Rainbow Electronics ADC12281 - Manual (Page 15)

Applications Information
(Continued)
6.0 COMMON APPLICATION PITFALLS
Driving the inputs (analog or digital) beyond the power
supply rails. For proper operation, all inputs should not go
more than 100 mV beyond the supply rails (more than
100 mV below the ground pins or 100 mV above the supply
pins). Exceeding these limits on even a transient basis may
cause faulty or erratic operation. It is not uncommon for high
speed digital circuits (e.g., 74F and 74AC devices) to exhibit
overshoot or undershoot that goes above the power supply
or more than a volt below ground. A resistor of about 50
Ω to
100 Ω in series with the offending digital input will eliminate
the problem.
Do not allow input voltages to exceed the supply voltage
during power up.
Be careful not to overdrive the inputs of the ADC12281 with
a device that is powered from supplies outside the range of
the ADC12281 supply. Such practice may lead to conversion
inaccuracies and even to device damage.
Attempting to drive a high capacitance digital data bus.
The more capacitance the output drivers must charge for
each conversion, the more instantaneous digital current
flows through V
D
I/O and DGND I/O. These large charging
current spikes can couple into the analog circuitry, degrading
dynamic performance. Adequate bypassing and maintaining
separate analog and digital ground planes will reduce this
problem. The digital data outputs should be buffered (with
74ACQ541, for example). Dynamic performance can also be
improved by adding series resistors at each digital output,
close to the ADC12281, which reduces the energy coupled
back into the converter output pins by limiting the output
current. A reasonable value for these resistors is 47
Ω.
Using an inadequate amplifier to drive the analog input.
As explained in Section 1.2, the capacitance seen at the
input alternates between 12 pF and 28 pF, depending upon
the phase of the clock. This dynamic load is more difficult to
drive than is a fixed capacitance.
If the amplifier exhibits overshoot, ringing, or any evidence of
instability, even at a very low level, it will degrade perfor-
mance. The CLC409 has been found to be a good amplifier
to drive the ADC12281. A small series resistor at the ampli-
fier output, followed by a capacitor to ground (as shown in
Figure 5), will improve performance.
Operating with the reference pins outside of the speci-
fied range. As mentioned in Section 1.1, V
REF
should be in
the range of 1.8V ≤ V
REF
≤ 2.2V. Operating outside of these
limits could lead to output distortion.
Using a clock source with excessive jitter, using exces-
sively long clock signal trace, or having other signals
coupled to the clock signal trace. This will cause the
sampling interval to vary, causing excessive output noise
and a reduction in SNR and SINAD performance.
10102729
FIGURE 6. Example of a Suitable Layout
ADC12281
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