ADC10662CIWMX/NOPB National Semiconductor, ADC10662CIWMX/NOPB Datasheet - Page 14

• Manufacturer Part Number: ADC10662CIWMX/NOPB
• Manufacturer: National Semiconductor

Applications Information
the Absolute Maximum Rating. If an analog input pin is
forced beyond these voltages, the current flowing through
the pin should be limited to 5 mA or less to avoid permanent
damage to the IC. The sum of all the overdrive currents into
all pins must be less than the Absolute Maximum Rating for
Package Input Current. When the input signal is expected to
4.0 INHERENT SAMPLE-AND-HOLD
Because the ADC10662 and ADC10664 sample the input
signal once during each conversion, they are capable of
measuring relatively fast input signals without the help of an
external sample-hold. In a non-sampling successive-
approximation A/D converter, regardless of speed, the input
signal must be stable to better than
conversion cycle or significant errors will result. Conse-
quently, even for many relatively slow input signals, the
signals must be externally sampled and held constant during
each conversion if a SAR with no internal sample-and-hold is
used.
Because they incorporate a direct sample/hold control input,
the ADC10662 and ADC10664 are suitable for use in DSP-
based systems. The S/H input allows synchronization of the
A/D converter to the DSP system’s sampling rate and to
other ADC10662s, and ADC10664s.
5.0 POWER SUPPLY CONSIDERATIONS
The ADC10662 and ADC10664 are designed to operate
from a +5V (nominal) power supply. There are two supply
pins, AV
bypass capacitors for the analog and digital portions of the
circuit. To guarantee accurate conversions, the two supply
pins should be connected to the same voltage source, and
each should be bypassed with a 0.1 µF ceramic capacitor in
parallel with a 10 µF tantalum capacitor. Depending upon the
circuit board layout and other system considerations, more
bypassing may be necessary.
The ADC10662 and ADC10664 have separate analog and
digital ground pins for separate bypassing of the analog and
digital supplies. Their ground pins should be connected to
the same potential, and all grounds should be “clean” and
free of noise.
CC
and DV
CC
. These pins allow separate external
±
1/2 LSB during each
(Continued)
FIGURE 4. Typical Connection
13
extend beyond this limit, an input protection scheme should
be used. A simple input protection network using diodes and
resistors is shown in Figure 4. Note the multiple bypass
capacitors on the reference and power supply pins. If V
is not grounded, it should also be bypassed to analog ground
using multiple capacitors (see 5.0 “Power Supply Consider-
ations”). AGND and DGND should be at the same potential.
V
In systems with multiple power supplies, careful attention to
power supply sequencing may be necessary to avoid over-
driving inputs. The A/D converter’s power supply pins should
be at the proper voltage before digital or analog signals are
applied to any of the other pins.
6.0 LAYOUT AND GROUNDING
In order to ensure fast, accurate conversions from the
ADC10662 and ADC10664, it is necessary to use appropri-
ate circuit board layout techniques. The analog ground re-
turn path should be low-impedance and free of noise from
other parts of the system. Noise from digital circuitry can be
especially troublesome.
All bypass capacitors should be located as close to the
converter as possible and should connect to the converter
and to ground with short traces. The analog input should be
isolated from noisy signal traces to avoid having spurious
signals couple to the input. Any external component (e.g., a
filter capacitor) connected across the converter’s input
should be connected to a very clean ground return point.
Grounding the component at the wrong point will result in
reduced conversion accuracy.
7.0 DYNAMIC PERFORMANCE
Many applications require the A/D converter to digitize AC
signals, but conventional DC integral and differential nonlin-
earity specifications don’t accurately predict the A/D convert-
er’s performance with AC input signals. The important speci-
fications for AC applications reflect the converter’s ability to
digitize AC signals without significant spectral errors and
without adding noise to the digitized signal. Dynamic char-
acteristics such as signal-to-noise ratio (SNR) and total har-
monic distortion (THD), are quantitative measures of this
capability.
IN0
is shown with an input protection network.
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