AD9649 Analog Devices, AD9649 Datasheet - Page 17

• Manufacturer Part Number: AD9649
• Description: 14-Bit, 20/40/65/80 MSPS, 1.8 V Analog-to-Digital Converter
• Manufacturer: Analog Devices

Specifications of AD9649

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 80MSPS
• Interface: Par
• Analog Input Type: Diff-Uni
• Ain Range: 2 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

THEORY OF OPERATION
The AD9649 architecture consists of a multistage, pipelined ADC.
Each stage 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 pipelined architecture permits the first stage to operate with
a new input sample, whereas the remaining stages operate with pre-
ceding 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 consists of a flash ADC.
The output staging block aligns the data, corrects errors, and
passes the data to the CMOS output buffers. The output buffers
are powered from a separate (DRVDD) supply, allowing adjust-
ment of the output voltage swing. During power-down, the output
buffers go into a high impedance state.
ANALOG INPUT CONSIDERATIONS
The analog input to the AD9649 is a differential switched-
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 35). 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 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 differ-
ential 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
VIN+
VIN–
Figure 35. Switched-Capacitor Input Circuit
C
C
PAR
PAR
H
H
S
S
C
C
SAMPLE
SAMPLE
S
S
H
H
Rev. 0 | Page 17 of 32
high IF frequencies. Either a shunt capacitor or two single-ended
capacitors can be placed on the inputs to provide a matching pas-
sive network. This ultimately creates a low-pass filter at the input
to limit unwanted broadband noise. See the AN-742 Application
Note, the AN-827 Application Note, and the Analog Dialogue
article,
Converters” (Volume 39, April 2005) for more information. In
general, the precise values depend on the application.
Input Common Mode
The analog inputs of the AD9649 are not internally dc-biased.
Therefore, in ac-coupled applications, the user must provide an
external dc bias. Setting the device so that VCM = AVDD/2
is recommended for optimum performance, but the device can
function over a wider range with reasonable performance, as
shown in Figure 36 and Figure 37.
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.
100
100
90
80
70
60
50
90
80
70
60
50
“Transformer-Coupled Front-End for Wideband A/D
0.5
0.5
Figure 36. SNR/SFDR vs. Input Common-Mode Voltage,
Figure 37. SNR/SFDR vs. Input Common-Mode Voltage,
0.6
0.6
INPUT COMMON-MODE VOLTAGE (V)
INPUT COMMON-MODE VOLTAGE (V)
0.7
0.7
f
f
IN
IN
= 32.1 MHz, f
= 10.3 MHz, f
0.8
0.8
SNR (dBFS)
SNR (dBFS)
SFDR (dBc)
SFDR (dBc)
0.9
0.9
S
S
= 80 MSPS
= 20 MSPS
1.0
1.0
1.1
1.1
1.2
1.2
AD9649
1.3
1.3