Circuit Note
CN-0051
Circuit Designs Using Analog Devices Products
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Devices Connected/Referenced
ADA4937-1
Ultralow Distortion Differential ADC
Driver
AD9246
14-Bit, 80 MSPS/105 MSPS/125 MSPS ADC
Driving the AD9233/AD9246/AD9254 ADCs in AC-Coupled Baseband Applications
CIRCUIT FUNCTION AND BENEFITS
CIRCUIT DESCRIPTION
The circuit described in this document and shown in Figure 1
uses the ADA4937-1 ADC driver to convert an ac-coupled,
single-ended input signal to a differential signal suitable for
driving the AD9246 14-bit, 125 MSPS analog-to-digital converter
(ADC). The ADA4937-1 is a low noise, ultralow distortion, high
speed differential amplifier with low dc offset and excellent
dynamic performance. It is an ideal choice for driving high
performance ADCs with resolutions up to 16 bits from dc to
100 MHz and is well suited for a wide variety of data acquisition
and signal processing applications. Combined with power and
cost savings over previously available ADCs, this circuit is
suitable for applications in communications, instrumentation,
and medical imaging. The ADA4937-2 is a dual version of the
ADA4937-1 that can be used when driving a dual ADC.
The AD9246 is a monolithic, single 1.8 V supply, 14-bit, 80 MSPS/
105 MSPS/125 MSPS ADC, featuring a high performance
sample-and-hold amplifier (SHA) and on-chip voltage reference.
The wide bandwidth, truly differential SHA allows a variety of
user-selectable input ranges and offsets, including single-ended
applications. The device can be applied in multiplexed systems
that switch full-scale voltage levels in successive channels and
for sampling single-channel input frequencies well beyond the
Nyquist frequency of the ADC.
200Ω
49.9Ω
10µF
90.9Ω
VIN
10µF
(VIN+) – (VIN–)
=2
VIN
1.8V
33Ω
+
+IN
VOCM
90.9Ω –IN
200Ω
ADA4937
10pF
200Ω
10µF
76.8Ω
AVDD
VIN–
DRVDD
AD9246
VIN+
33Ω
AGND SENSE CML
08610-001
VIN
10µF
GAIN =
5V
76.8Ω
50Ω
The AD9246 achieves its optimum performance when driven
differentially. The ADA4937-1 not only provides the singleended-to-differential conversion but also provides gain and
level shifting. The output common voltage of the ADA4937-1
is set by connecting a resistive divider to the VOCM pin of the
ADA4937-1. If the pin is left floating, the VOCM voltage is
approximately midsupply and is set by an internal divider.
200Ω
Figure 1. ADA4937-1 Driving the AD9246 14-Bit ADC
(Simplified Schematic: Decoupling and All Connections Not Shown)
Rev. A
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engineers. Standard engineering practices have been employed in the design and construction of
each circuit, and their function and performance have been tested and verified in a lab environment
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suitability and applicability for your use and application. Accordingly, in no event shall Analog
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Fax: 781.461.3113
©2009 Analog Devices, Inc. All rights reserved.
CN-0051
Circuit Note
The ADA4937-1 is powered with a single 5 V supply and is
configured for a gain-of-2 for a single-ended, input-to-differential output. The 76.8 Ω termination resistor, in parallel with
the single-ended input impedance of 137 Ω, provides a 50 Ω ac
termination for the source. The additional 49.9 Ω resistor, 10 µF
capacitor, and 76.8 Ω resistor connected to the 90.9 Ω resistor
on the inverting input balance the ac impedance driving the
noninverting input. A detailed analysis of this configuration can
be found in MT-076 Tutorial.
The signal generator has a symmetric, ground-referenced, bipolar
output. The VOCM pin of the ADA4937-1 is left unconnected;
therefore, the internal divider sets the output common-mode
voltage to midsupply. A portion of this is fed back to the summing
nodes, biasing −IN and +IN at 1.14 V. For a common-mode
voltage of 2.5 V, each ADA4937-1 output swings between 2.0 V
and 3.0 V, providing a 2 V p-p differential output for a 1 V p-p
single-ended input.
The output of the ADC driver is ac-coupled to a single-pole,
low-pass noise filter. The low-pass filter reduces the noise bandwidth at the ADC input and provides a degree of isolation from
the switched capacitor inputs of the ADC and the driver. In any
configuration, the optimum value of the shunt capacitor, C, is
dependent on the input frequency and source impedance and
may need to be optimized. Table 1 displays recommended values
for the RC network. However, these values are dependent on the
input signal frequency and may require further optimization.
