Accurately measure ceramic capacitors by extending VNA range
Ceramic capacitors play an oversized role in circuit performance as a major contributor to voltage regulator control loop stability and power delivery network (PDN) impedance. When designing with these circuit staples, you need good impedance data over a wide frequency range. Proper measurement technique is the key to making accurate measurements. Here’s how to use two simple techniques that result in accurate, wideband, frequency measurements by extending the range of a vector-network analyzer (VNA).
Ceramic capacitor values are difficult to measure because of their high dynamic range of the impedance. Larger tantalum and aluminum electrolytic capacitors present a much lower dynamic range and are easier to measure.
Consider the measurement of a 100 nF ceramic capacitor. At 1 Hz, the capacitive reactance is approximately 1.6 M?. At series resonance, the equivalent series resistance (ESR) is typically about 10 m?. Accurately measuring this capacitor over a wide frequency range requires a dynamic range—the ratio of the capacitive impedance at the lowest frequency to the ESR at the low impedance—of at least 164 dB.
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