16 equivalent circuit for adc inputs, 17 power consumption – Freescale Semiconductor 56F8122 User Manual
Page 125
Equivalent Circuit for ADC Inputs
56F8322 Technical Data, Rev. 10.0
Freescale Semiconductor
125
Preliminary
10.16 Equivalent Circuit for ADC Inputs
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 closed & S3 open, one input of the sample and hold
circuit moves to (V
REFH
-V
REFLO
)/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
REFLO
)/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
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 10-21 Equivalent Circuit for A/D Loading
10.17 Power Consumption
See
for a list of IDD requirements for the device. 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:
A, the internal [static component], is comprised of the DC bias currents for the oscillator, leakage currents,
PLL, and voltage references. These sources operate independently of processor state or operating
frequency.
Total power =
A: internal [static component]
+B: internal [state-dependent component]
+C: internal [dynamic component]
+D: external [dynamic component]
+E: external [static]
1
2
3
Analog Input
4
S1
S2
S3
C1
C2
S/H
C1 = C2 = 1pF
(V
REFH
- V
REFLO )
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