Electrical characteristics
MPC5604B/C Microcontroller Data Sheet, Rev. 9
Freescale Semiconductor
69
possible, ideally infinite. This capacitor contributes to attenuating the noise present on the input pin; furthermore, it sources
charge during the sampling phase, when the analog signal source is a high-impedance source.
A real filter can typically be obtained by using a series resistance with a capacitor on the input pin (simple RC filter). The RC
filtering may be limited according to the value of source impedance of the transducer or circuit supplying the analog signal to
be measured. The filter at the input pins must be designed taking into account the dynamic characteristics of the input signal
(bandwidth) and the equivalent input impedance of the ADC itself.
In fact a current sink contributor is represented by the charge sharing effects with the sampling capacitance: CS being
substantially a switched capacitance, with a frequency equal to the conversion rate of the ADC, it can be seen as a resistive path
to ground. For instance, assuming a conversion rate of 1 MHz, with CS equal to 3 pF, a resistance of 330 k is obtained (REQ
= 1 / (fc*CS), where fc represents the conversion rate at the considered channel). To minimize the error induced by the voltage
partitioning between this resistance (sampled voltage on CS) and the sum of RS + RF + RL + RSW + RAD, the external circuit
Eqn. 4
Equation 4 generates a constraint for external network design, in particular on a resistive path. Internal switch resistances (RSW and RAD) can be neglected with respect to external resistances.
Figure 20. Input equivalent circuit (precise channels)
VA
RS RF RL RSW RAD
++
+
REQ
---------------------------------------------------------------------------
1
2
---LSB
RF
CF
RS
RL
RSW1
CP2
CS
VDD
Sampling
Source
Filter
Current Limiter
EXTERNAL CIRCUIT
INTERNAL CIRCUIT SCHEME
CP1
RAD
Channel
Selection
VA
RS: Source impedance
RF: Filter resistance
CF: Filter capacitance
RL: Current limiter resistance
RSW1: Channel selection switch impedance
RAD: Sampling switch impedance
CP: Pin capacitance (two contributions, CP1 and CP2)
CS: Sampling capacitance