Datasheet

56F8356 Technical Data, Rev. 13
164 Freescale Semiconductor
Preliminary
10.17 Equivalent Circuit for ADC Inputs
Figure 10-23 illustrates the ADC input circuit during sample and hold. S1 and S2 are always open/closed
at the same time that S3 is closed/open. When S1/S2 are closed & S3 is open, one input of the sample and
hold circuit moves to V
REFH
- V
REFH
/ 2, while the other charges to the analog input voltage. When the
switches are flipped, the charge on C1 and C2 are averaged via S3, with the result that a single-ended
analog input is switched to a differential voltage centered about V
REFH
- V
REFH
/ 2. The switches switch
on every cycle of the ADC clock (open one-half ADC clock, closed one-half ADC clock). Note that there
are additional capacitances associated with the analog input pad, routing, etc., but these do not filter into
the S/H output voltage, as S1 provides isolation during the charge-sharing phase.
One aspect of this circuit is that there is an on-going input current, which is a function of the analog input
voltage, V
REF
and the ADC clock frequency.
1. Parasitic capacitance due to package, pin-to-pin and pin-to-package base coupling; 1.8pf
2. Parasitic capacitance due to the chip bond pad, ESD protection devices and signal routing; 2.04pf
1
2
3
Analog Input
4
S1
S2
S3
C1
C2
S/H
C1 = C2 = 1pF
(V
REFH
- V
REFLO
) / 2