Datasheet

AD9648
Rev. 0 | Page 27 of 44
In any configuration, the value of Shunt Capacitor C is dependent
on the input frequency and source impedance and may need to
be reduced or removed. Table 11 displays the suggested values to
set the RC network. However, these values are dependent on the
input signal and should be used only as a starting guide.
Table 11. Example RC Network
Frequency Range (MHz)
R Series
(Ω Each)
C Differential (pF)
0 to 70 33 22
70 to 200 125 Open
Single-Ended Input Configuration
A single-ended input can provide adequate performance in
cost-sensitive applications. In this configuration, SFDR and
distortion performance degrade due to the large input common-
mode swing. If the source impedances on each input are matched,
there should be little effect on SNR performance. Figure 48
shows a typical single-ended input configuration.
1V p-p
R
R
C
49.9Ω
0.1µF
10µF
10µF
0.1µF
AVDD
1kΩ
1kΩ
1kΩ
1kΩ
ADC
AVDD
VIN+x
VIN–x
09975-052
Figure 48. Single-Ended Input Configuration
VOLTAGE REFERENCE
A stable and accurate 1.0 V voltage reference is built into the
AD9648. The VREF can be configured using either the internal
1.0 V reference or an externally applied 1.0 V reference voltage.
The various reference modes are summarized in the sections that
follow. The Reference Decoupling section describes the best
practices PCB layout of the reference.
Internal Reference Connection
A comparator within the AD9648 detects the potential at the
SENSE pin and configures the reference into two possible
modes, which are summarized in Table 12. If SENSE is grounded,
the reference amplifier switch is connected to the internal resistor
divider (see Figure 49), setting VREF to 1.0 V.
VREF
SENSE
0.5V
ADC
SELECT
LOGIC
0.1µF
1.0µF
VIN–A/VIN–B
VIN+A/VIN+B
ADC
CORE
09975-055
Figure 49. Internal Reference Configuration
If the internal reference of the AD9648 is used to drive multiple
converters to improve gain matching, the loading of the reference
by the other converters must be considered. Figure 50 shows
how the internal reference voltage is affected by loading.
0
–3.0
0 2.0
LOAD CURRENT (mA)
REFERENCE VOLTAGE ERROR (%)
–0.5
–1.0
–1.5
–2.0
–2.5
0.2 0.4 0.6 0.8 1.0 1.4 1.6 1.81.2
INTERNAL V
REF
= 1.00V
09975-078
Figure 50. V
REF
Accuracy vs. Load Current
Table 12. Reference Configuration Summary
Selected Mode SENSE Voltage (V) Resulting VREF (V) Resulting Differential Span (V p-p)
Fixed Internal Reference AGND to 0.2 1.0 internal 2.0
Fixed External Reference AVDD 1.0 applied to external VREF pin 2.0