Rainbow Electronics ADC11L066 - Manual (Page 26)

Applications Information
(Continued)
pins). Exceeding these limits on even a transient basis may
cause faulty or erratic operation. It is not uncommon for high
speed digital components (e.g., 74F and 74AC devices) to
exhibit overshoot or undershoot that goes above the power
supply or below ground. A resistor of about 50
Ω to 100Ω in
series with any offending digital input, close to the signal
source, will eliminate the problem.
Do not allow input voltages to exceed the supply voltage,
even on a transient basis. Not even during power up or
power down.
Be careful not to overdrive the inputs of the ADC11L066 with
a device that is powered from supplies outside the range of
the ADC11L066 supply. Such practice may lead to conver-
sion 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
DR
and DR GND. These large charging cur-
rent spikes can couple into the analog circuitry, degrading
dynamic performance. Adequate bypassing and maintaining
separate analog and digital areas on the pc board will reduce
this problem.
Additionally, bus capacitance beyond the specified 15 pF/pin
will cause t
OD
to increase, making it difficult to properly latch
the ADC output data. The result could, again, be a reduction
in dynamic performance.
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
ADC11L066, which reduces the energy coupled back into
the converter output pins by limiting the output current. A
reasonable value for these resistors is 100
Ω.
Using an inadequate amplifier to drive the analog input.
As explained in Section 1.3, the capacitance seen at the
input alternates between 8 pF and 7 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. A small series resistor at each amplifier output and a
capacitor across the analog inputs (as shown in Figures 5, 6)
will
improve performance. The LMH6702, LMH6628,
LMH6622 and LMH6655 have been successfully used to
drive the analog inputs of the ADC11L066.
Also, it is important that the signals at the two inputs have
exactly the same amplitude and be exactly 180
o out of phase
with each other. Board layout, especially equality of the
length of the two traces to the input pins, will affect the
effective phase between these two signals. Remember that
an operational amplifier operated in the non-inverting con-
figuration will exhibit more time delay than will the same
device operating in the inverting configuration.
Operating with the reference pins outside of the speci-
fied range. As mentioned in Section 1.2, V
REF
should be in
the range of
0.8V ≤ V
REF
≤ 1.5V
Operating outside of these limits could lead to performance
degradation.
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.
ADC1
1L066
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