Record Length, Vertical Range and Offset, Input Coupling, and Probe Effect
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Return to Fundamentals of High-Speed Digitizers
This tutorial recommends tips and techniques for using National Instruments high-speed digitizers to build the most effective data sampling system possible. In this tutorial, you will learn fundamental information about the underlying theory of sampling with a high-speed digitizer and various methods to optimize the performance of your data sampling. This section of the tutorial covers the topics below.
Table of Contents
Record Length and Deep Memory
Record length refers to the amount of memory dedicated to storing digitized samples for postprocessing or display for a single acquisition. In a digitizer, record length limits the maximum duration of a single-shot acquisition. For example, with a 1,000-sample record and a sample rate of 20 MHz, the duration of the acquisition is 50 µs (the number of points multiplied by the acquisition time per sample, or 1,000 x 50 ns). With a 100,000-sample record and a sample rate of 20 MHz, the duration of acquisition is 5 ms (100,000 x 50 ns).
In many cases, measurement quality depends on the digitizer's ability to take a sustained acquisition while maintaining high sampling rates. In these cases, the amount of acquisition memory determines the fidelity of the acquired signal. High-speed digitizers with deep onboard acquisition memory have the ability to take enhanced time and frequency-domain measurements. For more information, refer to Benefits of Deep Memory in High-Speed Digitizers.
Vertical Range and Offset
Vertical range is the peak-to-peak voltage span that a digitizer can measure at the input connector. Most digitizers have several choices for vertical range.
Vertical offset is the voltage the vertical range is centered on. Vertical offset positions a waveform around a DC value. Using this offset allows you to examine small changes in the input signal, which can improve the accuracy of your measurement.
For example, imagine that you are acquiring the waveform shown in Figure 1 that outputs 0.75 V to 1.25 V. Without using vertical offset, you would need to specify a range of 2.5 V (±1.25 V) to capture the waveform. However, with vertical offset, you would only need to specify a range of 0.5 V (1.25 V - 0.75 V).
On many digitizers, you can configure the input channels to be DC coupled, AC coupled, or GND coupled. 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. Activating AC coupling inserts a capacitor in series with the input. This feature can be exploited to zoom in on AC signals with large DC offsets, such as switching noise on a 12 V power supply. GND coupling disconnects the input and internally connects the channel to ground to provide a ground, zero-voltage reference.
Refer to the specifications for your specific digitizer for input limits that must be observed regardless of coupling.
See Also:
Input Coupling Selector
Coupling with DMMs
Probes and Their Effect
Probes work with digitizers as part of your measurement system. Signals travel from the tip of the probe to the input of the high-speed digitizer and are then digitized by the ADC. This signal path makes the probe an important electrical system component that can affect the accuracy of the measurement. A probe can potentially influence measured amplitude and phase, and the signal can pick up additional noise on its way to the input stage. Although NI high-speed digitizers do not ship with probes, several types of probes are available, including passive, active, and current probes.
Passive Probes
The passive probe is the most widely used general-purpose probe. Passive probes are specified by bandwidth (or rise time), attenuation ratio, compensation range, and mechanical design aspects. Probes with attenuation X10, X100, or X1000, have a tunable capacitor that can reduce capacitive effects at the input. The ability to cancel or minimize effective capacitance improves the probe’s bandwidth and rise time. Figure 2 shows a typical X10 probe model.
Figure 2. Passive Probes
Adjust the tunable capacitor, Cp, to obtain a flat frequency response. Cp is the probe capacitance, Rp is the probe resistance, Cin is the input capacitance, Rin is the input resistance of the digitizer.
Analytically, obtaining a flat frequency response means that
Rin/(Rin + Rp) = Cp/(Cp + Cin + Cc)
When tuned for flat response, it can be shown that
Rin(Cin + Cc) = CpRp
or the time constant of the probe equals the time constant of the digitizer input.
Active Probes
Active probes, such as differential and field-effect transistor (FET) probes, contain active circuitry in the probe itself to reject noise and amplify the signal. FET probes are useful for low-voltage measurements at high frequencies and differential probes are noted for their high common-mode rejection ratio (CMRR) and nongrounded reference.
Current Probes
Current probes magnetically measure AC and/or DC current flowing in a conductor instead of using a series resistance in the loop to measure current. This lack of series resistance causes very little interference in the circuit being tested.
Probe Compensation
To maximize the bandwidth of the attenuating probes, the probe capacitor must be adjusted such that the input capacitance of the digitizer is exactly canceled. Precise adjustment of the tunable probe capacitor to get a flat frequency response is called probe compensation. Refer to the Figure 3 below as you perform the following probe compensation procedure:
Figure 3. Probe Compensation Illustration
1. Connect the BNC end of the probe to CH 0 and select X10 attenuation on the body of the probe tip.
2. Attach the BNC adapter (probe accessory) to the tip of the probe.
3. Connect the SMB 100 probe-compensation cable to PFI 1.
4. Attach the probe with the BNC adapter to the BNC female end of the SMB 100 cable.
5. Open Scope Soft Front Panel (Start Menu->Programs->National Instruments->NI-SCOPE->SCOPE Soft Front Panel).
6. Select the appropriate digitizer for probe compensation.
7. Activate the probe compensation signal from the Utility menu in the Scope Soft Front Panel toolbar. The probe compensation signal can also be activated programmatically using the NI-SCOPE instrument driver.
Figure 4. Probe Compensation Signal in NI-Scope
8. Adjust the tunable capacitor to make the waveform as square as possible.
Figure 5. Various Compensations of Probe Compensation Signals
9. Repeat steps 1-8 for CH 1 and the other probe.
For the most accurate measurements, compensate probes for each channel (CH 0 and CH 1) and use them on that channel only. Recompensate when using the same probe on a different channel.
See Also:
Oscilloscope Probes
Related Links:
Fundamentals of High-Speed Digitizers
Getting Started with NI-SCOPE
Differential Measurements Using High-Speed Digitizers
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