AD9257 Analog Devices, AD9257 Datasheet - Page 18

• Manufacturer Part Number: AD9257
• Description: Octal, 14-Bit, 40/65 MSPS Serial LVDS 1.8 V A/D Converter
• Manufacturer: Analog Devices

AD9257
THEORY OF OPERATION
The
provides sufficient overlap to correct for flash errors in the
preceding stage. The quantized outputs from each stage are
combined into a final 14-bit result in the digital correction
logic. The serializer transmits this converted data in a 14-bit
output. The pipelined architecture permits the first stage to
operate with a new input sample while the remaining stages
operate with preceding samples. Sampling occurs on the rising
edge of the clock.
Each stage of the pipeline, excluding the last, consists of a low
resolution flash ADC connected to a switched-capacitor DAC
and an interstage residue amplifier (for example, a multiplying
digital-to-analog converter (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 simply consists of a flash ADC.
The output staging block aligns the data, corrects errors, and
passes the data to the output buffers. The data is then serialized
and aligned to the frame and data clocks.
ANALOG INPUT CONSIDERATIONS
The analog input to the
capacitor circuit designed for processing differential input
signals. This circuit can support a wide common-mode range
while maintaining excellent performance. By using an input
common-mode voltage of midsupply, users can minimize
signal-dependent errors and achieve optimum performance.
The clock signal alternately switches the input circuit between
sample mode and hold mode (see Figure 43). When the input
circuit is switched to sample mode, the signal source must be
capable of charging the sample capacitors and settling within
one-half of a clock cycle. A small resistor in series with each
input can help reduce the peak transient current injected from
AD9257
VIN+ x
VIN– x
is a multistage, pipelined ADC. Each stage
Figure 43. Switched-Capacitor Input Circuit
C
C
PAR
PAR
AD9257
H
H
S
S
C
C
SAMPLE
SAMPLE
is a differential switched-
S
S
H
H
Rev. 0 | Page 18 of 40
the output stage of the driving source. In addition, low Q inductors
or ferrite beads can be placed on each leg of the input to reduce
high differential capacitance at the analog inputs and, therefore,
achieve the maximum bandwidth of the ADC. Such use of low
Q inductors or ferrite beads is required when driving the converter
front end at high IF frequencies. Either a differential capacitor or
two single-ended capacitors can be placed on the inputs to provide
a matching passive network. This ultimately creates a low-pass
filter at the input to limit unwanted broadband noise. See the
AN-742
Analog Dialogue article
Wideband A/D
information. In general, the precise values depend on the
application.
Input Common Mode
The analog inputs of the
Therefore, in ac-coupled applications, the user must provide
this bias externally. Setting the device so that V
recommended for optimum performance, but the device can
function over a wider range with reasonable performance, as
shown in Figure 44.
An on-board, common-mode voltage reference is included in
the design and is available from the VCM pin. The VCM pin
must be decoupled to ground by a 0.1 μF capacitor, as described
in the Applications Information section.
Maximum SNR performance is achieved by setting the ADC to
the largest span in a differential configuration. In the case of the
AD9257, the largest input span available is 2 V p-p.
100
90
80
70
60
50
40
30
20
Application Note, the
0.5
Figure 44. SNR/SFDR vs. Common-Mode Voltage,
Converters” (Volume 39, April 2005) for more
f
IN
0.7
= 9.7 MHz, f
“Transformer-Coupled Front-End for
AD9257
SNRFS
SFDR
AN-827
V
SAMPLE
CM
0.9
are not internally dc-biased.
(V)
= 65 MSPS
Application Note, and the
1.1
Data Sheet
CM
= AVDD/2 is
1.3