ADC08D500CIYB/NOPB National Semiconductor, ADC08D500CIYB/NOPB Datasheet - Page 31

• 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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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 ade-
quate 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
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 volt-
age must be provided at the differential inputs. This common
mode voltage should track the V
V
mon 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
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 min-
imize power. Keep the input common voltage within 50
mV of V
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
CMO
CMO
CMO
V
V
TABLE 5. Differential Input To Output Relationship
V
V
CM
CM
CMO
CM
CM
, is provided on-chip when a.c. input coupling is used
output potential will change with temperature. The com-
voltage to be connected to the inputs through on-chip
− 217.5mV
+ 217.5mV
+ 109 mV
.
V
− 109mV
(Non-Extended Control Mode, FSR High)
V
CMO
IN
CM
+
FIGURE 11. Differential Input Drive
.
V
V
V
V
CM
CM
Figure
CM
CM
+ 217.5mV
− 217.5mV
V
+ 109mV
− 109mV
V
IN
CM
−
11. This causes the on-chip
CMO
output pin. Note that the
20121444
CMO
CMO
Output Code
0111 1111 /
0000 0000
0100 0000
1000 0000
1100 0000
1111 1111
output must be
voltage when
CMO
. This is
30
If d.c. coupling is used, it is best to servo the input common
mode voltage with V
An example of this type of circuit is shown in
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
the bypass capacitor to isolate the input signal from V
R
pacitor "C" in
ponent of the input signal from affecting V
Be sure that the current drawn from the V
exceed 100 μA.
The Input impedance in the d.c. coupled mode (V
grounded) consists of a precision 100Ω resistor between V
+ and V
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 pro-
cess 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 ac-
complish single-ended to differential signal conversion is with
an appropriate balun-connected transformer, as shown in
Figure
The 100 Ohm external resistor placed across the output ter-
minals of the balun in parallel with the ADC08D1000's on-chip
100 Ohm resistor makes a 50 Ohms differential impedance
FIGURE 12. Example of Servoing the Analog Input with
D3
IN
, R
are used to divide the V
conversion with a balun-connected transformer
FIGURE 13. Single-Ended to Differential signal
D2
13.
IN
and R
− and a capacitance from each of these inputs to
CMO
Figure 12
D3
from the ADC. R
will divide the input signal, if necessary. Ca-
CMO
CMO
potential.
IN
should be chosen to keep any com-
− input. In that figure, R
to maintain optimum performance.
V
CMO
CMO
potential so that, after being
D1
and R
CMO
D2
CMO
are split to allow
Figure
output does not
.
IN
D1
+ input and
CMO
20121443
, R
12.
pin not
D2
20121455
CMO
and
IN
.