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How to increase range in 2-port impedance measurements

Posted: 18 Dec 2015 ?? ?Print Version ?Bookmark and Share

Keywords:impedance? op amps? AD8048? VNA? DC blocker?

Measuring the output impedance of wideband op amps presents a formidable challenge because of their high DC gain, which results in low closed-loop output impedance at low frequencies. Above the frequency of the op amp's internal dominant pole, however, its output impedance becomes inductive. Thus, the output impedance increases at 6 dB/octave and can increase to 100 次 or more at the closed-loop bandwidth frequency. Fortunately, you can overcome this problem with some simple techniques.

For the purposes of this article, we'll consider the AD8048 250MHz, General Purpose Voltage Feedback Op Amp from Analog Devices. At a gain of 2, the output impedance graph in the manufacturer's datasheet ranges from less than 0.1 次 to more than 100 次 as shown in figure 1.

Figure 1: The AD8048 output impedance graph from the manufacturer's datasheet shows a minimum impedance of approximately 85 m次, which is below the minimum limit of the 1-port measurement and a maximum impedance of 100 次 at 1GHz. That impedance is above the maximum limit of the 2-port measurement.

A summary of the impedance measurement ranges for various methods is shown in figure 2. The impedance to be measured is within the impedance limits of a 3-port FRA (frequency response analyser) measurement, but exceeds the frequency range of all common FRAs. The 1-port measurement can measure the impedance above 5MHz or so and up to the 1GHz maximum frequency, but won't provide the low frequency measurement results. Similarly, the 2-port impedance measurement can provide the impedance from low frequency up to a few hundred megahertz, at which point the impedance is greater than the 2-port technique can accurately measure.

Figure 2: The impedance measurement ranges for various measurement techniques. Note that the 3-port measurement covers the required impedance range, but not the maximum frequency, while the 1-port and 2-port measurements can accommodate the frequency range but not the impedance range, unless the impedance is measured in 2 parts.

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