adc12v170hfeb National Semiconductor Corporation, adc12v170hfeb Datasheet - Page 19

• Manufacturer Part Number: adc12v170hfeb
• Description: 12-bit, 170 Msps, 1.1 Ghz Bandwidth A/d Converter With Lvds Outputs
• Manufacturer: National Semiconductor Corporation

100 ohm termination resistor as close to the receiving circuit
as possible. See Figure 4.
5.0 POWER SUPPLY CONSIDERATIONS
The power supply pins should be bypassed with a 0.1 µF ca-
pacitor and with a 0.01 µF ceramic chip capacitor close to
each power pin. Leadless chip capacitors are preferred be-
cause they have low series inductance.
As is the case with all high-speed converters, the ADC12V170
is sensitive to power supply noise. Accordingly, the noise on
the analog supply pin should be kept below 100 mV
No pin should ever have a voltage on it that is in excess of the
supply voltages, not even on a transient basis. Be especially
careful of this during power turn on and turn off.
The V
operated from a supply in the range of 1.6V to 2.0V. This en-
ables lower power operation, reduces the noise coupling
effects from the digital outputs to the analog circuitry and sim-
plifies interfacing to lower voltage devices and systems. Note,
however, that t
6.0 LAYOUT AND GROUNDING
Proper grounding and proper routing of all signals are essen-
tial to ensure accurate conversion. Maintaining separate ana-
log and digital areas of the board, with the ADC12V170
between these areas, is required to achieve specified perfor-
mance.
The ground return for the data outputs (DRGND) carries the
ground current for the output drivers. The output current can
exhibit high transients that could add noise to the conversion
process. To prevent this from happening, the DRGND pins
should NOT be connected to system ground in close proximity
to any of the ADC12V170's other ground pins.
Capacitive coupling between the typically noisy digital circuit-
ry and the sensitive analog circuitry can lead to poor perfor-
mance. The solution is to keep the analog circuitry separated
FIGURE 4. Application Circuit using Transformer Drive Circuit (If 14-bit compatibility is not required do not connect pin
DR
pin provides power for the output drivers and may be
OD
increases with reduced V
DR
.
P-P
.
17 and 18)
19
from the digital circuitry, and to keep the clock line as short as
possible.
Since digital switching transients are composed largely of
high frequency components, total ground plane copper
weight will have little effect upon the logic-generated noise.
This is because of the skin effect. Total surface area is more
important than is total ground plane area.
Generally, analog and digital lines should cross each other at
90° to avoid crosstalk. To maximize accuracy in high speed,
high resolution systems, however, avoid crossing analog and
digital lines altogether. It is important to keep clock lines as
short as possible and isolated from ALL other lines, including
other digital lines. Even the generally accepted 90° crossing
should be avoided with the clock line as even a little coupling
can cause problems at high frequencies. This is because oth-
er lines can introduce jitter into the clock line, which can lead
to degradation of SNR. Also, the high speed clock can intro-
duce noise into the analog chain.
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 and trans-
formers. Mutual inductance can change the characteristics of
the circuit in which they are used. Inductors and transformers
should not be placed side by side, even with just a small part
of their bodies beside each other. For instance, place trans-
formers for the analog input and the clock input at 90° to one
another to avoid magnetic coupling.
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 between
the converter's input pins and ground or to the reference input
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