ADC08351CIMTC National Semiconductor, ADC08351CIMTC Datasheet - Page 10

• Manufacturer Part Number: ADC08351CIMTC
• Description: 8-Bit/ 42 MSPS/ 40 mW A/D Converter
• Manufacturer: National Semiconductor

www.national.com
Applications Information
the analog supply pin should be minimized, keeping it below
200 mV
the power supply should be even more severely limited.
No pin should ever have a voltage on it that is in excess of
the supply voltages. This can be a problem upon application
of power to a circuit. Be sure that the supplies to circuits driv-
ing the CLK, OE, analog input and reference pins do not
come up any faster than does the voltage at the ADC08351
power pins.
3.0 LAYOUT AND GROUNDING
Proper grounding and proper routing of all signals is essen-
tial to ensure accurate conversion. Separate analog and
digital ground planes that are connected beneath the
Generally, analog and digital lines should cross each other at
90 degrees to avoid getting digital noise into the analog path.
To maximize accuracy in video (high frequency) systems,
however, avoid crossing analog and digital lines altogether.
Furthermore, it is important to keep any clock lines isolated
from ALL other lines, including other digital lines. Even the
generally accepted 90 degree crossing should be avoided as
even a little coupling can cause problems at high frequen-
cies.
Best performance at high frequencies and at high resolution
is obtained with a straight signal path. That is, the signal path
through all components should form a straight line wherever
possible.
Be especially careful with the layout of inductors. Mutual in-
ductance can change the characteristics of the circuit in
which they are used. Inductors should not be placed side by
side, even with just a small part of their bodies beside each
other.
FIGURE 3. Layout example showing separate analog and digital ground planes connected below the ADC08351.
P-P
at 100 kHz. Of course, higher frequency noise on
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10
ADC08351 are required to meet data sheet limits. The ana-
log and digital grounds may be in the same layer, but should
be separated from each other and should never overlap
each other.
Capacitive coupling between the typically noisy digital
ground plane and the sensitive analog circuitry can lead to
poor performance that may seem impossible to isolate and
remedy. The solution is to keep the analog circuitry well
separated from the digital circuitry and from the digital
ground plane.
The analog input should be isolated from noisy signal traces
to avoid coupling of spurious signals into the input. Any ex-
ternal component (e.g., a filter capacitor) connected be-
tween the converter’s input and ground should be connected
to a very clean point in the analog ground plane.
Figure 3 gives an example of a suitable layout. All analog cir-
cuitry (input amplifiers, filters, reference components, etc.)
should be placed on or over the analog ground plane. All
digital circuitry and I/O lines should be placed over the digital
ground plane.
All ground connections should have a low inductance path to
ground.
4.0 DYNAMIC PERFORMANCE
The ADC08351 is ac tested and its dynamic performance is
guaranteed. To meet the published specifications, the clock
source driving the CLK input must be free of jitter. For best
DS100895-25