
High order DC Noise Rejection gives high attenuation to frequencies above about 4 times the sampling rate, and the response is attenuated by 0.7 dB at half the sampling rate, in addition to any attenuation present at the maximum sampling rate of 1.8 MS/s. For example, with a selected sampling rate of 1 kS/s, response is down 0.7 dB at 500 Hz, shown in the first following graph, and is very small at frequencies above 4 kHz as shown in the second following graph:




Other Considerations
The default DC Noise Rejection selection is Normal, and it gives the lowest noise at very low frequencies. However, it does not reject out-of-band signals very well and has considerable attenuation at half the sample rate. It also yields a non-flat noise spectrum due to the notches at multiples of the sample rate. 2nd order DC Noise Rejection gives the lowest overall noise over a broad range of sampling rates, roughly from 500 S/s to 100 kS/s. It does a better job rejecting out-of-band signals, while reducing in-band attenuation, and it yields a flat noise spectrum. High-order DC Noise Rejection offers the flattest in-band frequency response and is useful for rejecting strong out-of-band noise signals. It also yields a flat noise spectrum. However, it yields higher overall noise than 2nd order DC Noise Rejection.

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9. Transient Measurement Considerations


Cabling
When using the NI 4070 Digital Multimeter to acquire waveforms, you should choose your connecting cable carefully. Any unshielded cable or test leads can pick up common electromagnetic and electrostatic interference which corrupts waveform measurements and introduces spurs in the frequency domain. Shielded coaxial cables are often used for waveform measurement. You can use a female-BNC-to-dual-banana-plug adapter, such as Pomona P/N 1269, to easily connect the NI 4070 Digital Multimeter inputs to signal sources using coaxial shielded cable. National Instruments recommends the Belden 83317 shielded cable. This cable works well for DMM measurements, within the 300 kHz bandwidth (3 db) of the NI 4070 Digital Multimeter, and in situations where BNC connectivity is not required.

There are two alternative ways to connect the shield. If high frequency (> 1 kHz) common-mode AC signals are present, better results can be obtained if the shield is connected to the DMM LO terminal. If no common-mode AC signal is present or if the common-mode AC signal is low (< 100 mV), then connecting the shield to PXI chassis ground is preferable. The following figures show the DMM connected to a device under test (DUT) with the Belden 83317 cable.

As shown in the figure below, the best HI signal shielding results with a common-mode AC signal > 1 kHz. The shield potential must remain within 42 VAC/DC of LO:



As shown in the figure below, good shielding results if common-mode AC signals are less than 100 mV, and the shielding is safest if a large DC common-mode voltage is present.



Caution  Always observe proper safety practices when connecting to signals. Be aware that if the LO signal is connected to a source of hazardous voltage (either AC or DC) then any exposed BNC connector shells or shields connected to this voltage will also be at this hazardous voltage.


Slew Rate Limitations
The NI 4070 Digital Multimeter can measure almost any signal within its 300 kHz bandwidth. However, some signals may have enough high-frequency energy to introduce significant distortion. In general, the rate of change of the input signal voltage or current (amps), also known as its slew rate, should stay below the following limits to avoid excessive distortion:

Input Range
Maximum Slew Rate
300 V 2000 V/µs
100 V 200 V/µs
10 V 20 V/µs
1 V 2 V/µs
100 mV 0.4 V/µs
1 A 8 A/µs
200 mA 0.8 A/µs
20 mA 0.8 A/µs


If you suspect your slew rate may be too high, you can switch to a higher range to see if the waveform shape changes significantly.

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10. Current Waveforms


The NI 4070 Digital Multimeter can acquire current (amps) waveforms at frequencies up to 400 kHz. At high frequencies, the apparent burden voltage increases from that listed in the specifications due to internal inductance, which is approximately 5 µH. The increase in burden voltage is noticeable for frequencies above 30 kHz, depending on the current (amps) being measured. Apparent burden voltage is even higher when using long cable runs, which add more inductance and resistance (ohms). Minimize cable length or use a shunt resistor local to the DUT if wide bandwidth current (amps) waveform acquisition is desired. The following figure shows the burden voltage effect due to inductance:

Total VBURDEN AC = I*(R + RFUSE + 2ΠF(LIN + LLEAD1 + LLEAD2))

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