™
™
CMD
ADC
+
+
±
±
CMD = Common-mode detector
V
S
R
S
V
I
R
I
V
C
C
I
V = Input voltage at pin A
V = External source voltage
R = External source resistance
R = Internal MUX-on input resistance
C = Input capacitance
V = Capacitance-charging voltage
I
S
S
I
I
C
x
pint
Pext
C
= Parasitic capacitance, internal
C
= Parasitic capacitance, external
MSP432P4xx
C
pint
C
pext
Precision ADC Operation
849
SLAU356I – March 2015 – Revised June 2019
Copyright © 2015–2019, Texas Instruments Incorporated
Precision ADC
Figure 22-5. Analog Input Equivalent Circuit
The resistance of the source R
S
and R
I
affect t
sample
. Use
to calculate the minimum sampling
time t
sample
for a n-bit conversion, where n equals the bits of resolution.
t
sample
≥
(n + 1) × ln(2) × [(R
S
+ R
I
) × C
I
+ R
S
× (C
pext
+ C
pint
)], R
S
< 100 k
Ω
(11)
See the device-specific data sheet for R
I
, C
I
, and C
pint
values. C
pint
value is specified in the data sheet as
part of the digital inputs electrical specification.
Consider the following example of the minimum sample time calculation for a 14-bit analog-to-digital
conversion.
R
I
= 1 k
Ω
, C
I
= 15 pF, C
pint
= 5 pF, n = 14
R
S
= 10 k
Ω
, C
pext
= 10 pF
Substituting these values into
, the minimum sample time required is 3.28 µs.
Precision ADC supports fully-differential-input mode. As shown in
, when differential mode is
selected (ADC14DIF = 1), input common-mode voltage (V
CM
) at both inputs is automatically detected by
the Common Mode Detector (CMD). The CMD has a unique common-mode voltage detection circuit that
allows V
CM
to be set to any value from 0 V to V
REF
without degrading device performance. The detected
common-mode voltage is rejected from each input signal. The common-mode voltage includes common-
mode noise, which is rejected from each input signal in differential mode and makes Precision ADC a true
differential-input analog-to-digital converter when ADC14DIF = 1.
Figure 22-6. Precision ADC Differential Input Structure (Conceptual Diagram)
22.2.7 Conversion Memory
There are 32 ADC14MEMx conversion memory registers to store conversion results. Each ADC14MEMx
is configured with an associated ADC14MCTLx control register. The ADC14VRSEL bits define the voltage
reference and the ADC14INCHx and ADC14DIF bits select the input channels. The ADC14EOS bit
defines the end of sequence when a sequential conversion mode is used. A sequence rolls over from
ADC14MEM31 to ADC14MEM0 when the ADC14EOS bit in ADC14MCTL31 is not set.
The CSTARTADDx bits define the first ADC14MCTLx used for any conversion. If the conversion mode is
single-channel or repeat-single-channel, the CSTARTADDx points to the single ADC14MCTLx to be used.
If the conversion mode selected is either sequence-of-channels or repeat-sequence-of-channels,
CSTARTADDx points to the first ADC14MCTLx location to be used in a sequence. A pointer, not visible to
software, is incremented automatically to the next ADC14MCTLx in a sequence when each conversion
completes. The sequence continues until an ADC14EOS bit in ADC14MCTLx is processed; this is the last
control byte processed.