Texas Instruments MSP430x1xx - Manual (Page 297)

Analog Inputs and Multiplexer
15-7
ADC12
15.3.2 Input Signal Considerations
During sampling, the analog input signal is applied to the internal capacitor
array of the A/D core. Therefore, the charge of the capacitor array is supplied
directly by the source. The capacitor array has to be charged completely dur-
ing the sampling period. Therefore the external source resistances, dynamic
impedances, and capacitance of the capacitor array must be matched with the
sampling period so the analog signal can settle to within 12-bit accuracy.
Additionally, source impedances also affect the accuracy of the converter. The
source signal can drop at the input of the device due to leakage current or
averaged dc-input currents (due to input switching currents). For a 12-bit
converter, the error in LSBs due to leakage current is:
Error(LSBs) = 4.096 x (
µ
A of leakage current)
×
(k
Ω
of source
resistance)/(V
R+
– V
R–
)
For example, a 50-nA leakage current with a 10-k
Ω
source resistance and a
1.5-V V
REF
gives 1.4 LSBs of error.
These errors due to source impedance also apply to the output impedance of
any external-voltage reference source applied to Ve
REF+
. The output imped-
ance must be low enough to enable the transients to settle within 0.2/ADCLK
and generate leakage current induced errors of <<1LSB.
See the Sampling section for more details on sample timing and sampling con-
siderations.
15.3.3 Using the Temperature Diode
To use the on-chip temperature diode, the user simply selects the analog input
channel to 10. Any other configuration is done as if an external channel was
selected, including reference selection, conversion-memory selection, etc.
Selecting the diode channel automatically turns on the on-chip reference
generator (see Figure 15–1) as a voltage source for the temperature diode.
However, it does not enable the V
REF+
output or affect the reference selections
for the conversion; so, reference selections are the same as with any other
channel.
See the device data sheet for the temperature diode specifications.