NI Oscilloscope and Digitizer devices and modules have specifications for analog signals. These specifications can be unique for each device or model, so be sure to check the specifications for your specific model. This section is organized in six sections to cover the common specifications: Impedance and Coupling, Voltage Levels, Accuracy, Bandwidth and Transient Response, Spectral Characteristics, and Noise.
Impedance and Coupling
Digitizer inputs have parasitic capacitance that can potentially alter the signal being measured. A probe with an adjustable capacitance can be used to compensate for the input capacitance and achieve a flat frequency response. When frequencies are low, the capacitance has a very high reactance which does not cause meaningful loading. However, as the frequency increases, the loading becomes much greater due to a decrease in the probe's impedance.
Figure 1 shows a correctly compensated, undercompensated, and overcompensated probe for low frequencies. It should be noted that the cable capacitance is also compensated when the probe is compensated.
The PXIe-5172 has a typical input capacitance of 16 pF ± 1.2 pF for 1 MΩ input impedance. The probe used needs to be able to compensate from 14.8 pF to 17.2 pF for best results
You can specify an input channel to be DC-coupled, AC-coupled, or ground coupled. DC-coupling allows DC and low frequency components of a signal to pass through without being attenuated. AC-coupling removes DC offsets and low frequency components, only allowing high frequency signals. Ground coupling disconnects the input and internally connects the channel to ground to provide a ground, zero-voltage reference.
In Figure 2, Plot A shows DC-coupling that shows the waveform with the DC portion included. Plot B shows the same waveform with AC-coupling that removes the DC component.
The PXIe-5164 has both AC and DC coupling to allow both low and high frequency signal measurements.
Input impedance is a measure of how the input circuitry impedes current from flowing through to analog input ground. For NI Oscilloscopes, the common input impedance is 50 Ω or 1 MΩ.
Typically, the 1 MΩ, or high Z, impedance is used with a probe for high voltage measurements. For some applications, such as RF, 50 Ω input impedance is used to match the source impedance to minimize reflections that can distort the signal being measured.
The PXIe-5164 has both 50 Ω and 1 MΩ input impedances. Choosing the appropriate input impedance is important for taking good measurements.
Select the Right Oscilloscope Probe for Your Application: Loading Effects
Input Return Loss
Input return loss indicates the decrease in power of a signal that is reflected due to mismatched impedances. The equation for return loss can be found below.
The PXIe-5162 has an input return loss of roughly -20 dB for a frequency of 1 GHz. This means that a signal 20 dB less than the input signal is reflected due to impedance mismatch.
Input Voltage Standing-Wave Ratio (VSWR)
VSWR is the ratio of reflected vs. transmitted waves. VSWR can be used to determine how much of the input signal is being reflected. Depending on whether the reflected wave is in or out of phase with the input signal, it could either increase or decrease the net amplitude. VSWR is the ratio of this maximum net amplitude and minimum net amplitude. Different ways to find VSWR are shown in the equations and figures below.
The PXIe-5162 has a VSWR of roughly 1.1 at 500 MHz. This gives the ratio of the maximum amplitude and minimum amplitude due to the phase of the reflected signal.
Chapter 1: Understanding Key RF Switch Specifications: Voltage Standing-Wave Ratio
Input offset, or vertical offset, is the voltage on which the voltage range is centered. Vertical offset positions the vertical range around a user-defined DC value. Using this offset allows for examination of small changes in the input signal, which can lead to improved measurement accuracy.
Figure 6 shows the relationship between the input range and offset and how it can affect the resolution.
The PXIe-5164 has an input offset of ±5 V for the 0.25 V input range. This allows the oscilloscope to measure a signal that is carried by a ±5 DC voltage to fully maximize the ADC's performance.
Input range, or vertical range, is the peak-to-peak voltage span that a digitizer can measure at the input connector. Many digitizers have multiple vertical ranges to maximize the ADC's performance and get better resolution.
The PXIe-5171 has input ranges of 0.2 Vpp, 0.4 Vpp, 1 Vpp, 2 Vpp and 5 Vpp. A user has these five ranges to choose from to get the best possible resolution. If the signal being measured is 0.5 Vpp, it would be best to use the 1 Vpp range rather than the 5 Vpp range to maximize the ADC.
Maximum Input Overload
This is the maximum input voltage that a device can handle. Exceeding this voltage may cause damage to the device.
The PXIe-5162 has a maximum input overload of |Peaks| ≤ 42 V for 1 MΩ. The maximum input range for the device is 50 Vpp with a ±15 V offset. This means input signals with peaks of ±40 V can be measured and still be within the input overload specification of 42 V.
