AD9257 Analog Devices, AD9257 Datasheet - Page 19

• Manufacturer Part Number: AD9257
• Description: Octal, 14-Bit, 40/65 MSPS Serial LVDS 1.8 V A/D Converter
• Manufacturer: Analog Devices

Data Sheet
Differential Input Configurations
There are several ways to drive the
passively. However, optimum performance is achieved by driving
the analog input differentially. Using a differential double balun
configuration to drive the
and a flexible interface to the ADC (see Figure 46) for baseband
applications.
For applications where SNR is a key parameter, differential trans-
former coupling is the recommended input configuration (see
Figure 47), because the noise performance of most amplifiers is
not adequate to achieve the true performance of the AD9257.
Regardless of the configuration, the value of the shunt capacitor,
C, is dependent on the input frequency and may need to be
reduced or removed.
It is not recommended to drive the
VOLTAGE REFERENCE
A stable and accurate 1.0 V voltage reference is built into the
AD9257. 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 VREF pin should be externally decoupled to ground
with a low ESR, 1.0 μF capacitor in parallel with a low ESR,
0.1 μF ceramic capacitor.
AD9257
2V p-p
provides excellent performance
Figure 46. Differential Double Balun Input Configuration for Baseband Applications
AD9257
AD9257
0.1µF
2V p-p
either actively or
input single-ended.
Figure 47. Differential Transformer-Coupled Configuration
ET1-1-I3
49.9Ω
1:1 Z RATIO
ADT1-1WT
0.1μF
for Baseband Applications
Rev. 0 | Page 19 of 40
0.1µF
0.1µF
C
C
*C1 IS OPTIONAL
33Ω
33Ω
R
R
33Ω
33Ω
0.1µF
*C1
C
5pF
Internal Reference Connection
A comparator within the
SENSE pin and configures the reference into two possible
modes, which are summarized in Table 9. If SENSE is grounded,
the reference amplifier switch is connected to the internal resistor
divider (see Figure 45), setting VREF to 1.0 V.
Table 9. Reference Configuration Summary
Selected Mode
Fixed Internal
Fixed External
*C1
33Ω
33Ω
Reference
Reference
R
R
*C1 IS OPTIONAL
200Ω
C
200Ω
R
*C1
VIN+ x
VIN– x
5pF
*C1
1.0µF
ADC
Figure 45. Internal Reference Configuration
VCM
VIN+ x
VIN– x
0.1µF
SENSE
Voltage (V)
AGND to 0.2
AVDD
0.1µF
SENSE
VIN+ x
VIN– x
VREF
ADC
AD9257
C
VCM
0.1µF
SELECT
LOGIC
Resulting
VREF (V)
1.0 internal
1.0 applied
to external
VREF pin
detects the potential at the
ADC
0.5V
CORE
ADC
Resulting
Differential
Span (V p-p)
2.0
2.0
AD9257