C8051F340/1/2/3/4/5/6/7
5.3.3. Settling Time Requirements
When the ADC0 input configuration is changed (i.e., a different AMUX0 selection is made), a minimum tracking time is required before an accurate conversion can be performed. This tracking time is determined by the AMUX0 resistance, the ADC0 sampling capacitance, any external source resistance, and the accu- racy required for the conversion. Note that in
Figure 5.5 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 5.1. When measuring the Temperature Sensor output or VDD with respect to GND, RTOTAL reduces to RMUX. See Table 5.1 for ADC0 minimum settling time requirements.
⎛ 2n ⎞ ⋅
t = ln ⎝
SA
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
RTOTAL is the sum of the AMUX0 resistance and any external source resistance. n is the ADC resolution in bits (10).
Differential Mode |
|
MUX
Select
Px.x
RMUX = 5k
RCInput= RMUX * CSAMPLE
Px.x
RMUX = 5k
MUX Select
MUX Select
Px.x
RMUX = 5k
CSAMPLE = 5pF
RCInput= RMUX * CSAMPLE
CSAMPLE = 5pF
CSAMPLE = 5pF
Figure 5.5. ADC0 Equivalent Input Circuits
Rev. 0.5 | 47 |