The input common-mode voltage to the ADC is set by the CML
pin of the AD9246 and the pair of 200 Ω resistors. In other
applications, the CML pin of the ADC and the VOCM pin of the
ADA4937-1 are used to set the input common-mode voltage to
the ADC (see the Common Variations section). The ADA4937-1
is fabricated using the Analog Devices, Inc., proprietary silicongermanium (SiGe), complementary bipolar process, enabling it
to achieve very low levels of distortion with an input voltage noise
of only 2.2 nV/ √Hz.
The circuit shown in Figure 1 was tested with a −1 dBFS signal
at various input frequencies. Figure 2 shows a plot of the second
and third harmonic distortion (HD2/HD3) vs. input frequency.
–75
–80
HD3
It is required that the exposed pad on the underside of both the
AD9246 and the ADA4937-1 (LFCSP packages) be connected
to a large area ground plane to achieve the best electrical and
thermal performance. The copper plane should have several
vias to achieve the lowest possible resistive thermal path for
heat dissipation to flow through the bottom of the PCB. These
vias should be solder filled or plugged.
COMMON VARIATIONS
The AD9246 (14-bit, 80 MSPS/105 MSPS/125 MSPS) ADC is pin
compatible with both the AD9233 (12-bit, 80 MSPS/105 MSPS/
125 MSPS) and the AD9254 (14-bit, 150 MSPS).
There are a few other amplifier configurations to consider when
driving ADCs. They are differential ac-coupled input to differential output, dc-coupled single-ended input to ac-coupled
differential output, dc-coupled single-ended input to differential
output, and dc-coupled differential input to differential output.
In dc-coupled systems, the driver output common-mode
voltage is set via the VOCM pin of the ADA4937-1. The
adjustable level of the output common-mode voltage allows the
ADA4937-1 output to match the input common-mode voltage
of the ADC. The internal common-mode feedback loop of the
ADA4937-1 also provides exceptional output balance and
suppression of even-order harmonic distortion products. Often
in these applications, the ADC CML pin is connected directly to
the VOCM pin of the driver to ensure that optimal ADC input
common-mode voltage is achieved. In other applications, the
VOCM pin can be driven from a low impedance source such as an
op amp. It can also be left floating but bypassed with a capacitor;
in this case the VOCM voltage is set at the midpoint of the voltage
applied to the two supply pins.
Table 1. RC Network Recommended Values
Input Frequency Range
(MHz)
R Series ( Ω) C Differential (pF)
0 to 70
33
15
70 to 200
33
5
200 to 300
15
5
>300
15
Open (no capacitor)
–85
HD2
–90
–95
–100
0
20
40
60
80
FREQUENCY (MHz)
100
120
08610-002
HARMONIC DISTORTION (dBc)
G = +2
The circuit must be constructed on a multilayer PC board
with a large area ground plane. Proper layout, grounding,
and decoupling techniques must be used to achieve optimum
performance (see MT-031 Tutorial, MT-101 Tutorial, and the
AD9246 evaluation board layout).
Figure 2. Second Harmonic Distortion (HD2) and Third Harmonic Distortion
(HD3) for the ADA4937-1 Driving the AD9246 ADC
Rev. A | Page 2 of 3
Circuit Note
CN-0051
LEARN MORE
Data Sheets and Evaluation Boards
Rob Reeder and Jim Caserta, Wideband A/D Converter FrontEnd Design Considerations II: Amplifier- or Transformer Drive
for the ADC? Ask The Application Engineer—36, Analog
Dialogue 41-02, February 2007.
ADA4937-1 Data Sheet.
AD9246 Data Sheet
AD9246 Evaluation Board.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND," Analog Devices.
REVISION HISTORY
MT-074 Tutorial, Differential Drivers for Precision ADCs,
Analog Devices.
Updated Format ................................................................. Universal
11/09—Rev. 0 to Rev. A
2/09—Revision 0: Initial Release
MT-075 Tutorial, Differential Drivers for High Speed ADCs
Overview, Analog Devices.
MT-076 Tutorial, Differential Driver Analysis, Analog Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
John Ardizonni and Jonathan Pearson, "Rules of the Road" for
High-Speed Differential ADC Drivers, Analog Dialogue,
Volume 43, May 2009, Analog Devices.
ADIsimDiffAmp (Differential Amplifier Tool), Analog Devices.
(Continued from first page) "Circuits from the Lab" are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may
use the "Circuits from the Lab" in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of
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from the Lab" at any time without notice, but is under no obligation to do so. Trademarks and registered trademarks are the property of their respective owners.
©2009 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN08610-0-11/09(A)
Rev. A | Page 3 of 3