Record Length, Vertical Range and Offset, and Input Coupling


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.


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. 


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).


Figure 1. AC coupling will remove any DC offset of a signal allowing you to use a smaller vertical range and collect higher resolution data.



Input Coupling

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:
Coupling with DMMs