AC Amplitude Accuracy/Frequence Response
Since real-world amplifiers cannot provide ideal performance, the gain is a product of the input frequency. Frequency response gives the magnitude response of a signal over a range of frequencies.
Consider a signal of 1.25 GHz acquired on the PXIe-5162 with a 50 Ω input impedance and 1 Vpp range, the equation above for AC amplitude accuracy and Figure 7, the frequency response graph, can be used to find the typical accuracy. Figure 7 shows an amplitude of roughly 2 dB for 1.25 GHz signal, taking the accuracy as 2 dB ± 0.5 dB, or the range 1.5 dB to 2.5 dB.
AC Amplitude Drift
Like DC drift, the AC amplitude can drift due to a change in temperature from the last calibration.
The amplitude drift of the PXIe-5162 can be found using the equation from its specification document. Taking the example from the AC Amplitude Accuracy section, if the board temperature is now 5 degrees from the calibrated temperature, the AC amplitude now falls within the range 1.48 dB to 2.52 dB. The AC amplitude accuracy changes by just 0.02 dB because the PXIe-5162 only takes temperature changes beyond ± 3·°C into account for AC amplitude drift.
Crosstalk is the measure of how much a signal on one channel can affect another channel. Ideally, acquiring a signal on one channel should not affect another signal being acquired, but this is not always the case due to unwanted conductive, capacitive, or inductive coupling from one part of the digitizer to another.
The Channel-to-Channel Crosstalk table from the PXIe-5162 specification document shows that if a signal of 50 MHz is being sampled with an input impedance of 50 Ω, there is a characteristic isolation of -60 dB between channels when both are set to the same input range.
Accuracy determines how close the value given by the digitizer will be to the actual signal.
The equation below shows the DC accuracy specification from the PXIe-5162 specification document. An example of calculating the DC accuracy of a signal that is 0.6 V with 0 V vertical offset and a full-scale voltage of 1 V is shown below.
DC drift is used to find the accuracy of the digitizer when the onboard temperature of the device is more than ±X °C since it was last calibrated.
X varies from device to device; on the PXIe-5162 it is 3 °C. The equation above shows the DC drift spec from the PXIe-5162 Specification document. An example of calculating the DC drift of a 0.6 V signal with a 0 V vertical offset, full-scale voltage of 1 V, and a difference in onboard temperature of 5 ℃ from the last calibration is shown below.
Resolution is the smallest input voltage change a digitizer can ideally capture. Resolution can be expressed in bits (LSB), in proportions, or in percent full scale. Table 2 gives a few examples.
Resolution limits the precision of a measurement. The higher the resolution (number of bits), the more precise the measurement. An 8-bit ADC divides the vertical range of the input amplifier into 256 discrete levels. With a vertical range of 10 V, the 8-bit ADC cannot ideally resolve voltage differences smaller than 39 mV. In comparison, a 14-bit ADC with 16,384 discrete levels can ideally resolve voltage differences as small as 610 µV.
Figure 8 shows a sine wave measured by a 3-bit ADC and a 16-bit ADC.
The PXIe-5160 has a 10-bit resolution, which gives it 1,024 discrete levels. Given a 10 V range, this allows the device to measure changes as small as 9.77 mV. In comparison the 3-bit resolution in Figure 8 can only measure changes of 1.25 V.
Bandwidth and Transient Response
The AC-coupling cutoff gives the -3 dB point of the high-pass filter when using AC coupling.
The PXIe-5162 has a cutoff of 170 kHz for 50 Ω and 17 Hz for 1 MΩ. When using 1 MΩ input impedance with AC coupling, frequencies below 17 Hz will be attenuated by more than 3 dB.
Bandwidth is defined as the point when the measured signal’s power is half of the original signal’s power. When working with a voltage signal, the -3 dB point is when the measured voltage is times the original. Since NI Digitizers pass DC, the bandwidth is specified as the uppermost frequency that can be measured before the signal hits times the original value.
The PXIe-5162 has a bandwidth of 1.5 GHz in the 50 Ω input impedance setting and 300 MHz in the 1 MΩ setting. The warranted 50 Ω case indicates that the input signal will not be attenuated to 70.7% when the frequency of the input signal is under 1.5GHz. Figure 9 shows that the -3 dB point is roughly 150 MHz for a particular digitizer.
Acquiring an Analog Signal: Bandwidth, Nyquist Sampling Theorem, and Aliasing
Bandwidth filters are used to filter out undesirable spurs and noise to achieve improved resolution on the input signal. These filters can be thought of as low-pass filters that are used to reject unwanted high frequency content associated with the input signal. These filters can be analog or digital.
