Datasheet
AD9645 Data Sheet
Rev. 0 | Page 20 of 36
Differential Input Configurations
There are several ways to drive the AD9645 either actively or
passively. However, optimum performance is achieved by driving
the analog inputs differentially. Using a differential double balun
configuration to drive the AD9645 provides excellent performance
and a flexible interface to the ADC for baseband applications
(see Figure 55).
For applications where SNR is a key parameter, differential trans-
former coupling is the recommended input configuration (see
Figure 56) because the noise performance of most amplifiers is
not adequate to achieve the true performance of the AD9645.
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 AD9645 inputs single-ended.
VOLTAGE REFERENCE
A stable and accurate 1.0 V voltage reference is built into the
AD9645. 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.
If the internal reference of the AD9645 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. Figure 54
shows the typical drift characteristics of the internal reference
in 1.0 V mode.
The internal buffer generates the positive and negative full-scale
references for the ADC core.
0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
–4.5
–5.0
0 3.02.52.01.51.00.5
V
REF
ERROR (%)
LOAD CURRENT (mA)
INTERNAL V
REF
= 1V
10537-048
Figure 53. V
REF
Error vs. Load Current
4
–8
–40 85
V
REF
ERROR (mV)
TEMPERATURE (°C)
–6
–4
–2
0
2
–15 10 35 60
10537-049
Figure 54. Typical V
REF
Drift
ADC
R0.1µF
0.1µF
2V p-p
VCM
C
*C1
*C1
C
R
0.1µF
0.1µF 0.1µF
33Ω
200Ω
33Ω
33Ω
33Ω
VINx+
VINx–
ET1-1-I3
C
C
5pF
R
*C1 IS OPTIONAL
10537-046
Figure 55. 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
VINx–
ADC
VINx+
*C1
C
VCM
33Ω
33Ω
200Ω
0.1µF
5pF
10537-047
Figure 56. Differential Transformer-Coupled Configuration for Baseband Applications