Instruction Manual

Applications Information (Continued)
The full scale error in LSB for a sine wave input can be
described as approximately
E
FS
=4096(1-sin(90˚ + dev))
Where dev is the angular difference between the two signals
having a 180˚ relative phase relationship to each other (see
Figure 3). Drive the analog inputs with a source impedance
less than 100Ω.
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 (which will result in reduced performance),
one of the analog inputs should be connected 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 distor-
tion performance, better performance with a single-ended
input can be obtained by reducing the reference voltage
while maintaining a full-range output. Table 1 and Table 2
indicate the input to output relationship of the ADC12L080.
The V
IN
+ and the V
IN
− inputs of the ADC12L080 consist of
an analog switch followed by a switched-capacitor amplifier.
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
minimize this, use 33Ω series resistors at each of the signal
inputs with a 51 pF capacitor to ground, 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 oppor-
tunity to filter the input. The 51 pF capacitor value is for
Nyquist applications and should be replaced with a smaller
capacitor for undersampling applications. The resulting pole
should be at 1.7 to 2.0 times the highest input frequency.
When determining this capacitor value, take into consider-
ation the 8 pF ADC input capacitance.
Table 3 gives component values for Figure 5 to convert a
signals to a range 1.0V
±
0.5V at each of the differential input
pins of the ADC12L080.
TABLE 3. Resistor values for Circuit of Figure 5
SIGNAL
RANGE
R1 R2 R3 R4 R5, R6
0 - 0.25V 0Ω open 200Ω 1780Ω 1000Ω
0 - 0.5V 0Ω open 249Ω 1400Ω 499Ω
±
0.5V 100Ω 1210Ω 100Ω 1210Ω 499Ω
3.0 DIGITAL INPUTS
Digital inputs consist of CLK, OF and PD. All digital inputs
are 3V CMOS compatible.
3.1 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 indicated in the Electrical Table 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
minimum sample rate.
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 ADC12L080 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 indicated in the Electrical Table.
The clock line should be terminated at its source in the
characteristic impedance of that line. Take care to maintain a
constant clock line impedance throughout the length of the
line. Refer to Application Note AN-905 for information on
setting characteristic impedance.
It is highly desirable that the the source driving the ADC CLK
pin only drive that pin. However, if that source is used to
drive other things, each driven pin should be a.c. terminated
with a series RC to ground, as shown in Figure 4, such that
the resistor value is equal to the characteristic impedance of
the clock line and the capacitor value is
where t
PD
is the signal propagation rate down the clock line,
"L" is the line length and Z
O
is the characteristic impedance
of the clock line. This termination should be as close as
possible to the ADC clock pin but beyond it as seen from the
clock source. Typical t
PD
is about 150 ps/inch (60 ps/cm) on
FR-4 board material. The units of "L" and t
PD
should be the
same (inches or centimeters).
3.2 OF
The OF pin is used to determine the digital data output
format. When this pin is high, the output formant is two’s
complement. When this pin is low the output format is offset
binary. Changing this pin while the device is operating will
result in uncertainty of the data for a few conversion cycles.
20061012
FIGURE 3. Angular Errors Between the Two Input
Signals Will Reduce the Output Level or Cause
Distortion
ADC12L080
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