ADC08351CIMTC National Semiconductor, ADC08351CIMTC Datasheet - Page 9

• Manufacturer Part Number: ADC08351CIMTC
• Description: 8-Bit/ 42 MSPS/ 40 mW A/D Converter
• Manufacturer: National Semiconductor

Timing Diagram
Functional Description
The ADC08351 achieves 6.8 effective bits at 21 MHz input
frequency with 42 MHz clock frequency digitizing to eight bits
the analog signal at V
range of 0.5 V
Input voltages below 0.0665 times the reference voltage will
cause the output word to consist of all zeroes, while input
voltages above
put word to consist of all ones. For example, with a V
2.4V, input voltages below 160 mV will result in an output
word of all zeroes, while input voltages above 1.79V will re-
sult in an output word of all ones.
The output word rate is the same as the clock frequency.
Data is acquired at the falling edge of the clock and the digi-
tal equivalent of that data is available at the digital outputs
2.5 clock cycles plus t
long as the clock signal is present at pin 12, but the data will
not appear at the outputs unless the OE pin 1 is low. The
digital outputs are in the high impedance state when the OE
pin or when the PD pin is high.
Applications Information
1.0 THE ADC REFERENCE AND THE ANALOG INPUT
The capacitance seen at the input changes with the clock
level, appearing as 4 pF when the clock is low, and 11 pF
when the clock is high. Since a dynamic capacitance is more
difficult to drive than is a fixed capacitance, choose an ampli-
fier that can drive this type of load. The CLC409, CLC440,
LM6152, LM6154, LM6181 and LM6182 are good devices
for driving analog input of the ADC08351. Do not drive the in-
put beyond the supply rails.
The maximum peak-to-peak input level without clipping of
the reconstructed output is determined by the values of the
resistor string between V
reference ladder has a voltage of 0.0665 times V
the top of the reference ladder has a voltage of 0.7468 times
V
be about 68% of the value of V
the input peak-to-peak voltage and V
We do not recommend opertaing with input levels below
1 V
REF
P-P
. The maximum peak-to-peak input level works out to
because the signal-to-noise ratio will degrade consid-
P-P
3
⁄
4
to 0.68 V
of the reference voltage will cause the out-
OD
IN
later. The ADC08351 will convert as
REF
that is within the nominal voltage
A
(Continued)
.
and AGND. The bottom of the
REF
. The relationship between
REF
is
FIGURE 2. t
REF
, while
REF
of
EN
, t
9
DIS
erably due to the quantization noise. However, the
ADC08351 will give adequate results in many applications
with signal levels down to about 0.5 V
Very good performance can be obtained with reference volt-
ages up to the supply voltage (V
As with all sampling ADCs, the opening and closing of the
switches associated with the sampling causes an output of
energy from the analog input, V
has switches associated with it, so the reference source
must be able to supply sufficient current to hold V
The analog input of the ADC08351 is self-biased with an
18 k pull-up resistor to V
to AGND. This allows for either a.c. or d.c. coupling of the in-
put signal. These two resistors provide a convenient way to
ensure a signal that is less than full scale will be centered
within the input common mode range of the converter. How-
ever, the high values of these resistors and the energy com-
ing from this input means that performance will be improved
with d.c. coupling.
The driving circuit at the signal input must be able to sink and
source sufficient current at the signal frequency to prevent
distortion from being introduced at the input.
2.0 POWER SUPPLY CONSIDERATIONS
A tantalum or aluminum electrolytic capacitor of 5 µF to
10 µF should be placed within a centimeter of each of the
A/D power pins, with a 0.1 µF ceramic chip capacitor placed
within
capacitors are preferred because they provide lower lead in-
ductance than do their leaded counterparts.
While a single voltage source should be used for the analog
and digital supplies of the ADC08351, these supply pins
should be decoupled from each other to prevent any digital
noise from being coupled to the analog power pins. A ferrite
bead between the analog and digital supply pins would help
to isolate the two supplies.
The converter digital supply should not be the supply that is
used for other digital circuitry on the board. It should be the
same supply used for the A/D analog supply, decoupled from
the A/D analog supply pin, as described above. A common
analog supply should be used for both V
of these pins should be separately bypassed with a 0.1 µF
ceramic capacitor and with low ESR a 10 µF capacitor.
As is the case with all high speed converters, the ADC08351
is sensitive to power supply noise. Accordingly, the noise on
Test Circuit
DS100895-24
1
⁄
2
centimeter of each of the power pins. Leadless chip
REF
and a 12 k pull-down resistor
IN
A
. The reference ladder also
= V
REF
P-P
A
and V
(V
= 3V, 2.04 V
REF
D
www.national.com
REF
= 0.735V).
, and each
steady.
P-P
).