Rainbow Electronics ADC081000 - Manual (Page 28)

Applications Information
(Continued)
8.0 LAYOUT AND GROUNDING
Proper grounding and proper routing of all signals are es-
sential to ensure accurate conversion. A single ground plane
should be used, as opposed to splitting the ground plane into
analog and digital areas.
Since digital switching transients are composed largely of
high frequency components, the skin effect tells us that total
ground plane copper weight will have little effect upon the
logic-generated noise. Total surface area is more important
than is total ground plane volume. Coupling between the
typically noisy digital circuitry and the sensitive analog cir-
cuitry can lead to poor performance that may seem impos-
sible to isolate and remedy. The solution is to keep the
analog circuitry well separated from the digital circuitry.
High power digital components should not be located on or
near any linear component or power supply trace or plane
that services analog or mixed signal components as the
resulting common return current path could cause fluctuation
in the analog input “ground” return of the ADC, causing
excessive noise in the conversion result.
Generally, we assume that analog and digital lines should
cross each other at 90˚ to avoid getting digital noise into the
analog path. In high frequency systems, however, avoid
crossing analog and digital lines altogether. Clock lines
should be isolated from ALL other lines, analog AND digital.
The generally accepted 90˚ crossing should be avoided as
even a little coupling can cause problems at high frequen-
cies. Best performance at high frequencies is obtained with a
straight signal path.
The analog input should be isolated from noisy signal traces
to avoid coupling of spurious signals into the input. This is
especially important with the low level drive required of the
ADC081000. Any external component (e.g., a filter capaci-
tor) connected between the converter’s input and ground
should be connected to a very clean point in the analog
ground plane. All analog circuitry (input amplifiers, filters,
etc.) should be separated from any digital components.
9.0 DYNAMIC PERFORMANCE
The ADC081000 is a.c. tested and its dynamic performance
is guaranteed. To meet the published specifications and
avoid jitter-induced noise, the clock source driving the CLK
input must exhibit low rms jitter. The allowable jitter is a
function of the input frequency and the input signal level, as
described in Section 4.0.
It is good practice to keep the ADC clock line as short as
possible, to keep it well away from any other signals and to
treat it as a transmission line. Other signals can introduce
jitter into the clock signal. The clock signal can also introduce
noise into the analog path if not isolated from that path.
Best dynamic performance is obtained when the exposed
pad at the back of the package has a good connection to
ground. This is because this path from the die to ground is a
lower impedance than that offered by the package pins.
10.0 COMMON APPLICATION PITFALLS
Allowing loose power supply voltage tolerance. The
ADC081000 is specified for operation between 1.8 Volts to
2.0 Volts. Using a 1.8 Volt power supply then implies the
need for no negative tolerance. The best solution is to use an
adjustable linear regulator such as the LM317 or LM1086 set
for 1.9V as discussed in Section 7.1.
Driving the inputs (analog or digital) beyond the power
supply rails. For device reliability, all inputs should not go
more than 150 mV below the ground pins or 150 mV above
the supply pins. Exceeding these limits on even a transient
basis may not only cause faulty or erratic operation, but may
impair device reliability. It is not uncommon for high speed
digital circuits to exhibit undershoot that goes more than a
volt below ground. Controlling the impedance of high speed
lines and terminating these lines in their characteristic im-
pedance should control overshoot.
Care should be taken not to overdrive the inputs of the
ADC081000. Such practice may lead to conversion inaccu-
racies and even to device damage.
Incorrect analog input common mode voltage in the d.c.
coupled mode. As discussed in Sections 1.3 and 3.0, the
Input common mode voltage must remain within 50 mV of
the V
CMO
output and track that output, which has a variability
with temperature that must also be tracked. Distortion per-
formance will be degraded if the input common mode volt-
age is more than 50 mV from V
CMO
.
Using an inadequate amplifier to drive the analog input.
Use care when choosing a high frequency amplifier to drive
the ADC081000 as many high speed amplifiers will have
higher distortion than will the ADC081000, resulting in over-
all system performance degradation.
Driving the V
BG
pin to change the reference voltage. As
mentioned in Section 1.3, the reference voltage is intended
to be fixed to provide one of two different full-scale values
(600 mV
P-P
and 800 mV
P-P
). Over driving this pin will not
change the full scale value, but can otherwise upset opera-
tion.
Driving the clock input with an excessively high level
signal. The ADC clock level should not exceed the level
described in the Operating Ratings Table or the input offset
error could increase.
Inadequate clock levels. As described in Section 4.0, insuf-
ficient clock levels can result in poor performance. Excessive
clock levels could result in the introduction of an input offset.
Using an excessively long clock signal trace, or having
other signals coupled to the clock signal trace. This will
cause the sampling interval to vary, causing excessive out-
put noise and a reduction in SNR performance.
Failure to provide adequate heat removal. As described in
Section 7.2, it is important to provide an adequate heat
removal to ensure device reliability. This can either be done
with adequate air flow or the use of a simple heat sink built
into the board. The backside pad should be grounded for
best performance.
ADC081000
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