AD9643 Analog Devices, AD9643 Datasheet - Page 23

• Manufacturer Part Number: AD9643
• Description: 14-Bit, 170/210/250 MSPS, 1.8 V Dual Analog-to-Digital Converter (ADC)
• Manufacturer: Analog Devices

Specifications of AD9643

• Resolution (bits): 14bit
• # Chan: 2
• Sample Rate: 250MSPS
• Interface: LVDS,Par
• Analog Input Type: Diff-Bip
• Ain Range: 1.75 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

Data Sheet
THEORY OF OPERATION
The AD9643 has two analog input channels and two digital
output channels. The intermediate frequency (IF) signal passes
through several stages before appearing at the output port(s).
The dual ADC design can be used for diversity reception of signals,
where the ADCs operate identically on the same carrier but from
two separate antennae. The ADCs can also be operated with
independent analog inputs. The user can sample frequencies
from dc to 300 MHz using appropriate low-pass or band-pass
filtering at the ADC inputs with little loss in ADC performance.
Operation to 400 MHz analog input is permitted but occurs at
the expense of increased ADC noise and distortion.
Synchronization capability is provided to allow synchronized
timing between multiple devices.
Programming and control of the AD9643 are accomplished
using a 3-pin, SPI-compatible serial interface.
ADC ARCHITECTURE
The AD9643 architecture consists of a dual front-end sample-
and-hold circuit, followed by a pipelined switched-capacitor
ADC. The quantized outputs from each stage are combined into
a final 14-bit result in the digital correction logic. The pipelined
architecture permits the first stage to operate on a new input
sample and the remaining stages to operate on the 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 digital-
to-analog converter (DAC) and an interstage residue amplifier
(MDAC). The MDAC magnifies the difference between the recon-
structed 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 input stage of each channel contains a differential sampling
circuit that can be ac- or dc-coupled in differential or single-
ended modes. The output staging block aligns the data, corrects
errors, and passes the data to the output buffers. The output buffers
are powered from a separate supply, allowing digital output noise to
be separated from the analog core. During power-down, the
output buffers go into a high impedance state.
ANALOG INPUT CONSIDERATIONS
The analog input to the AD9643 is a differential switched-
capacitor circuit that has been designed for optimum performance
while processing a differential input signal.
The clock signal alternatively switches the input between sample
mode and hold mode (see the configuration shown in Figure 46).
When the input is switched into sample mode, the signal source
must be capable of charging the sampling capacitors and settling
within 1/2 clock cycle.
Rev. B | Page 23 of 36
A small resistor in series with each input can help reduce the
peak transient current required from the output stage of the
driving source. A shunt capacitor can be placed across the
inputs to provide dynamic charging currents. This passive
network creates a low-pass filter at the ADC input; therefore,
the precise values are dependent on the application.
In intermediate frequency (IF) undersampling applications, the
shunt capacitors should be reduced. In combination with the
driving source impedance, the shunt capacitors limit the input
bandwidth. Refer to the
Domain Response of Switched-Capacitor ADCs; the
Application
Switched-Capacitor ADCs; and the Analog Dialogue article,
“Transformer-Coupled Front-End for Wideband A/D
for more information on this subject.
For best dynamic performance, the source impedances driving
VIN+ and VIN− should be matched, and the inputs should be
differentially balanced.
Input Common Mode
The analog inputs of the AD9643 are not internally dc biased.
In ac-coupled applications, the user must provide this bias
externally. Setting the device so that V
0.9 V) is recommended for optimum performance. An on-board
common-mode voltage reference is included in the design and is
available from the VCM pin. Using the VCM output to set the
input common mode is recommended. Optimum performance
is achieved when the common-mode voltage of the analog input
is set by the VCM pin voltage (typically 0.5 × AVDD). The
VCM pin must be decoupled to ground by a 0.1 μF capacitor, as
described in the Applications Information section. This
decoupling capacitor should be placed close to the pin to
minimize the series resistance and inductance between the part
and this capacitor.
Differential Input Configurations
Optimum performance is achieved while driving the AD9643
in a differential input configuration. For baseband applications,
the AD8138, ADA4937-2, ADA4938-2, and
VIN+
VIN–
C
C
PAR1
PAR1
Note, A Resonant Approach to Interfacing Amplifiers to
S
S
Figure 46. Switched-Capacitor Input
C
C
PAR2
PAR2
AN-742 Application
H
C
C
S
S
BIAS
BIAS
CM
S
S
S
= 0.5 × AVDD (or
ADA4930-2
Note, Frequency
C
C
FB
FB
AN-827
Converters, ”
AD9643
S