ADC08D500CIYB/NOPB National Semiconductor, ADC08D500CIYB/NOPB Datasheet - Page 30

• Manufacturer Part Number: ADC08D500CIYB/NOPB
• Description: IC ADC 8BIT 500MSPS DUAL 128LQFP
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of ADC08D500CIYB/NOPB

• Number Of Bits: 8
• Sampling Rate (per Second): 500M
• Data Interface: Serial
• Number Of Converters: 2
• Power Dissipation (max): 1.78W
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 128-LQFP Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *ADC08D500CIYB; *ADC08D500CIYB/NOPB; ADC08D500CIYB

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2.0 Applications Information
The buffered analog inputs simplify the task of driving these
inputs and the RC pole that is generally used at sampling
ADC inputs is not required. If it is desired to use an amplifier
circuit before the ADC, use care in choosing an amplifier with
adequate noise and distortion performance and adequate
gain at the frequencies used for the application.
Note that a precise d.c. common mode voltage must be
present at the ADC inputs. This common mode voltage,
V
and the input signal is a.c. coupled to the ADC.
When the inputs are a.c. coupled, the V
grounded, as shown in Figure 11. This causes the on-chip
V
50k-Ohm resistors.
IMPORTANT NOTE: An Analog input channel that is not
used (e.g. in DES Mode) should be left floating when the
inputs are a.c. coupled. Do not connect an unused analog
input to ground.
When the d.c. coupled mode is used, a common mode
voltage must be provided at the differential inputs. This
common mode voltage should track the V
Note that the V
perature. The common mode output of the driving device
should track this change.
IMPORTANT NOTE: An analog input channel that is not
used (e.g. in DES Mode) should be tied to the V
when the inputs are d.c coupled. Do not connect unused
analog inputs to ground.
Full-scale distortion performance falls off rapidly as the
input common mode voltage deviates from V
a direct result of using a very low supply voltage to
minimize power. Keep the input common voltage within
50 mV of V
(Continued)
CMO
CMO
V
V
RELATIONSHIP (Non-Extended Control Mode, FSR
V
V
CM
CM
CM
CM
, is provided on-chip when a.c. input coupling is used
TABLE 5. DIFFERENTIAL INPUT TO OUTPUT
voltage to be connected to the inputs through on-chip
− 217.5mV
+ 217.5mV
+ 109 mV
V
− 109mV
V
IN
CM
FIGURE 11. Differential Input Drive
+
CMO
CMO
.
output potential will change with tem-
V
V
V
V
CM
CM
CM
CM
High)
+ 217.5mV
− 217.5mV
V
+ 109mV
V
− 109mV
IN
CM
−
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CMO
Output Code
CMO
0111 1111 /
0000 0000
0100 0000
1000 0000
1100 0000
1111 1111
output must be
CMO
CMO
output pin.
. This is
voltage
30
Performance is as good in the d.c. coupled mode as it is
in the a.c. coupled mode, provided the input common
mode voltage at both analog inputs remain within 50 mV
of V
If d.c. coupling is used, it is best to servo the input common
mode voltage, using the V
performance. An example of this type of circuit is shown in
Figure 12.
FIGURE 12. Example of Servoing the Analog Input with
One such circuit should be used in front of the V
another in front of the V
R
gained up by the amplifier, the input common mode voltage
is equal to V
allow the bypass capacitor to isolate the input signal from
V
essary. Capacitor "C" in Figure 12 should be chosen to keep
any component of the input signal from affecting V
Be sure that the current drawn from the V
not exceed 100 µA.
The Input impedance in the d.c. coupled mode (V
grounded) consists of a precision 100Ω resistor between
V
to ground. In the a.c. coupled mode the input appears the
same except there is also a resistor of 50K between each
analog input pin and the V
Driving the inputs beyond full scale will result in a saturation
or clipping of the reconstructed output.
2.2.1 Handling Single-Ended Input Signals
There is no provision for the ADC08D500 to adequately
process single-ended input signals. The best way to handle
single-ended signals is to convert them to differential signals
before presenting them to the ADC. The easiest way to
accomplish single-ended to differential signal conversion is
with an appropriate balun-connected transformer, as shown
in Figure 13.
CMO
IN
D3
+ and V
conversion with a balun-connected transformer
CMO
are used to divide the V
FIGURE 13. Single-Ended to Differential signal
. R
.
IN
, R
IN
− and a capacitance from each of these inputs
CMO
D2
and R
from the ADC. R
D3
IN
− input. In that figure, R
will divide the input signal, if nec-
CMO
V
CMO
CMO
CMO
potential.
potential so that, after being
pin, to maintain optimum
D1
and R
CMO
D2
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IN
output does
D1
CMO
+ input and
are split to
20121455
, R
CMO
D2
pin not
.
and