Problem testing an ADC with a DAC and voltage divider
There could be a couple potential causes for this depending on the magnitude of the DAC output and how this signal is driven to the ADS1299. The DAC may not have the required drive strength  capability, so added impedance to the signal path may cause this problem. You may need to buffer the DAC's output.

I have posted your question to TI's E2E forum at the link below. My hope is that other community members will see it and that you can provide more information about your design.

http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/t/328451.aspx

Best Regards,

Ryan