There are a few factors to consider that leads to the increased current consumption. The AC and DC performance of the AD7403 is much better in the AD7403 when compared to the AD7401A. For example the minimum SINAD for the AD7401A is 72dB however the AD7403 has a much improved SINAD of 81dB min. To achieve this level of improvement the AD7403 modulator uses more current.
Secondly, the AD7403 also has much better regulatory approved isolation ratings. The Viorm of the AD7401A is 891 Vpeak while the AD7403 achieves an improved Viorm of 1250 Vpeak. In order to improve on the isolation specifications and enhance the robustness of the transmission across the isolation barrier there is more current used.
However due to the improved accuracy of the AD7403 smaller shunt resistors can be used while maintaining the same level of ADC accuracy as that of the AD7401A. This should help reduce the power losses through the shunt and improve the overall system efficiency. If the same shunt resistor value is used ADC accuracy is improved and therefore it is possible to improve the overall system efficiency