Applications Information (Continued)
For differential operation, each analog input signal should
have a peak-to-peak voltage equal to the input reference
voltage, V
REF
, and be centered around V
CM
. For single
ended operation, one of the analog inputs should be con-
nected to the d.c. common mode voltage of the driven input.
The peak-to-peak differential input signal should be twice the
reference voltage to maximize SNR and SINAD performance
(Figure 2b). For example, set V
REF
to 1.0V, bias V
IN
−to 1.0V
and drive V
IN
+with a signal range of 0V to 2.0V. Because
very large input signal swings can degrade distortion perfor-
mance, better performance with a single-ended input can be
obtained by reducing the reference voltage while maintaining
a full-range output.
Tables 1, 2
indicate the input to output
relationship of the ADC12L066.
The V
IN
+and the V
IN
−inputs of theADC12L066 consist of an
analog switch followed by a switched-capacitor amplifier.
The capacitance seen at the analog input pins changes with
the clock level, appearing as 8 pF when the clock is low, and
7 pF when the clock is high.Although this difference is small,
a dynamic capacitance is more difficult to drive than is a
fixed capacitance, so choose the driving amplifier carefully.
LMH6702, LMH6628, LMH6622, LMH6655 are good ampli-
fiers for driving the ADC12L066.
The internal switching action at the analog inputs causes
energy to be output from the input pins.As the driving source
tries to compensate for this, it adds noise to the signal. To
prevent this, use 18Ωseries resistors at each of the signal
inputs with a 10 pF capacitor across the inputs, as can be
seen in
Figure 5
and
Figure 6
. These components should be
placed close to the ADC because the input pins of the ADC
is the most sensitive part of the system and this is the last
opportunity to filter the input. The 10 pF capacitor value is for
undersampling application and should be replaced with a
68 pF capacitor for Nyquist application.
2.0 DIGITAL INPUTS
Digital inputs consist of CLK, OE and PD.
2.1 CLK
The CLK signal controls the timing of the sampling process.
Drive the clock input with a stable, low jitter clock signal in
the range of 10 MHz to 80 MHz with rise and fall times of less
than 2 ns. The trace carrying the clock signal should be as
short as possible and should not cross any other signal line,
analog or digital, not even at 90˚.
The CLK signal also drives an internal state machine. If the
CLK is interrupted, or its frequency is too low, the charge on
internal capacitors can dissipate to the point where the ac-
curacy of the output data will degrade. This is what limits the
lowest sample rate to 10 MSPS.
The duty cycle of the clock signal can affect the performance
of any A/D Converter. Because achieving a precise duty
cycle is difficult, the ADC12L066 is designed to maintain
performance over a range of duty cycles. While it is specified
and performance is guaranteed with a 50% clock duty cycle,
performance is typically maintained over a clock duty cycle
range of 40% to 60%.
The clock line should be series terminated at the clock
source in the characteristic impedance of that line. If the
clock line is longer than
where t
r
is the clock rise time and t
prop
is the propagation rate
of the signal along the trace, the CLOCK pin should be a.c.
terminated with a series RC such that the resistor value is
equal to the characteristic impedance of the clock line and
the capacitor value is
where ’I’ is the line length in inches and Z
o
is the characteric
impedance of the clock line. This termination should be
located as close as possible to, but within one centimeter of,
the ADC12L066 clock pin as shown in
Figure 4
.
2.2 OE
The OE pin, when high, puts the output pins into a high
impedance state. When this pin is low the outputs are in the
active state. The ADC12L066 will continue to convert
whether this pin is high or low, but the output can not be read
while the OE pin is high.
The OE pin should NOT be used to multiplex devices to-
gether to drive a common bus as this will result in excessive
capacitance on the data output pins, reducing SNR and
SINAD performance of the converter. See Section 3.0.
2.3 PD
The PD pin, when high, holds the ADC12L066 in a power-
down mode to conserve power when the converter is not
being used. The power consumption in this state is 50 mW.
The output data pins are undefined in this mode. Power
consumption during power-down is not affected by the clock
frequency, or by whether there is a clock signal present. The
data in the pipeline is corrupted while in the power down
mode.
3.0 OUTPUTS
TheADC12L066 has 12 TTL/CMOS compatible Data Output
pins. The offset binary data is present at these outputs while
the OE and PD pins are low. While the t
OD
time provides
information about output timing, a simple way to capture a
valid output is to latch the data on the
rising edge
of the
conversion clock (pin 10).
Be very careful when driving a high capacitance 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 current
spikes can cause on-chip ground noise and couple into the
analog circuitry, degrading dynamic performance. Adequate
bypassing, limiting output capacitance and careful attention
to the ground plane will reduce this problem. Additionally,
20032812
FIGURE 3. Angular Errors Between the Two Input
Signals Will Reduce the Output Level or Cause
Distortion
ADC12L066
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