AD5624R output shutoff

I have a strange issue with the AD5624R on my first test board.   I'm using a 3.3v supply and I'm trying to use the internal reference.  The output of DAC_A is loaded with a series 100 ohm resistor that feeds into an opamp.


To activate the internal reference, I first send 0x380001 via my SPI interface, following the formatting in Table 17 of the datasheet. Then I send 0x18fff0 to set DAC_A to full scale, following the formatting in Figure 55 of the datasheet.  With my oscilloscope I observe the output go to 2.5v, which is expected. But it goes back to zero at 40mS.  If I apply 3/4 full scale with 0x18cff0 I get the output at 2v, but it goes back to zero at 80mS.  If I apply 1/2 scale with 0x188ff0 I get 1.5v output for 1.3s before it shuts off.   Why is the output shutting down?

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