Texas Instruments MSP430x1xx - Manual (Page 270)

Comparator_A Description
14-4
A comparator output will oscillate if the voltage difference across the input ter-
minals is small. Internal and external parasitic effects and cross coupling on
and between signal lines, power-supply lines, and other parts of the system
are responsible for this behavior (see Figure 14–2). The comparator output os-
cillation reduces accuracy and resolution of the comparison result. Selecting
the output filter can reduce errors associated with comparator oscillation.
Figure 14–2. RC-Filter Response at the Output of the Comparator
+ Terminal
– Terminal Comparator Inputs
Comparator Output
Unfiltered at CAOUT
Comparator Output
Filtered at CAOUT
14.2.5 The Voltage Reference Generator
The voltage reference generator is used to generate V
CAREF
. V
CAREF
can be
applied to either of the comparator input terminals. Control bits CAREF0 and
CAREF1 control the output of the voltage generator. Control bit CARSEL
selects the comparator terminal to which V
CAREF
is applied. If external signals
are applied to the comparator input terminals, the internal reference generator
should be shut off to reduce current consumption. The divider in the voltage
reference generator can generate a fraction of the device’s V
CC
, or a fixed
transistor-threshold voltage. This threshold voltage tolerance is specified in
the specific device’s data sheet.
Ratiometric measurement principles that compare unknown values, such as
resistive or capacitive sensors, with a known value such as a precision resistor
or capacitor, can use an internal reference and achieve accurate results with-
out an absolute V
CC
. V
CC
needs to be stable, but not necessarily known. The
accuracy of ratiometric measurements is determined by the accuracy of the
known resistor or capacitor value.
Absolute measurement principles require a stable V
CC
to ensure that the volt-
age reference generated produces accurate reference-voltage levels.