ADC08D1020DEV/NOPB National Semiconductor, ADC08D1020DEV/NOPB Datasheet - Page 36

• Manufacturer Part Number: ADC08D1020DEV/NOPB
• Description: BOARD DEV FOR ADC08D1020
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of ADC08D1020DEV/NOPB

• Number Of Adc's: 2
• Number Of Bits: 8
• Sampling Rate (per Second): 1G
• Data Interface: Serial
• Inputs Per Adc: 1 Differential
• Input Range: 870 mVpp
• Power (typ) @ Conditions: 1.6W @ 1GSPS
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: ADC08D1020
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: ADC08D1020DEV

www.national.com
Figure 13
tial signal conversion using a balun. The circuitry specific to
the balun will depend upon 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. A designer
should match the impedance of their analog source to the
ADC081020's on-chip 100Ω differential input termination re-
sistor. The range of this termination resistor is described in
the electrical table as the specification R
Also, the phase and amplitude balance are important. The
lowest possible phase and amplitude imbalance is desired
In
offset that can be measured at the ADC inputs V
LMH6555's input terminated to ground as shown but not driv-
en and with no R
offset with reference to V
reduced with a resistor in the R
adjusted negative offset with reference to V
|15mV| should be reduced with a resistor in the R
Table 9
unadjusted differential offsets to bring the V
back to within |15mV|.
Figure
FIGURE 13. Single-Ended to Differential Signal
gives suggested R
14, R
is a generic depiction of a single-ended to differen-
ADJ-
Conversion Using a Balun
ADJ
and R
resistors applied. An unadjusted positive
ADJ+
IN-
ADJ-
are used to adjust the differential
greater than |15mV| should be
ADJ-
and R
FIGURE 14. Example of Servoing the Analog Input with V
position. Likewise, an un-
ADJ+
IN
.
values for various
IN-
IN+
ADJ+
IN+
greater than
/ V
20206243
/ V
IN-
position.
IN-
offset
with
36
when selecting a balun. The phase imbalance should be no
more than ±2.5° and the amplitude imbalance should be lim-
ited to less than 1dB at the desired input frequency range.
Finally, when selecting a balun, the VSWR (Voltage Standing
Wave Ratio), bandwidth and insertion loss of the balun should
also be considered. The VSWR aids in determining the overall
transmission line termination capability of the balun when in-
terfacing to the ADC input. The insertion loss should be
considered so that the signal at the balun output is within the
specified input range of the ADC as described in the Con-
verter Electrical Characteristics as the specification V
2.2.1.2. d.c. Coupled Input
When d.c. coupling to the ADC08D1020 analog inputs is re-
quired, single-ended to differential conversion may be easily
accomplished with the LMH6555, as shown
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 ADC08D1020. Connecting the ADC08D1020
V
propriate buffer, will ensure that the common mode input
voltage is as needed for optimum performance of the AD-
C08D1020. The LMV321 was chosen to buffer V
low voltage operation and reasonable offset voltage.
Be sure to limit output current from the ADC08D1020 V
pin to 100 μA
CMO
Unadjusted Offset Reading
pin to the V
TABLE 9. D.C. Coupled Offset Adjustment
91mV to 110mV
11mV to 30mV
31mV to 50mV
51mV to 70mV
71mV to 90mV
0mV to 10mV
CM_REF
pin of the LMH6555, through an ap-
CMO
20206255
no resistor needed
Resistor Value
20.0kΩ
10.0kΩ
6.81kΩ
4.75kΩ
3.92kΩ
inFigure
CMD
IN
14. In
for its
.
CMO