AD9286 Analog Devices, AD9286 Datasheet - Page 15

• Manufacturer Part Number: AD9286
• Description: 8-Bit, 500 MSPS, 1.8 V Analog-to-Digital Converter (ADC)
• Manufacturer: Analog Devices

Specifications of AD9286

• Resolution (bits): 8bit
• # Chan: 2
• Sample Rate: 500MSPS
• Interface: Par
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: 2 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

THEORY OF OPERATION
The AD9286 is a pipeline-type converter. The input buffers are
differential, and both sets of inputs are internally biased. This
allows the use of ac or dc input modes. A sample-and-hold
amplifier is incorporated into the first stage of the multistage
pipeline converter core. The output staging block aligns the data,
carries out error correction for the pipeline stages, and feeds
that data to the output interleave block and, finally, to the output
buffers. All user-selected options are programmed through
dedicated digital input pins or a serial port interface (SPI).
ADC ARCHITECTURE
Each interleaving channel of the AD9286 consists of a differential
input buffer followed by a sample-and-hold amplifier (SHA).
This SHA is followed by a pipeline switched-capacitor ADC.
The quantized outputs from each stage are combined into a final
8-bit result in the digital correction logic. The pipelined archi-
tecture permits the first stage to operate on a new input sample,
whereas the remaining stages operate on preceding samples.
Each stage of the pipeline, excluding the last, consists of a low
resolution flash ADC connected to a switched-capacitor DAC
and interstage residue amplifier (MDAC). The residue amplifier
magnifies the difference between the reconstructed DAC output
and the flash input for the next stage in the pipeline. One bit of
redundancy is used in each stage to facilitate digital correction
of flash errors. The last stage consists of a flash ADC.
The input stage contains a differential SHA that can be ac- or
dc-coupled in differential or single-ended mode. The output
staging block aligns the data, carries out the error correction,
and passes the data to the output buffers. The output buffers
are powered from a separate supply, allowing adjustment of the
output voltage swing. During power-down, the output buffers
go into a high impedance state.
The outputs from both interleaving channels are time interleaved
to achieve an effective 500 MSPS.
ANALOG INPUT CONSIDERATIONS
The analog inputs of the AD9286 are differentially buffered.
For best dynamic performance, the source impedances driving
VIN1+, VIN1−, VIN2+, and VIN2− should be matched such
that common-mode settling errors are symmetrical. Because
the AD9286 interleaves two ADC cores, special attention should
be given, during board layout, to the symmetry of the two analog
paths. Mismatch introduces undesired distortion. The analog
inputs are optimized to provide superior wideband performance
and must be driven differentially. SNR and SINAD performance
degrades significantly if the analog inputs are driven with
a single-ended signal.
A wideband transformer, such as Mini-Circuits® ADT1-1WT, can
provide the differential analog inputs for applications that require
a single-ended-to-differential conversion. Both analog inputs
are self-biased by an on-chip resistor divider to a nominal 1.4 V.
Rev. A | Page 15 of 28
Differential Input Configurations
Optimum performance is achieved when driving the AD9286
in a differential input configuration. For baseband applications,
the
and a flexible interface to the ADC (see Figure 24). The output
common-mode voltage of the AD9286 is easily set to 1.4 V, and
the driver can be configured in a Sallen-Key filter topology to
provide band limiting of the input signal.
1.2V p-p
The AD9286 can also be driven passively with a differential
transformer-coupled input (see Figure 25). To bias the analog
input, the VCM voltage can be connected to the center tap of
the secondary winding of the transformer.
The signal characteristics must be considered when selecting a
transformer. Most RF transformers saturate at frequencies below
a few megahertz (MHz). Excessive signal power can also cause
core saturation, which leads to distortion.
VOLTAGE REFERENCE
An internal differential voltage reference creates positive and
negative reference voltages that define the 1.2 V p-p fixed span
of the ADC core. This internal voltage reference can be adjusted
by means of SPI control. It can also be driven externally with
an off-chip stable reference. See the Memory Map Register
Descriptions section for more details.
RBIAS
The AD9286 requires the user to place a 10 kΩ resistor between
the RBIAS pin and ground. This resistor, which is used to set
the master current reference of the ADC core, should have a 1%
tolerance.
0.1µF
ADA4937-1
Figure 24. Differential Input Configuration Using the ADA4937-1
1.2V p-p
Figure 25. Differential Transformer-Coupled Configuration
61.9Ω
differential driver provides excellent performance
227.4Ω
200Ω
49.9Ω
0.1µF
ADA4937-1
–
+
200Ω
200Ω
4.7pF
33Ω
33Ω
4.7pF
33Ω
33Ω
AD9286
+
–
+
–
VIN1
VIN2
VCM
AD9286
AD9286
+
–
+
–
VIN1
VIN2
VCM