AD9271 Analog Devices, AD9271 Datasheet - Page 34

• Manufacturer Part Number: AD9271
• Description: Octal LNA/VGA/AAF/ADC and Crosspoint Switch
• Manufacturer: Analog Devices

Specifications of AD9271

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 50MSPS
• Interface: LVDS,Ser
• Analog Input Type: SE-Uni
• Ain Range: 0.25 V p-p,0.32 V p-p,0.4 V p-p
• Adc Architecture: Pipelined
• Pkg Type: QFP

AD9271
Internal Reference Operation
A comparator within the AD9271 detects the potential at the
SENSE pin and configures the reference. If SENSE is grounded,
the reference amplifier switch is connected to the internal
resistor divider (see Figure 66), setting VREF to 1 V.
The REFT and REFB pins establish their input span of the ADC
core from the reference configuration. The analog input full-
scale range of the ADC equals twice the voltage at the reference
pin for either an internal or an external reference configuration.
1µF
*OPTIONAL.
1µF*
REFERENCE
EXTERNAL
SENSE
0.1µF
SENSE
0.1µF*
VREF
VIN+
VIN–
VREF
AVDD
VIN+
VIN–
Figure 66. Internal Reference Configuration
Figure 67. External Reference Operation
SELECT
LOGIC
SELECT
LOGIC
CORE
ADC
CORE
ADC
0.5V
0.5V
REFT
REFB
REFT
REFB
0.1µF
0.1µF
0.1µF
0.1µF
0.1µF
0.1µF
+
+
4.7µF
4.7µF
Rev. B | Page 34 of 60
External Reference Operation
The use of an external reference may be necessary to enhance
the gain accuracy of the ADC or to improve thermal drift charac-
teristics. Figure 69 shows the typical drift characteristics of the
internal reference in 1 V mode.
When the SENSE pin is tied to AVDD, the internal reference is
disabled, allowing the use of an external reference. The external
reference is loaded with an equivalent 6 kΩ load. An internal
reference buffer generates the positive and negative full-scale
references, REFT and REFB, for the ADC core. Therefore, the
external reference must be limited to a nominal voltage of 1.0 V.
–0.02
–0.04
–0.06
–0.08
–0.10
–0.12
–0.14
–0.16
–0.18
–0.20
0.02
–10
–15
–20
–25
–5
5
0
0
–40
0
Figure 68. VREF Accuracy vs. Load, AD9271-50
Figure 69. Typical VREF Drift, AD9271-50
0.5
–20
1.0
0
CURRENT LOAD (mA)
TEMPERATURE (°C)
1.5
20
2.0
40
2.5
60
3.0
80
3
.5