The PXIe-5162 has two bandwidth limiting filters, 20 MHz and 175 MHz. When measuring a 15 MHz signal, the 20 MHz filter would be used to keep out unwanted high frequencies. For a signal of 150 MHz, the 175 MHz filter can be used.
Frequency response gives the magnitude response of a signal over a range of frequencies. This shows how the magnitude of input frequencies will vary for each oscilloscope.
Figure 10 shows the frequency response of the PXIe-5105 in the 50 Ω, 1 Vpp input range at full bandwidth with the anti-alias filter enabled.
Effective Number of Bits (ENOB)
ENOB is a specification that relates measurement or generation performance of a device to a common specification used in data converters: bits of resolution. Most data converters are designed to perform at a particular speed and resolution. Instrument vendors have always used this design element to define the measurement resolution of their devices. No instrument is ideal, so performance specifications show how close a device is to ideal. An ADC may specify a certain number of bits, but noise may add measurement uncertainty that exceeds the precision those bits could ideally achieve. For example, a 14-bit ADC may only have 12 usable bits: this is the ENOB of the device. ENOB is calculated directly from SINAD, discussed below, using values for ideal ADC noise and spurs as shown in the equation below. This calculation shows how close to an ideal instrument the device is performing. ADC performance declines as the input frequency increases due to high-frequency distortions; this causes ENOB to diminish with increasing input frequency.
The PXIe-5172 has an ENOB of 11.8 bits when the 20 MHz filter is enabled in the 5 V range. Even though it is a 14-bit product, under those conditions it has an ENOB of 11.8
SINAD is the ratio of signal power, including noise and distortion power, to noise and distortion power alone. An instrument with high SINAD can discern the fundamental frequency from spurs and noise better than an instrument with a low SINAD. The most useful approximation of SINAD is shown in the equation below.
Using ENOB, we can calculate SINAD. Using the example and equation in the ENOB section above gives the PXIe-5172 a SINAD of 72.796 dB.
Understanding Frequency Performance Specifications
Signal-to-Noise Ratio (SNR)
SNR, usually given in dB, is the ratio of the power of the input signal level to the noise power. The greater the SNR of a device, the better its ability to differentiate between the signal and noise, especially when the input signal has a low amplitude.
Based on the single-tone spectrum of the PXIe-5162 as shown in Figure 11, SNR can be approximated as 91 dB.
The single-tone spectrum uses a pure tone input signal to represent an oscilloscope's overall spectral performance.. A single-tone spectrum provides a good approximation of spectral characteristics such as THD, SNR, and so on for a specific configuration.
Figure 12 displays the single-tone spectrum of the PXIe-5162 when tested with an input signal of approximately 300 MHz.
Spurious-Free Dynamic Range (SFDR)
Spurious free dynamic range (SFDR), usually expressed in dBc, is the usable dynamic range before spurious noise interferes with or distorts the fundamental signal. The amplitude of the fundamental signal is usually -1 dBFS. SFDR is the measure of the ratio in amplitude between the fundamental signal and the largest harmonically or non-harmonically related spur from DC to the full Nyquist bandwidth (half the sampling rate). A spur is any frequency bin on a spectrum analyzer, or from a Fourier transform, of the analog signal above the noise floor. A device with a high SFDR can measure a signal with less effects from noise and spurs.
The PXIe-5171 has a characteristic SFDR of -70 dBc or better, depending on filters, voltage range, and input frequency. This means that the highest noise spur is at least 70 dBc below the fundamental frequency.
Total Harmonic Distortion (THD)
The THD of a signal is the ratio of the sum of the powers of the first five harmonics to the power of the fundamental frequency. The equation below shows the calculation for THD, where H is the amplitude of each harmonic and F is the amplitude of the fundamental frequency.
The PXIe-5172 has a THD of -77 dBc for an input frequency of 30 MHz or less in the 5 V input range.
Understanding Frequency Performance Specifications
All voltage and frequency components that are not present in the actual or ideal signal, spurs, or harmonics, but are present in the measurement of signals, are noise. Input signals not only carry the ideal signal that needs to be measured, but also contain noise. The noise floor is the amplitude of any noise in the device’s frequency range. RMS noise indicates the noise that can be seen depending on factors such as input impedance and input range. The RMS noise of an oscilloscope is the noise present without an input signal and measured with a 50 Ω terminator.
The RMS noise of the PXIe-5164 configured for 50 Ω of input impedance and 5 V input range is 0.030.