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FAQs - Digital Servo-loop Testing

Frequently Asked Question: "What is digital servo-loop testing?"

Discussion follows:

Digital servo-loop testing is used to test analog-to-digital converters (ADCs) with the goal being to calculate the ADC's integral non-linearity (INL), differential non-linearity (DNL), offset error, and gain error(s). In particular, this type of testing discovers, with very high precision, the "edge" between two adjacent digital output codes (the transition point between them). In order to do this, a very stable, very low-noise analog signal is adjusted until the converter's output codes are roughly 50% at the target code (or lower) and 50% at the target code plus one (or higher). The analog signal is generated by a "coarse" digital-to-analog converter (DAC) summed with a "fine" DAC. The "fine" DAC has a very small adjustment range relative to the coarse DAC so that the analog voltage can be adjusted in increments well below 1/10 of the "weight" (in voltage terms) of the ADC's "least significant bit" (the ideal voltage difference between two adjacent digital output codes).

While the theory is simple, the circuitry used for digital servo-loop testing can be very complex, but it could be considered less complex, in some ways, than the circuitry used for the analog servo-loop. There are many things to consider in order to optimize the adjustment of the two DACs for a particular ADC. A digital comparator is also required in order to create a feedback value for use in refining the settings of the two DACs. The digital comparator circuitry can be modified such that the internal "transition noise" (or "output noise") of the converter can also be measured, providing data that can be used to graph transition noise versus output code. One key aspect of servo-loop testing is that the feedback from the digital comparator to the coarse and fine DACs must be very fast. Servo-loop testing requires a "tightly coupled" system and is often difficult or impossible to do with "modular" test equipment.

The digital servo-loop test technique is a "code edge" test technique. This can be contrasted with a ramp test or an AC histogram test which are "code center" tests. Code center tests sweep an analog voltage over the ADC's transfer function while monitoring or recording the digital output codes that result. Code center tests can have an advantage in that they can be faster than code edge tests for some ADCs (particularly high-speed ADCs). However, code center tests are not typically as repeatable as code edge tests. This is particularly true for high-resolution ADCs. Code edge tests are considered "the gold standard" for ADC testing.

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