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ADXL213 possible noise sources?

I'm using an ADXL213 in an existing design, and I think I'm seeing more noise than I had expected. The ADXL213 is set up with Cx and Cy both 1uF, and Rset is 5.1M. Cdc is 0.1uF. 

I'm using a MCF54415 to measure the output PWM of the ADXL at rest, and taking 100 samples and averaging them. I'm seeing a RMS error in the duty cycle of 0.000715, and I was expecting to see a RMS of 0.0001316. The expected value is based on the 160ug/rtHz value, with BW=4.7, resulting in expected 438.76 ug. At 30% duty cycle per g, that gives me an expected RMS error in the duty cycle of 0.0001316.

I'm going to get the Vcc scoped out, to make sure there's no real noise there. What other common sources of noise can I look for? Or am I calculating the expected RMS error incorrectly?


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