PSoC3 ADC by itself is a differential amplifier ADC. The Delta Sigma ADC has a differential amplifier front-end. This differential amplifier feeds its output to the ADC’s differential inputs. The differential amplifier front-end can have a maximum gain of 8. The differential ADC can have a maximum gain of 16. A total gain of 128 can be implemented.
If gain greater than 128 is required, two PGA’s can be used back to back as shown in the figure below and then cascaded to the ADC along with its front-end buffers. If a gain of 256 is required, configure both PGAs for a gain of 2 and cascade it to the differential ADC/buffer with a gain of 128.
Please refer to the Application Note on Instrumentation Amplifier in PSoC 3/5 (AN60319)