AD9634-170EBZ Ver la hoja de datos (PDF) - Analog Devices
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AD9634-170EBZ Datasheet PDF : 31 Pages
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Data Sheet
THEORY OF OPERATION
The AD9634 can sample any fS/2 frequency segment from dc to
250 MHz using appropriate low-pass or band-pass filtering at
the ADC inputs with little loss in ADC performance.
Programming and control of the AD9634 are accomplished
using a 3-pin, SPI-compatible serial interface.
ADC ARCHITECTURE
The AD9634 architecture consists of a front-end sample-and-
hold circuit, followed by a pipelined, switched-capacitor ADC.
The quantized outputs from each stage are combined into a
final 12-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
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 input stage 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 AD9634 is a differential switched-capacitor
circuit that has been designed to attain optimum performance
when 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 ½ clock cycle.
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.
AD9634
In intermediate frequency (IF) undersampling applications, reduce
the shunt capacitors. In combination with the driving source
impedance, the shunt capacitors limit the input bandwidth.
Refer to the AN-742 Application Note, Frequency Domain
Response of Switched-Capacitor ADCs; the AN-827 Application
Note, A Resonant Approach to Interfacing Amplifiers to Switched-
Capacitor ADCs; and the Analog Dialogue article, “Transformer-
Coupled Front-End for Wideband A/D Converters” for more
information on this subject.
BIAS
VIN+
CPAR1
S
CS
CPAR2
S
CFB
H
S
S
VIN–
CPAR1
CS
CPAR2
S
S
CFB
BIAS
Figure 46. Switched-Capacitor Input
For best dynamic performance, match the source impedances
driving VIN+ and VIN− and differentially balance the inputs.
Input Common Mode
The analog inputs of the AD9634 are not internally dc biased. In
ac-coupled applications, the user must provide this bias externally.
Setting the device so that VCM = 0.5 × AVDD (or 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. Place this
decoupling capacitor close to the pin to minimize the series
resistance and inductance between the part and this capacitor.
Rev. B | Page 19 of 30
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