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)
0 to 70
70 to 200
C Differential (pF)
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.
Figure 48. Single-Ended Input Configuration
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.
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.
INTERNAL VREF = 1.00V
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
LOAD CURRENT (mA)
Figure 50. VREF Accuracy vs. Load Current
Table 12. Reference Configuration Summary
SENSE Voltage (V)
Fixed Internal Reference
AGND to 0.2
Fixed External Reference
Resulting VREF (V)
1.0 applied to external VREF pin
Resulting Differential Span (V p-p)
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