Datasheet
Data Sheet AD9637
Rev. A | Page 19 of 40
Differential Input Configurations
There are several ways to drive the AD9637 either actively or
passively. However, optimum performance is achieved by driving
the analog input differentially. Using a differential double balun
configuration to drive the AD9637 provides excellent performance
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 AD9637.
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 AD9637 input single-ended.
VOLTAGE REFERENCE
A stable and accurate 1.0 V voltage reference is built into the
AD9637. 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.
Internal Reference Connection
A comparator within the AD9637 detects the potential at 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
SENSE
Voltage (V)
Resulting
V
REF
(V)
Resulting
Differential
Span (V p-p)
Fixed Internal
Reference
AGND to 0.2 1.0 internal 2.0
Fixed External
Reference
AVDD 1.0 applied
to external
VREF pin
2.0
VREF
SENSE
0.5V
ADC
SELECT
LOGIC
0.1µF
1.0µF
VIN– x
VIN+ x
ADC
CORE
10215-044
Figure 45. Internal Reference Configuration
ADC
R
0.1µF
0.1µF
2V p-p
VCM
C
*C1
*C1
C
R
0.1µF
0.1µF 0.1µF
33Ω
200Ω
33Ω
33Ω
33Ω
VIN+ x
VIN– x
ET1-1-I3
C
C
5pF
R
*C1 IS OPTIONAL
10215-045
Figure 46. Differential Double Balun Input Configuration for Baseband Applications
2Vp-p
R
R
*C1
*C1 IS OPTIONAL
49.9Ω
0.1μF
ADT1-1WT
1:1 Z RATIO
VIN– x
ADC
VIN+ x
*C1
C
VCM
33Ω
33Ω
200Ω
0.1µF
5pF
10215-046
Figure 47. Differential Transformer-Coupled Configuration for Baseband Applications