ADC08D1520DEV/NOPB National Semiconductor, ADC08D1520DEV/NOPB Datasheet - Page 36

• Manufacturer Part Number: ADC08D1520DEV/NOPB
• Description: BOARD DEV FOR ADC08D1520
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of ADC08D1520DEV/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: 1.6W @ 1GSPS
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: ADC08D1520
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: ADC08D1520DEV

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IMPORTANT NOTE: An analog input channel that is not used
(e.g. in DES Mode) should be connected to a.c. ground (i.e.,
capacitors to ground) when the inputs are a.c. coupled. Do
not connect an unused analog input directly to ground.
FIGURE 11. V
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 remains within 50 mV of V
2.2.1 Single-Ended Input Signals
There is no provision for the ADC08D1520 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
CMO
FIGURE 12. Single-Ended to Differential Signal
output potential will change with temperature. The com-
12.
CMO
.
CMO
Conversion Using a Balun
Drive for A.C. Coupled Differential Input
CMO
output pin. Note that the
20193144
CMO
voltage when
CMO
20193143
. This is
CMO
.
36
Figure 12
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 in order to aid in
designing the best performing single-ended to differential
conversion circuit.
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. The
impedance of the analog source should be matched to the
ADC08D1520's on-chip 100Ω differential input termination
resistor. The range of this termination resistor is described in
the Converter Electrical Characteristics as the specification
R
Also, the phase and amplitude 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 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; see V
Electrical Characteristics.
2.2.2. D.C. Coupled Input Signals
When d.c. coupling to the ADC08D1520 analog inputs, sin-
gle-ended to differential conversion may be easily accom-
plished with the LMH6555, as shown in
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 operation
of the ADC08D1520. Connecting the ADC08D1520 V
to the V
will ensure that the common mode input voltage meets the
requirements for optimum performance of the ADC08D1520.
The LMV321 was chosen to buffer V
operation and reasonable offset voltage.
The output current from the ADC08D1520 V
be limited to 100 μA.
IN
.
CM_REF
is a generic depiction of a single-ended to differen-
pin of the LMH6555, via an appropriate buffer,
CMD
IN
Figure
for its low voltage
in the Converter
CMO
13. In such
pin should
CMO
pin