AD9251 Analog Devices, AD9251 Datasheet - Page 23

• Manufacturer Part Number: AD9251
• Description: 14-Bit, 20 MSPS/40 MSPS/65 MSPS/80 MSPS, 1.8 V Dual Analog-to-Digital Converter
• Manufacturer: Analog Devices

Specifications of AD9251

• Resolution (bits): 14bit
• # Chan: 2
• Sample Rate: 80MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p,2 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

VOLTAGE REFERENCE
A stable and accurate 1.0 V voltage reference is built into the
AD9251. 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 AD9251 detects the potential at the
SENSE pin and configures the reference into two possible
modes, which are summarized in Table 10. If SENSE is grounded,
the reference amplifier switch is connected to the internal resistor
divider (see Figure 46), setting VREF to 1.0 V.
Table 10. Reference Configuration Summary
Selected Mode
Fixed Internal Reference
Fixed External Reference
1.0µF
Figure 46. Internal Reference Configuration
VIN+A/VIN+B
VIN–A/VIN–B
0.1µF
SENSE
VREF
SELECT
LOGIC
SENSE Voltage (V)
AGND to 0.2
AVDD
ADC
0.5V
CORE
ADC
Resulting VREF (V)
1.0 internal
1.0 applied to external VREF pin
Rev. A | Page 23 of 36
If the internal reference of the AD9251 is used to drive multiple
converters to improve gain matching, the loading of the reference
by the other converters must be considered. Figure 47 shows
how the internal reference voltage is affected by loading.
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
0
0
0.2
Figure 47. V
0.4
Resulting Differential Span (V p-p)
2.0
2.0
0.6
REF
LOAD CURRENT (mA)
INTERNAL V
Accuracy vs. Load Current
0.8
1.0
REF
1.2
= 0.993V
1.4
1.6
1.8
AD9251
2
.0