AD9627-11 Analog Devices, AD9627-11 Datasheet - Page 25

• Manufacturer Part Number: AD9627-11
• Description: 11-Bit, 105 MSPS/150 MSPS, 1.8 V Dual Analog-to-Digital Converter
• Manufacturer: Analog Devices

Specifications of AD9627-11

• Resolution (bits): 11bit
• # Chan: 2
• Sample Rate: 150MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p,1 V p-p,2 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

VOLTAGE REFERENCE
A stable and accurate voltage reference is built into the
AD9627-11. The input range can be adjusted by varying the
reference voltage applied to the AD9627-11, using either the
internal reference or an externally applied reference voltage.
The input span of the ADC tracks reference voltage changes
linearly. 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 AD9627-11 detects the potential at the
SENSE pin and configures the reference into four possible
modes, which are summarized in Table 11. If SENSE is grounded,
the reference amplifier switch is connected to the internal
resistor divider (see Figure 51), setting VREF to 1.0 V. Connecting
the SENSE pin to VREF switches the reference amplifier output
to the SENSE pin, completing the loop and providing a 0.5 V
reference output.
If a resistor divider is connected external to the chip, as shown
in Figure 52, the switch again sets to the SENSE pin. This puts
the reference amplifier in a noninverting mode with the VREF
output defined as follows:
Table 11. Reference Configuration Summary
Selected Mode
External Reference
Internal Fixed Reference
Programmable Reference
Internal Fixed Reference
VREF
1.0µF

0
Figure 51. Internal Reference Configuration
5 .
VIN+A/VIN+B
VIN–A/VIN–B

0.1µF
 


1
SENSE
VREF
R2
R1



SELECT
LOGIC
SENSE Voltage
AVDD
VREF
0.2 V to VREF
AGND to 0.2 V
AD9627-11
0.5V
CORE
ADC
Resulting VREF (V)
N/A
0.5
1.0
0
5 .

Rev. B | Page 25 of 72



1

R2
R1



(see Figure 52)
The input range of the ADC always equals twice the voltage at
the reference pin for either an internal or an external reference.
If the internal reference of the AD9627-11 is used to drive multiple
converters to improve gain matching, the loading of the reference
by the other converters must be considered. Figure 53 shows
how the internal reference voltage is affected by loading.
–0.25
–0.50
–0.75
–1.00
–1.25
1.0µF
0
0
Figure 52. Programmable Reference Configuration
0.1µF
VREF
Figure 53. VREF Accuracy vs. Load
VIN+A/VIN+B
VIN–A/VIN–B
Resulting Differential Span (V p-p)
2 × external reference
1.0
2 × VREF
2.0
0.5
SENSE
R2
R1
LOAD CURRENT (mA)
VREF = 1.0V
1.0
SELECT
LOGIC
AD9627-11
VREF = 0.5V
1.5
0.5V
AD9627-11
CORE
ADC
2.0