10.17 Equivalent Circuit for ADC Inputs
Figure 10-24 illustrates the ADC input circuit during sample & 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
- V
/ 2, while the other charges to the analog input voltage. When the
REFH
REFH
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 - V / 2. The switches switch
REFH
REFH
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
and the ADC clock frequency.
REF
3
Analog Input
4
S1
C1
C2
S/H
S3
V
REFH - VREFH / 2
S2
2
1
C1 = C2 = 1pF
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
3. Equivalent resistance for the ESD isolation resistor and the channel select mux; 500 ohms
4. 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
Figure 10-24 Equivalent Circuit for A/D Loading
10.18 Power Consumption
This section provides additional detail which can be used to optimize power consumption for a given
application.
Power consumption is given by the following equation:
Total power =
A: internal [static component]
+B: internal [state-dependent component]
+C: internal [dynamic component]
+D: external [dynamic component]
+E: external [static]
A, the internal [static component], is comprised of the DC bias currents for the oscillator, leakage current,
PLL, and voltage references. These sources operate independently of processor state or operating
frequency.
B, the internal [state-dependent component], reflects the supply current required by certain on-chip
resources only when those resources are in use. These include RAM, Flash memory and the ADCs.
56F8367 Technical Data, Rev. 9
164
Freescale Semiconductor
Preliminary
FREESCALE [ Freescale ]
