Rainbow Electronics ADC14061 - Manual (Page 18)

Applications Information
(Continued)
6.0 DYNAMIC PERFORMANCE
The ADC14061 can achieve impressive dynamic perfor-
mance. To achieve the best dynamic performance with the
ADC14061, the clock source driving the CLK input must be
free of jitter. For best ac performance, isolate the ADC clock
from any digital circuitry with buffers, as with the clock tree
shown in
Figure 10 .
As mentioned in section 5.0, it is good practice to keep the
ADC clock line as short as possible and to keep it well away
from any other signals. Other signals can introduce phase
noise (jitter) into the clock signal, which can lead to in-
creased distortion. Even lines with 90˚ crossings have ca-
pacitive coupling, so try to avoid even these 90˚ crossings of
the clock line.
7.0 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 under-
shoot that goes more than a volt below ground. A resistor of
about 50 to 100
Ω in series with the offending digital input will
eliminate the problem.
Do not allow input voltages to exceed the supply voltage dur-
ing power up.
Be careful not to overdrive the inputs of the ADC14061 with
a device that is powered from supplies outside the range of
the ADC14061 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 of the
ADC14061, degrading dynamic performance. Adequate by-
passing 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 resis-
tors at each digital output, close to the ADC14061, which re-
duces 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 1.3, the capacitance seen at the in-
DS100103-23
FIGURE 9. Example at a suitable layout.
DS100103-24
FIGURE 10. Isolating the ADC clock from other
circuitry with a clock tree.
www.national.com 18