AD9239 Analog Devices, AD9239 Datasheet - Page 19

• Manufacturer Part Number: AD9239
• Description: Quad, 12-Bit, 170 MSPS/210 MSPS/250 MSPS Serial Output 1.8 V ADC
• Manufacturer: Analog Devices

Specifications of AD9239

• Resolution (bits): 12bit
• # Chan: 4
• Sample Rate: 250MSPS
• Interface: Ser
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: 1.25 V p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

THEORY OF OPERATION
The AD9239 architecture consists of a differential input buffer,
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 12-bit result in the digital
correction logic. The pipelined 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 (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 input stage contains a differential SHA that can be ac- or
dc-coupled in differential or single-ended mode. The output of
the pipeline ADC is put into its final serial format by the data
serializer, encoder, and CML drivers block. The data rate multiplier
creates the clock used to output the high speed serial data at the
CML outputs.
ANALOG INPUT CONSIDERATIONS
The analog input to the AD9239 is a differential buffer. This
input is optimized to provide superior wideband performance
and requires that the analog inputs be driven differentially. SNR
and SINAD performance degrades if the analog input is driven
with a single-ended signal.
For best dynamic performance, the source impedances driving
VIN + x and VIN − x should be matched such that common-
mode settling errors are symmetrical. These errors are reduced
by the common-mode rejection of the ADC. A small resistor in
GENERATOR
GENERATOR
SIGNAL
SIGNAL
1.25V p-p
1.25V p-p
Figure 46. Differential Amplifier Configuration for DC-Coupled Baseband Applications
Figure 45. Differential Amplifier Configuration for AC-Coupled Baseband Applications
62Ω
0.1µF
62Ω
0.1µF
10kΩ
10kΩ
27Ω
200Ω
200Ω
27Ω
200Ω
200Ω
1.65V
V
OCM
V
OCM
ADA4937
–V
G = UNITY
+V
3.3V
ADA4937
G = UNITY
–V
3.3V
+V
S
S
S
205Ω
205Ω
Rev. B | Page 19 of 40
S
205Ω
205Ω
24Ω
24Ω
24Ω
24Ω
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 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 intermediate
frequency (IF). Either a shunt capacitor or two single-ended capac-
itors 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-827 Application Note
and the Analog Dialogue article “Transformer-Coupled Front-End
for Wideband A/D Converters” (Volume 39, April 2005) for
more information on this subject. In general, the precise values
depend on the appli
Maximum SNR performance is achieved by setting the ADC to
the largest span in a differential configuration. In the case of the
AD9239, the default input span is 1.25 V p-p. To configure the
ADC for a different input span, see Register 18. For the best
performance, an input span of 1.25 V p-p or greater should be
used (see Table 15 for details).
Differential Input Configurations
There are several ways to drive the AD9239 either actively or
passively; in either case, optimum performance is achieved by
driving the analog input differentially. For example, using the
ADA4937 differential amplifier to drive the AD9239 provides
excellent performance and a flexible interface to the ADC (see
Figure 45 and Figure 46) for baseband and second Nyquist
(~100 MHz IF) applications. In either application, 1% resistors
should be used for good gain matching. It should also be noted
that the dc-coupled configuration will show some degradation
in spurious performance. For further reference, consult the
ADA4937
OPTIONAL C
0.1µF
0.1µF
1.4V
OPTIONAL C
33Ω
33Ω
33Ω
33Ω
data sheet.
VIN + x
VIN – x
VIN + x
VIN – x
R
cation.
R
C
AVDD
VCMx
1.8V
IMPEDANCE
C
ADC INPUT
AD9239
AVDD
1.8V
IMPEDANCE
ADC INPUT
AD9239
DRVDD
1.8V
DRVDD
1.8V
AD9239