This could be caused by a few things, but the most common is not interfacing with the MAX1258's SPI interface correctly. The MAX1258 does things differently on its SPI bus if you're talking to the ADC or DAC: for ADC you need to use a CPOL =1, CPHA= 1 SPI configuration. For the DAC commands, use CPOL=0, CPHA=1 configuration.