MC56F8346 Freescale Semiconductor, Inc, MC56F8346 Datasheet - Page 164

• Manufacturer Part Number: MC56F8346
• Description: 56f8300 16-bit Digital Signal Controllers
• Manufacturer: Freescale Semiconductor, Inc

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
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
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
164
1.
2.
3.
4.
Parasitic capacitance due to package, pin-to-pin and pin-to-package base coupling; 1.8pf
Parasitic capacitance due to the chip bond pad, ESD protection devices and signal routing; 2.04pf
Equivalent resistance for the ESD isolation resistor and the channel select mux; 500 ohms
Sampling capacitor at the sample and hold circuit. Capacitor C1 is normally disconnected from the input and is only
connected to it at sampling time; 1pf
REF
Analog Input
and the ADC clock frequency.
1
REFH
Figure 10-23 Equivalent Circuit for A/D Loading
- V
REFH
2
56F8346 Technical Data, Rev. 15
/ 2, while the other charges to the analog input voltage. When the
(V
REFH
3
- V
REFLO
) / 2
S1
S2
4
S3
REFH
C1
C2
S/H
- V
REFH
C1 = C2 = 1pF
/ 2. The switches switch
Freescale Semiconductor
Preliminary