AD9481 Analog Devices, AD9481 Datasheet - Page 17

• Manufacturer Part Number: AD9481
• Description: 8-Bit, 250 MSPS, 3.3 V A/D Converter
• Manufacturer: Analog Devices

Specifications of AD9481

• Resolution (bits): 8bit
• # Chan: 1
• Sample Rate: 250MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: (Vref) p-p,1 V p-p
• Adc Architecture: Pipelined
• Pkg Type: QFP

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.
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.
Figure 28. Driving the ADC with an RF Transformer
49.9Ω
0.1µF
(R, C OPTIONAL)
33Ω
33Ω
10pF
VIN+
VIN–
AD9481
AGND
AVDD
Rev. 0 | Page 17 of 28
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.
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.
500mV
0.1µF
10µF
2.0V
1.3kΩ
2kΩ
49.9Ω
+
SENSE = GND
VIN+
VIN–
Figure 31. Internal Reference Equivalent Circuit
Figure 29. Driving the ADC with the AD8138
0.1µF
DIGITALOUT = ALL 1s
SENSE
VREF
VIN+
VIN–
Figure 30. Analog Input Full Scale
499Ω
523Ω
7kΩ
7kΩ
499Ω
499Ω
AD8138
SELECT
LOGIC
33Ω
33Ω
20pF
DIGITALOUT = ALL 0s
0.5V
CORE
ADC
VIN+
VIN–
AD9481
AVDD
AGND
AD9481
2.0V