AFE5818: Ramp mode step size

Part Number: AFE5818

I'm trying to verify the data coming from an AFE5818 on a custom board by putting it into ramp mode. I am expecting the values to increase by 1 each time we receive a new sample. However, when I run it, I see increases of 64.

I can see from the datasheet that:

The step size of the ramp pattern is function of ADC resolution (N) and serialization factor (S) and is given by 2^(S-N) .

I have set the ADC resolution to 14 and the serialization factor to 14 so I would expect the step size to be 1. Even so, there doesn't seem to be a combination of possible ADC resolution (12 bit or 14 bit) and serialization factor (12x, 14x or 16x) that would result in a step size of 64.

Are there other registers I need to set in order to get the ramp mode working correctly?