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AD7490 Sampling Rate & Artifacts


I am testing a design with an AD7490 and I'm a bit unsure on how to determine the sampling rate. Given VDD of 5V, per the datasheet a 20 MHz SCLK should be equivalent to a 1MSPS sampling rate. Does that mean it's a 20:1 ratio for the sampling rate?

I'm seeing some artifacts in my sampled waveform that I'm unsure what the cause is. I am sampling the following 1 kHz sine wave on VIN8 (this is me probing directly on the ADC VIN8 pin):

After collecting the data with a SPI<->USB Master (Totalphase's Cheetah device) at an SCLK frequency of 16 MHz and importing to MATLAB, I plot this for 1021 samples:

The artifacts appear to have some periodicity to them, occurring every 25-30 samples on each wave. Also, trough to trough is about 500 samples, which gives a sampling rate of (500 samples/1 millisecond) = 500 kSPS, which is a 32:1 ratio with SCLK = 16 MHz.

I would appreciate any insight, thank you very much in advance!

Also here are a couple of screenshots of the SPI datastream:

Zoomed in:

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