Datasheet

AD9481
Rev. 0 | Page 17 of 28
APPLICATIONS
The AD9481 uses a 1.5 bit per stage architecture. The analog
inputs drive an integrated high bandwidth track-and-hold
circuit that samples the signal prior to quantization by the 8-bit
core. For ease of use, the part includes an on-board reference
and input logic that accepts TTL, CMOS, or LVPECL levels. The
digital output logic levels are CMOS-compatible.
ANALOG INPUTS
The analog input to the AD9481 is a differential buffer. For best
dynamic performance, impedances at VIN+ and VIN− should
match. Optimal performance is obtained when the analog
inputs are driven differentially. SNR and SINAD performance
can degrade if the analog input is driven with a single-ended
signal. The analog inputs self-bias to approximately 1.9 V; this
common-mode voltage can be externally overdriven by
approximately ±300 mV if required.
A wideband transformer, such as the Mini-Circuits ADT1-1WT,
can provide the differential analog inputs for applications that
require a single-ended-to-differential conversion. Note that the
filter and center-tap capacitor on the secondary side is optional
and dependent on application requirements. An RC filter at the
secondary side helps reduce any wideband noise getting aliased
by the ADC.
05045-029
AD9481
VIN+
AVDD
(R, C OPTIONAL)
AGND
0.1µF
10pF
33Ω
49.9Ω
33Ω
VIN–
Figure 28. Driving the ADC with an RF Transformer
For dc-coupled applications, the AD8138/AD8139 or AD8351
can serve as a convenient ADC driver, depending on
requirements. Figure 29 shows an example with the AD8138.
The AD9481 PCB has an optional AD8351 on board, as shown
in Figure 39 and Figure 40. The AD8351 typically yields better
performance for frequencies greater than 30 MHz to 40 MHz.
The AD9481’s linearity and SFDR start to degrade at higher
analog frequencies (see the Typical Performance Characteristics
section). For higher frequency applications, the AD9480 with
LVDS outputs and superior AC performance should be
considered.
05045-030
AD9481
AD8138
VIN+
AVDD
AGND
0.1µF
20pF
33Ω
33Ω
VIN–
49.9Ω
2kΩ
1.3kΩ
499Ω
499Ω
523Ω
499Ω
Figure 29. Driving the ADC with the AD8138
The AD9481 can be easily configured for different full-scale
ranges. See the Voltage Reference section for more information.
Optimal performance is achieved with a 1 V p-p analog input.
05045-031
SENSE = GND
VIN+
2.0V500mV 2.0V
VIN–
DIGITALOUT = ALL 1s DIGITALOUT = ALL 0s
Figure 30. Analog Input Full Scale
VOLTAGE REFERENCE
A stable and accurate 1.0 V reference is built into the AD9481.
Users can choose this internal reference or provide an external
reference for greater accuracy and flexibility. Figure 32 shows
the typical reference variation with temperature. Table 8
summarizes the available reference configurations.
SELECT
LOGIC
ADC
CORE
0.5V
7kΩ
VREF
0.1µF10µF
+
SENSE
VIN+
VIN–
05045-032
7kΩ
Figure 31. Internal Reference Equivalent Circuit