Agilent Measurement Journal - Issue 4 - 2008 - (Page 61)

S Selecting the best digitizer solution for any application is a complex task. At a minimum, it involves an in-depth evaluation of what must be measured, how it can be measured and the required degree of accuracy. These factors must then be used in the comparison of a multitude of available acquisition schemes and the associated individual device speciﬁcations. Digitizer manufacturers typically present a device’s “banner speciﬁcations” such as bandwidth, resolution and sampling rate. Although these are offered as an indication of instrument quality, they often have little or no impact on the ultimate measurement ﬁdelity in many applications. There is no simple answer to the question, “Which digitizer should I use?” Instead, each application and device must be reviewed on a case-by-case basis. No system is ideal: It will add noise to the analog signal being sampled, create signal distortion (harmonics) and suffer from some amount of clock jitter. These elements and more contribute to system performance and should be reviewed within each set of device speciﬁcations. Figure 1. Example of high-speed PCI digitizer that carries detailed performance speciﬁcations. Understanding accuracy and resolution Resolution is intended to deﬁne the ﬁneness of detail that can be distinguished. Usually quoted as a number of bits, it indicates the number of discrete levels used in the encoding of the input signal. As such, a resolution of 8, 10, 12 or 14 bits should represent the ability to distinguish one part in 256, 1,024, 4,096 or 16,384, respectively (Figure 2). 1-bit 2-bit 3-bit 4-bit 5-bit 6-bit 7-bit 8-bit 9-bit 10-bit Comparing devices Great care should be taken when comparing devices between manufacturers because they often use different test procedures and limits. For example, one manufacturer may quote a harmonic distortion value that includes the ﬁrst six harmonics while another manufacturer includes only the ﬁrst ﬁve, thereby producing a lower distortion value. In such cases it is unclear as to which device presents the lowest distortion properties for any given input signal. As a rule, device speciﬁcations should be compared like to like. This is not always possible, however, so speciﬁcations and the deﬁnitions of those speciﬁcations must be reviewed for each device (Figure 1). 2 4 8 16 32 64 128 256 512 1024 Figure 2. Resolution quoted as bits indicates the number of discrete measurement levels used. Agilent Measurement Journal 61

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 4 - 2008

Agilent Measurement Journal - Issue 4 - 2008 Delivering Confidence through Compliance with Standards Contents Emerging Innovations Trying Early Device Implementations at the IEEE 1588 PlugFest Achieving Greater Confidence in Measurement Accuracy through Consistency in Calibration Services Overcoming the Challenges of RFID Component Testing 3GPP LTE: Introducing Single- Carrier FDMA Ensuring Reliable Operation and Performance in Converged IP Networks Testing Storage Area Networks and Devices at 8.5-Gb/s Fibre Channel Rates Choosing an Appropriate Calibration Method for Vector Network Analysis Making Traceable EVM Measurements with Digital Oscilloscopes Exploring Terahertz Measurement, Imaging and Spectroscopy: The Electromagnetic Spectrum’s Final Frontier Interpreting Quoted Specifications when Selecting Digitizers

Agilent Measurement Journal - Issue 4 - 2008

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