ADC08D1500DEV/NOPB National Semiconductor, ADC08D1500DEV/NOPB Datasheet - Page 31

• Manufacturer Part Number: ADC08D1500DEV/NOPB
• Description: BOARD DEV FOR ADC08D1500
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of ADC08D1500DEV/NOPB

• Mfg Application Notes: Clocking High-Speed A/D Converters AppNote
• Number Of Adc's: 2
• Number Of Bits: 8
• Sampling Rate (per Second): 1.5G
• Data Interface: Serial
• Inputs Per Adc: 1 Differential
• Input Range: 870 mVpp
• Power (typ) @ Conditions: 1.8W @ 1.5GSPS
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: ADC08D1500
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: ADC08D1500DEV

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 of the ADC08D1500 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
2.2.1 Handling Single-Ended Input Signals
There is no provision for the ADC08D1500 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 12.
2.2.1.1 A.C. Coupled Input
The easiest way to accomplish single-ended a.c. input to dif-
ferential a.c. signal is with an appropriate balun-connected
transformer, as shown in Figure 12.
Figure 12 is a generic depiction of a single-ended to differen-
tial signal conversion using a balun. The circuitry specific to
the balun will depend on the type of balun selected and the
overall board layout. It is recommended that the system de-
signer contact the manufacturer of the balun they have se-
lected to aid in designing the best performing single-ended to
differential conversion circuit using that particular balun.
When selecting a balun, it is important to understand the input
architecture of the ADC. There are specific balun parameters
of which the system designer should be mindful. They should
match the impedance of their analog source to the
ADC08D1500's on-chip 100 differential input termination re-
sistor. The range of this termination resistor is described in
the electrical table as the specification R
Also, as a result of the ADC architecture, the phase and am-
plitude balance are important. The lowest possible phase and
amplitude imbalance is desired when selecting a balun. The
phase imbalance should be no more than ±2.5° and the am-
plitude imbalance should be limited to less than 1dB at the
desired input frequency range. Finally, when selecting a
balun, the VSWR (Voltage Standing Wave Ratio), bandwidth
CMO
FIGURE 12. Single-Ended to Differential Signal
output potential will change with temperature. The com-
CMO
.
Conversion Using a Balun
CMO
.
CMO
output pin. Note that the
IN
.
CMO
voltage when
CMO
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. This is
31
and insertion loss of the balun should also be considered. The
VSWR aids in determining the overall transmission line ter-
mination capability of the balun when interfacing to the ADC
input. The insertion loss should be considered so that the sig-
nal at the balun output is within the specified input range of
the ADC as described in the Converter Electrical Character-
istics as the specification V
2.2.1.2 D.C. Coupled Input
When d.c. coupling to the ADC08D1500 analog inputs is re-
quired, single-ended to differential conversion may be easily
accomplished with the LMH6555, as shown in Figure 13. In
such applications, the LMH6555 performs the task of single-
ended to differential conversion while delivering low distortion
and noise, as well as output balance, that supports the oper-
ation of the ADC08D1500. Connecting the ADC08D1500
V
propriate buffer, will ensure that the ADC08D1500 common
mode input voltage is as needed for optimum performance of
the ADC08D1500. See Figure 13. The LMV321 was chosen
as the buffer in Figure 13 for its low voltage operation and
reasonable offset voltage.
Be sure to limit output current from the ADC08D1500 V
pin to 100 μA.
Figure 13, R
offset that can be measured at the ADC inputs V
unadjusted positive offset with reference to V
|15mV| should be reduced with a resistor in the R
Likewise, an unadjusted negative offset with reference to
V
the R
for various unadjusted differential offsets to bring the V
V
FIGURE 13. Example of Servoing the Analog Input with
CMO
IN-
IN-
greater than |15mV| should be reduced with a resistor in
offset back to within |15mV|.
ADJ+
Unadjusted Offset
pin to the V
91mV to 110mV
TABLE 6. D.C. Coupled Offset Adjustment
11mV to 30mV
31mV to 50mV
51mV to 70mV
71mV to 90mV
0mV to 10mV
position. gives suggested R
Reading
ADJ-
CM_REF
and R
ADJ+
pin of the LMH6555, through the ap-
IN
V
are used to adjust the differential
.
CMO
no resistor needed
Resistor Value
ADJ-
20.0kΩ
10.0kΩ
6.81kΩ
4.75kΩ
3.92kΩ
and R
IN-
ADJ-
greater than
IN+
www.national.com
ADJ+
/ V
position.
20152155
values
IN-
. An
IN+
CMO
/