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of capacitors that still have good high frequency characteristics). This capacitor has two
effects:
• It helps attenuate any noise that may exist on the input.
• It sources charge during the sample period when the analog signal source is a
high-impedance source.
Series resistance can be used with the capacitor on an input signal to implement a simple
RC filter. The maximum level of filtering at the input signals is application dependent and
is based on the bandpass characteristics required to accurately track the dynamic
characteristics of an input. Simple RC filtering at the signal may be limited by the source
impedance of the transducer or circuit supplying the analog signal to be measured. Refer to
Section 13.7.5.3, "Error Resulting from Leakage," for more information. In some cases, the
size of the capacitor at the signal may be very small.
Figure 13-50 is a simplified model of an input channel. Refer to this model in the following
discussion of the interaction between the external circuitry and the circuitry inside the
QADC64E.
Source
External Filter
R
SRC
V
SRC
V
= Source Voltage
SRC
R
= Source Impedance
SRC
R F
= Filter Impedance
C F
= Filter Capacitor
C P
= Internal Parasitic Capacitance
C
= Sample Capacitor
SAMP
V I
= Internal Voltage Source during Sample and Hold
Figure 13-50. Electrical Model of an A/D Input Signal
In Figure 13-50, R
, R
F
parasitic capacitor. C
V
is an internal voltage source used to provide charge to C
I
The following paragraphs provide a simplified description of the interaction between the
QADC64E and the external circuitry. This circuitry is assumed to be a simple RC low-pass
MOTOROLA Chapter 13. Queued Analog-to-Digital Converter Legacy Mode Operation
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
Internal Circuit Model
S1
R F
C F
and C
comprise the external filter circuit. C
SRC
F
is the capacitor array used to sample and hold the input voltage.
SAMP
QADC64E Integration Requirements
S2
AMP
C P
QADC64E Sample AMP Model
SAMP
S3
C
SAMP
VI
is the internal
P
during sample phase.
13-73
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