Analog-to-Digital Converter (ADC)
871
SPRUH22I – April 2012 – Revised November 2019
Copyright © 2012–2019, Texas Instruments Incorporated
Analog Subsystem
10.3.10 Internal/External Reference Voltage Selection
10.3.10.1 Internal Reference Voltage
The ADC can operate in two different reference modes, selected by the ADCCTL1.ADCREFSEL bit. By
default the internal bandgap is chosen to generate the reference voltage for the ADC. This will convert the
voltage presented according to a fixed scale 0 to 3.3v range. The equation governing conversions in this
mode is:
Digital Value = 0
when Input
≤
0v
Digital Value = 4096 [(Input – VREFLO)/3.3v]
when 0v < Input < 3.3v
Digital Value = 4095,
when Input
≥
3.3v
*All fractional values are truncated
**VREFLO must be tied to ground in this mode. This is done internally on some devices.
10.3.10.2 External Reference Voltage
To convert the voltage presented as a ratiometric signal, the external VREFHI/VREFLO pins should be
chosen to generate the reference voltage. In contrast with the fixed 0 to 3.3v input range of the internal
bandgap mode, the ratiometric mode has an input range from VREFLO to VREFHI. Converted values will
scale to this range. For instance, if VREFLO is set to 0.5v and VREFHI is 3.0v, a voltage of 1.75v will be
converted to the digital result of 2048. See the device datasheet for the allowable ranges of VREFLO and
VREFHI. On some devices VREFLO is tied to ground internally, and hence limited to 0v. The equation
governing the conversions in this mode is:
Digital Value = 0
when Input
≤
VREFLO
Digital Value = 4096 [(Input – VREFLO)/(VREFHI – VREFLO)]
when VREFLO < Input < VREFHI
Digital Value = 4095,
when Input
≥
VREFHI
*All fractional values are truncated