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FAQs - Analog Servo-loop Testing

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

Discussion follows:

Analog 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 completely or partially generated by a simple integrator and this integrator includes feedback from the test circuitry that will drive the analog voltage up or down depending on the ADC's digital result. This explains both the "analog" and "servo-loop" portions of the name for this test technique.

While the theory is simple, the circuitry used for analog servo-loop testing can be very complex. There are many things to consider in order to optimize the servo-loop for a particular ADC. The circuitry has also evolved over the years to improve test time. Older analog servo-loop tests used a simple integrator for the testing. Modern analog servo-loops include a "pedestal DAC" to generate an analog voltage that is close to the desired voltage plus an integrator to make up the difference between the DAC voltage and the actual input voltage that causes the ADC's digital output to be balanced between two adjacent codes. A digital comparator is also required in order to create the feedback signal to the integrator. 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 integrator 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 analog 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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