ADAU1761 data format and VU-Meter

The ADAU1761's ADC input levels depend on a number of things, including the PGA settings on the analog front-end, the input signal configuration, and the AVDD supply voltage.

With all PGAs set to 0 dB, a full scale single-ended input will be about 2.83 V peak-to-peak for AVDD = 3.3 V. However, you'll also see in the datasheet that there is some gain error (typically 12%, but varies slightly part-to-part), so if you input 2.83 V peak-to-peak, you'll actually see something like 0.88 FFS in the DSP itself. I would recommend against exceeding the datasheet's listed full-scale input in order to compensate for the typical gain error, because this could cause clipping in a situation where the part has less gain error than typical. Any way, the gain error is only 1 dB and you should see very little variation from part to part. You can find more information in the datasheet.

The ADCs should saturate at full scale, so if you exceed 2.83 V peak-to-peak, the peak value should clip and never exceed 0 dBFS inside the DSP. Note that by clipping the input signal you are still increasing the RMS level of the signal. You're using a Mean-Squared envelope, which is following the average signal level. That means that a full-scale sine tone will register much lower than a severely clipped sine tone (which is approximately a square wave and therefore has a much higher RMS level).

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