SPC564BXX Datasheet, PDF (89/123 Pages) STMicroelectronics – Up to 80 MHz and 75 MIPs operation

SPC564Bxx-SPC56ECxx
Electrical Characteristics
This relation can again be simplified considering C_S as an additional worst condition. In
reality, transient is faster, but the A/D converter circuitry has been designed to be robust
also in very worst case: the sampling time T_s is always much longer than the internal time
constant.
Equation 6
₁  RSW + RAD  CS « TS
The charge of C_P1 and C_P2 is redistributed on C_S,determining a new value of the voltage
V_A1 on the capacitance according to the following equation
Equation 7
VA1  CS + CP1 + CP2 = VA  CP1 + CP2
A second charge transfer involves also C_F (that is typically bigger than the on-chip
capacitance) through the resistance RL: again considering the worst case in which C_P2 and
C_S were in parallel to C_P1 (since the time constant in reality would be faster), the time
constant is:
Equation 8
₂  RL  CS + CP1 + CP2
In this case, the time constant depends on the external circuit: in particular imposing that the
transient is completed well before the end of sampling time T_S, a constraints on R_L sizing is
obtained:
Equation 9
8.5  ₂ = 8.5  RL  CS + CP1 + CP2  TS
Of course, R_L shall be sized also according to the current limitation constraints, in
combination with R_S (source impedance) and R_F (filter resistance). Being C_F definitively
bigger than C_P1, C_P2 and C_S, then the final voltage V_A2 (at the end of the charge transfer
transient) will be much higher than V_A1. The following equation must be respected (charge
balance assuming now C_S already charged at V_A1):
Equation 10
VA2  CS + CP1 + CP2 + CF = VA CF + VA1 CP1 + CP2 + CS
The two transients above are not influenced by the voltage source that, due to the presence
of the R_FC_F filter, is not able to provide the extra charge to compensate the voltage drop on
C_S with respect to the ideal source V_A; the time constant R_FC_F of the filter is very high with
respect to the sampling time (T_S). The filter is typically designed to act as anti-aliasing
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