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Functional Description
832
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Analog Comparators
The internal reference can be programmed in one of two modes (low range or high range) depending on
the RNG bit in the ACREFCTL register. When RNG is clear, the internal reference is in high-range mode,
and when RNG is set the internal reference is in low-range mode.
In each range, the internal reference, V
IREF
, has 16 preprogrammed thresholds or step values. The
threshold to be used to compare the external input voltage against is selected using the VREF field in the
ACREFCTL register.
In the high-range mode, the V
IREF
threshold voltages start at the ideal high-range starting voltage of V
DDA
/
4.2 and increase in ideal constant voltage steps of V
DDA
/ 29.4.
In the low-range mode, the V
IREF
threshold voltages start at 0 V and increase in ideal constant voltage
steps of V
DDA
/ 22.12.
summarizes the ideal V
IREF
step voltages for each mode and their
dependence on the RNG and VREF fields.
Table 12-1. Internal Reference Voltage and ACREFCTL Field Values
ACREFCTL Register
Output Reference Voltage Based on VREF Field Value
EN Bit Value
RNG Bit Value
EN = 0
RNG = X
0 V (GND) for any value of VREF. It is recommended that RNG = 1 and VREF = 0 to minimize
noise on the reference ground.
EN = 1
RNG = 0
V
IREF
High Range: 16 voltage threshold values indexed by VREF = 0x0.. 0xF
Ideal starting voltage (VREF =0): V
DDA
/ 4.2
Ideal step size: V
DDA
/ 29.4
Ideal V
IREF
threshold values: V
IREF
(VREF) = V
DDA
/ 4.2 + VREF × (V
DDA
/ 29.4), for VREF = 0x0..
0xF
For minimum and maximum V
IREF
threshold values, see
RNG = 1
V
IREF
Low Range: 16 voltage threshold values indexed by VREF = 0x0.. 0xF
Ideal starting voltage (VREF =0): 0 V
Ideal step size: V
DDA
/ 22.12
Ideal V
IREF
threshold values: V
IREF
(VREF) = VREF × (V
DDA
/ 22.12), for VREF = 0x0.. 0xF
For minimum and maximum V
IREF
threshold values, see
Note that the values shown in
are the ideal values of the V
IREF
thresholds. These values
actually vary between minimum and maximum values for each threshold step, depending on process and
temperature. The minimum and maximum values for each step are given by:
•
V
IREF
(VREF) [Min] = Ideal V
IREF
(VREF) (Ideal Step size 2 mV) / 2
•
V
IREF
(VREF) [Max] = Ideal V
IREF
(VREF) + (Ideal Step size 2 mV) / 2
Examples of minimum and maximum V
IREF
values for V
DDA
= 3.3 V for high and low ranges, are shown
in
and
. Note that these examples are only valid for V
DDA
= 3.3 V; values scale up
and down with V
DDA
.
Table 12-2. Analog Comparator Voltage Reference
Characteristics (V
DDA
= 3.3 V, EN = 1, and RNG = 0)
VREF Value
V
IREF
Min
Ideal V
IREF
V
IREF
Max
Unit
0x0
0.731
0.786
0.841
V
0x1
0.843
0.898
0.953
V
0x2
0.955
1.010
1.065
V
0x3
1.067
1.122
1.178
V
0x4
1.180
1.235
1.290
V
0x5
1.292
1.347
1.402
V
0x6
1.404
1.459
1.514
V
0x7
1.516
1.571
1.627
V
0x8
1.629
1.684
1.739
V
0x9
1.741
1.796
1.851
V
0xA
1.853
1.908
1.963
V