AD9281 Analog Devices, AD9281 Datasheet - Page 11

• Manufacturer Part Number: AD9281
• Description: Dual Channel 8-Bit Resolution CMOS ADC
• Manufacturer: Analog Devices

Specifications of AD9281

• Resolution (bits): 8bit
• # Chan: 2
• Sample Rate: 28MSPS
• Interface: Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (Vref) p-p,1 V p-p,2 V p-p,Uni (Vref),Uni 1.0V,Uni 2.0V
• Adc Architecture: Pipelined
• Pkg Type: SOP

REV. F
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
1.0 F
10 F
Figure 26. Reference Buffer Equivalent Circuit and
External Decoupling Recommendation
0.1 F
0.1 F
–13
–23
–33
–43
–53
–73
–35
–40
–45
–50
–55
–60
–70
–63
–65
–3
–0.5
–0.5
REFSENSE
0.1 F
0.1 F
1V
2V
a. Differential Input, 3 V Supplies
1V
b. Differential Input, 5 V Supplies
IREFT
AVSS
IREFB
2V
VREF
0
0
10k
10k
Figure 27. THD vs. CML Input Span and Power Supply (Analog Input = 1 MHz)
0.5
CORE
CML – V
CML – V
ADC
AD9281
0.5
1
INTERNAL
CONTROL
LOGIC
1V
1.5
1
QREFT
QREFB
0.1 F
2
2.5
1.5
0.1 F
10 F
0.1 F
–11–
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
2. Differential inputs are the most insensitive to common-mode
3. An AD9281 powered by AVDD = 3 V and a single ended
drive.
voltage.
input, should have a 1 V span with a common-mode voltage
of 0.75 V.
–15
–25
–35
–45
–55
–65
–15
–25
–35
–45
–55
–65
–0.5
–0.5
d. Single-Ended Input, 5 V Supplies
c. Single-Ended Input, 3 V Supplies
1V
2V
1V
0
2V
0
0.5
CML – V
CML – V
0.5
1
1.5
1
AD9281
2
1.5
2.5