C8051F336/7/8/9
7.2.3. Settling Time Requirements
A minimum tracking time is required before each conversion to ensure that an accurate conversion is per-
formed. This tracking time is determined by any series impedance, including the AMUX0 resistance, the
the ADC0 sampling capacitance, 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 applica-
tions, these three SAR clocks will meet the minimum tracking time requirements.
Figure 7.3 shows the equivalent ADC0 input circuits for both Differential and Single-ended modes. Notice
that the equivalent time constant for both input circuits is the same. The required ADC0 settling time for a
given settling accuracy (SA) may be approximated by Equation 7.1. When measuring the Temperature
Sensor output or V
with respect to GND, R
reduces to R
. See Table 6.8 for ADC0 minimum
DD
TOTAL
MUX
settling time requirements as well as the mux impedance and sampling capacitor values.
2n
SA
⎛
⎝
⎞
⎠
------
t = ln
× RTOTALCSAMPLE
Equation 7.1. 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).
Differential Mode
Single-Ended Mode
MUX Select
MUX Select
Px.x
Px.x
RMUX
RMUX
CSAMPLE
CSAMPLE
RCInput= RMUX * CSAMPLE
RCInput= RMUX * CSAMPLE
CSAMPLE
Px.x
RMUX
MUX Select
Figure 7.3. ADC0 Equivalent Input Circuits
Rev. 0.2
47
SILICON [ SILICON ]
