I'm looking at the datasheet for an MCP4921 12-bit DAC (PDF), which describes the following feature:
Selectable Unity or 2x Gain Output - The user can configure the full-scale range of the device to be VREF or 2*VREF by setting the gain selection option bit (gain of 1 of 2).
This doesn't give me much of an idea as to why anyone would need such a feature. It seems to me that one could just increase VREF, instead of using the gain selection. So I read further into the sheet:
In some applications, precision digital control of the output range is desirable. Example 6-4 illustrates how to use the DAC devices to achieve this in a bipolar or single-supply application. This circuit is typically used in Multiplier mode and is ideal for linearizing a sensor whose slope and offset varies. Refer to Section 6.9 “Using Multiplier Mode” for more information on Multiplier mode.
And, from section 6.9:
The MCP4921 uses an external reference, and these devices are ideally suited for use as a multiplier/divider in a signal chain. Common applications are: (a) precision programmable gain/attenuator amplifiers and (b) motor control feedback loops. The wide input range (0V – VDD) is in Unbuffered mode, and near rail-to-rail range in Buffered mode. Its bandwidth (> 400 kHz), selectable 1x/2x gain and low power consumption give maximum flexibility to meet the application’s needs.
This seems pretty vague - I don't see how having the output range go to 2*VREF would be a common enough requirement to warrant making it a feature of the device.
What am I missing? Is there a common use for DACs that makes such a feature useful / beneficial? Why wouldn't we just set VREF to a higher voltage if we wanted that increased range?