Output Impedance vs. Frequency Data Sheet Figure 1Consider this first example excerpt from a datasheet; a plot of the output impedance of a voltage reference with and without a capacitor. Identifying the manufacturer is not required as the issues that will be discussed are not manufacturer-specific problems. Similar issues can be found in data provided by most manufacturers.

While the graph of Fig. 1 seems straightforward, there are several issues that become apparent when one tries to make sense of what the data is saying. For example, the output impedance is a function of the reference output current, which is not specified. The dielectric of the 1µF capacitor is not specified, though the results would be significantly different for ceramic, tantalum and aluminum electrolytic capacitors.

The voltage reference output current will also influence these parameters and, therefore, it should be included in the data and also considered in the circuit design. The most surprising issue, however, is that the impedance with the 1µF capacitor can actually be used to determine that the capacitor is not a 1µF capacitor at all, but a 3.3µF capacitor!

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