Texas Instruments MSP430x4xx - Manual (Page 430)

SD16_A Operation
22-4 SD16_A
22.2 SD16_A Operation
The SD16_A module is configured with user software. The setup and
operation of the SD16_A is discussed in the following sections.
22.2.1 ADC Core
The analog-to-digital conversion is performed by a 1-bit, second-order
sigma-delta modulator. A single-bit comparator within the modulator quantizes
the input signal with the modulator frequency f
M
. The resulting 1-bit data
stream is averaged by the digital filter for the conversion result.
22.2.2 Analog Input Range and PGA
The full-scale input voltage range for each analog input pair is dependent on
the gain setting of the programmable gain amplifier of each channel. The
maximum full-scale range is
±
V
FSR
where V
FSR
is defined by:
V
FSR
+
V
REF
ń2
GAIN
PGA
For a 1.2V reference, the maximum full-scale input range for a gain of 1 is:
" V
FSR
+
1.2Vń2
1
+" 0.6V
Refer to the device-specific data sheet for full-scale input specifications.
22.2.3 Voltage Reference Generator
The SD16_A module has a built-in 1.2V reference. It is enabled by the
SD16REFON bit. When using the internal reference an external 100nF
capacitor connected from V
REF
to AV
SS
is recommended to reduce noise. The
internal reference voltage can be used off-chip when SD16VMIDON = 1. The
buffered output can provide up to 1mA of drive. When using the internal
reference off-chip, a 470nF capacitor connected from V
REF
to AV
SS
is
required. See device-specific data sheet for parameters.
An external voltage reference can be applied to the V
REF
input when
SD16REFON and SD16VMIDON are both reset.
22.2.4 Auto Power-Down
The SD16_A is designed for low power applications. When the SD16_A is not
actively converting, it is automatically disabled and automatically re-enabled
when a conversion is started. The reference is not automatically disabled, but
can be disabled by setting SD16REFON = 0. When the SD16_A or reference
are disabled, they consume no current.