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External Crystal on AD4115 and EVAL-AD4115SDZ

I just purchased an EVAL-AD4115SDZ so I could evaluate the AD4115. I've noticed that it works as expected after a reset using the internal clock - writing 0x0000 to the ADCMODE register and dropping CS indicates continuous conversions at 125Khz on the RD/MISO pin. But as soon as I write 0x00C0 to the ADCMODE register (enable external clock), the continuous data stream stops. When I look at the CLK test point with my oscilloscope, I am not seeing any oscillation. So my question is: did I receive a defective eval board or is there an undocumented issue with the AD4115. For the record, I also have the AD4112 eval board and the external XTAL oscillator works correctly. 

Fixed typo
[edited by: WallyBalls at 1:58 PM (GMT -5) on 15 Jan 2021]
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