AD9246-105EB Analog Devices Inc, AD9246-105EB Datasheet - Page 15

• Manufacturer Part Number: AD9246-105EB
• Description: BOARD EVAL FOR 105MSPS AD9246
• Manufacturer: Analog Devices Inc

Specifications of AD9246-105EB

• Number Of Adc's: 1
• Number Of Bits: 14
• Sampling Rate (per Second): 105M
• Data Interface: Serial
• Inputs Per Adc: 1 Differential
• Input Range: 2 Vpp
• Power (typ) @ Conditions: 373mW @ 105MSPS
• Voltage Supply Source: Single
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD9246-105
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

THEORY OF OPERATION
The AD9246 architecture consists of a front-end sample-and-
hold amplifier (SHA) 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 pipeline
architecture permits the first stage to operate on a new input
sample, while the remaining stages operate on 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 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 simply consists of a flash ADC.
The input stage contains a differential SHA that can be ac- or
dc-coupled in differential or single-ended modes. 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.
ANALOG INPUT CONSIDERATIONS
The analog input to the AD9246 is a differential switched
capacitor SHA that has been designed for optimum
performance while processing a differential input signal.
The clock signal alternately switches the SHA between sample
mode and hold mode (see Figure 36). When the SHA is
switched into 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 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 IF undersampling applications, any shunt capacitors should
be reduced. In combination with the driving source impedance,
these capacitors would limit the input bandwidth.
Rev. A | Page 15 of 44
For more information, see Application Notes AN-742, Frequency
Domain Response of Switched-Capacitor ADCs; and AN-827,
A Resonant Approach to Interfacing Amplifiers to Switched-
Capacitor ADCs, and the Analog Dialogue article,
Coupled Front-End for Wideband A/D Converters. ”
For best dynamic performance, the source impedances driving
VIN+ and VIN− should match such that common-mode
settling errors are symmetrical. These errors are reduced by the
common-mode rejection of the ADC.
An internal differential reference buffer creates two reference
voltages used to define the input span of the ADC core. The
span of the ADC core is set by the buffer to be 2 × VREF. The
reference voltages are not available to the user. Two bypass
points, REFT and REFB, are brought out for decoupling to
reduce the noise contributed by the internal reference buffer.
It is recommended that REFT be decoupled to REFB by a 0.1 μF
capacitor, as described in the Layout Considerations section.
Input Common Mode
The analog inputs of the AD9246 are not internally dc-biased.
In ac-coupled applications, the user must provide this bias
externally. Setting the device such that V
recommended for optimum performance; however, the device
functions over a wider range with reasonable performance (see
Figure 32). An on-board, common-mode voltage reference is
included in the design and is available from the CML pin.
Optimum performance is achieved when the common-mode
voltage of the analog input is set by the CML pin voltage
(typically 0.55 × AVDD). The CML pin must be decoupled to
ground by a 0.1 μF capacitor, as described in the Layout
Considerations section.
VIN+
VIN–
C
C
PIN, PAR
PIN, PAR
Figure 36. Switched Capacitor SHA Input
S
S
H
C
C
S
S
CM
= 0.55 × AVDD is
C
C
S
S
H
H
“Transformer-
AD9246