Rainbow Electronics ADC10464 - Manual (Page 12)

Applications Information
(Continued)
TL H 11108 – 14
FIGURE 4 Typical Connection Note the multiple bypass capacitors on the reference and power supply pins If V
REF b
is not grounded it should also be bypassed to analog ground using multiple capacitors (see 5 0 ‘‘Power Supply
Considerations’’) AGND and DGND should be at the same potential V IN0 is shown with an input protection network
Pin 17 is normally left open but optional ‘‘speedup’’ resistor R SA can be used to reduce the conversion time
4 0 INHERENT SAMPLE-AND-HOLD
Because the ADC10461 ADC10462 and ADC10464 sam-
ple 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 succes-
sive-approximation A D converter regardless of speed the
input signal must be stable to better than
g 1 2 LSB during
each conversion cycle or significant errors will result Con-
sequently even for many relatively slow input signals the
signals must be externally sampled and held constant dur-
ing each conversion if a SAR with no internal sample-and-
hold is used
Because they incorporate a direct sample hold control in-
put the ADC10461 ADC10462 and ADC10464 are suitable
for use in DSP-based systems The S H input allows syn-
chronization of the A D converter to the DSP system’s sam-
pling rate and to other ADC10461s
ADC10462s and
ADC10464s
The ADC10461 ADC10462 and ADC10464 can perform
accurate conversions of input signals with frequency com-
ponents from DC to over 250 kHz
5 0 POWER SUPPLY CONSIDERATIONS
The ADC10461 ADC10462 and ADC10464 are designed
to operate from a a5V (nominal) power supply There are
two supply pins AV
CC and DV CC These pins allow sepa-
rate external 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 volt-
age source and each should be bypassed with a 0 1 mF
ceramic capacitor in parallel with a 10 mF tantalum capaci-
tor Depending on the circuit board layout and other system
considerations more bypassing may be necessary
The ADC10461 has a single ground pin and the ADC10462
and ADC10464 each have separate analog and digital
ground pins for separate bypassing of the analog and digital
supplies The devices with separate analog and digital
ground pins should have their ground pins connected to the
same potential and all grounds should be ‘‘clean’’ and free
of noise
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 sig-
nals are applied to any of the other pins
6 0 LAYOUT AND GROUNDING
In order to ensure fast accurate conversions from the
ADC10461 ADC10462 and ADC10464 it is necessary to
use appropriate circuit board layout techniques The analog
ground return path should be low-impedance and free of
noise from other parts of the system Noise from digital cir-
cuitry can be especially troublesome so digital grounds
should always be separate from analog grounds For best
performance separate ground planes should be provided
for the digital and analog parts of the system
All bypass capacitors should be located as close to the con-
verter as possible and should connect to the converter and
to ground with short traces The analog input should be iso-
lated from noisy signal traces to avoid having spurious sig-
nals 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 con-
verter’s performance with AC input signals The important
specifications for AC applications reflect the converter’s
ability to digitize AC signals without significant spectral er-
rors and without adding noise to the digitized signal Dynam-
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