Si1010/1/2/3/4/5
5.2.4. Settling Time Requirements
A minimum amount of tracking time is required before each conversion can be performed, to allow the
sampling capacitor voltage to settle. This tracking time is determined by the AMUX0 resistance, the ADC0
sampling capacitance, any external source resistance, and the accuracy required for the conversion. Note
that in low-power tracking mode, three SAR clocks are used for tracking at the start of every conversion.
For many applications, these three SAR clocks will meet the minimum tracking time requirements, and
higher values for the external source impedance will increase the required tracking time.
Figure 5.4 shows the equivalent ADC0 input circuit. The required ADC0 settling time for a given settling
accuracy (SA) may be approximated by Equation 5.1. When measuring the Temperature Sensor output or
V
with respect to GND, R
reduces to R
. See Table 4.10 for ADC0 minimum settling time
DD
TOTAL
MUX
requirements as well as the mux impedance and sampling capacitor values.
2n
SA
------
t = ln
RTOTALCSAMPLE
Equation 5.1. ADC0 Settling Time Requirements
Where:
SA is the settling accuracy, given as a fraction of an LSB (for example, 0.25 to settle within 1/4 LSB)
t is the required settling time in seconds
R
is the sum of the AMUX0 resistance and any external source resistance.
TOTAL
n is the ADC resolution in bits (10).
MUX Select
P0.x
RMUX
CSAMPLE
RCInput= RMUX * CSAMPLE
Note: The value of CSAMPLE depends on the PGA Gain. See Table 4.10 for details.
Figure 5.4. ADC0 Equivalent Input Circuits
5.2.5. Gain Setting
The ADC has gain settings of 1x and 0.5x. In 1x mode, the full scale reading of the ADC is determined
directly by V . In 0.5x mode, the full-scale reading of the ADC occurs when the input voltage is V
x 2.
REF
REF
The 0.5x gain setting can be useful to obtain a higher input Voltage range when using a small V
volt-
REF
age, or to measure input voltages that are between V
trolled by the AMP0GN bit in register ADC0CF.
and V . Gain settings for the ADC are con-
DD
REF
82
Rev. 1.0
SILICON [ SILICON ]
