Datasheet

AD9281
–11–
QREFT
QREFB
IREFT
0.1mF10mF
0.1mF
0.1mF
AD9281
REFSENSE
AVSS
1V
0.1mF10mF
IREFB
VREF
0.1mF
0.1mF
10kV
10kV
ADC
CORE
INTERNAL
CONTROL
LOGIC
0.1mF1.0mF
Figure 26. Reference Buffer Equivalent Circuit and
External Decoupling Recommendation
For best results in both noise suppression and robustness
against crosstalk, the 4-capacitor buffer decoupling arrangement
shown in Figure 26 is recommended. This decoupling should
–3
–0.5
THD – dB
–13
–23
–33
–43
–53
–63
–73
0 0.5 1 1.5
CML – V
2V
1V
a. Differential Input, 3 V Supplies
–35
–0.5
THD – dB
–40
–45
–50
–55
–60
–65
–70
0 0.5 1 1.5
CML – V
2 2.5
1V
2V
b. Differential Input, 5 V Supplies
Figure 27. THD vs. CML Input Span and Power Supply (Analog Input = 1 MHz)
–15
–0.5
THD – dB
–25
–35
–45
–55
–65
0 0.5 1 1.5
CML – V
2V
1V
c. Single-Ended Input, 3 V Supplies
–15
–0.5
THD – dB
–25
–35
–45
–55
–65
0 0.5 1 1.5
CML – V
2V
1V
2 2.5
d. Single-Ended Input, 5 V Supplies
feature chip capacitors located close to the converter IC. The
capacitors are connected to either IREFT/IREFB or QREFT/
QREFB. A connection to both sides is not required.
COMMON-MODE PERFORMANCE
Attention to the common-mode point of the analog input volt-
age can improve the performance of the AD9281. Figure 27
illustrates THD as a function of common-mode voltage (center
point of the analog input span) and power supply.
Inspection of the curves will yield the following conclusions:
1. An AD9281 running with AVDD = 5 V is the easiest to
drive.
2. Differential inputs are the most insensitive to common-mode
voltage.
3. An AD9281 powered by AVDD = 3 V and a single ended
input, should have a 1 V span with a common-mode voltage
of 0.75 V.
REV. F