Hi Naveen, The two bits in the GAIN register are the BUFF-GAIN bit (which applies a 1x or 2x gain to the output buffer) and the REF-DIV bit (which will divide the reference by 1x or 2x). The REF DIV bit applies whether you are using the internal or external reference. When using a 2.5V reference and a VDD voltage under ~3.5V, you have to set the reference divider bit. This is due to the external buffer limitations, as it requires a certain amount of headroom between the reference and VDD to operate. In this configuration, you will effectively be providing a reference of 1.25V, as the device will divide the 2.5V by 2. This is why you see ~1.2V as the full-scale voltage. To set the full-scale range to 2.5V, you should set the BUFF-GAIN bit to 1. So, the full command to send to the Gain register will be 0x0101. This will give you a 0V - 2.5V output range. In a bit of a roundabout way, this is taking your 2.5V reference, dividing it to 1.25V, then multiplying it by 2 back to 2.5V. If for whatever reason you require a higher range, you will need to increase your VDD voltage. Additionally, the register that disables the internal reference is the CONFIG register (0x03). You are likely already setting this, but make sure to write 0x0100 to disable the internal reference if you intend to use an external reference. Thanks, Erin
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