I have built two identical prototype boards to evaluate the ADAU1978 4-channel ADC. The circuit is based directly on the example schematic in the ADAU1978 datasheet, and to test the noise floor all four channels are allowed to self-bias to mid-rail, and each channel has a 1K resistor between the +ve and -ve input pins.
At a sample rate of 48kSPS both boards work well, and the noise floor is close to the value expected from the datasheet. However, at higher sample rates both boards generate samples which include unexpected 'noise spikes'. The characteristics of these spikes is as follows:
* The 'spikes' consist of a sudden deviation from the mean value in one direction for one sample, immediately followed (on
the next sample) by a large swing in the opposite direction. Sometimes the third sample also shows a large deviation, in the same direction as the first one.
* The spikes occur on all four channels of each ADC, and on both boards. On some channels the 'spikes' appeared to be quite regular (although the repetition rate varies between channels, and over time), whilst on other channels the timing of the spikes appears to be random.
* At a sample rate of 192kSPS, the 'spikes' have absolute amplitudes of 2000 – 3000 peak-to-peak, typically, so the impulses are only about 75dB below the full-scale limit of the ADC. At 96kSPS the amplitude is reduced to about 85dB below full-scale, and at 48kSPS the spikes have gone completely (or are below the normal noise floor).
I have tried increasing the decoupling on all the supply rails, and on the reference voltage pin, and have also tried powering the ADC board from a separate battery supply to eliminate any noise on the supply rails from the processor board. Nothing that I have tried so far has any discernible effect on the spikes.
Has anyone else seen this behaviour from the ADAU197x ADCs at higher sampling rates, please? If so, did you find a way to reduce the amplitude of the 'spikes'? For my sensor application I would like to achieve 90dB of dynamic range at 192kSPS if at all possible, so it is unclear at the moment if the ADAU1978 can be used for this purpose.