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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
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 teste d and verified in a lab environment
at room temperature. However, you are solely responsible for testing the circuit and determining its
suitability and applicability for your use and application. Accordingly, in no event shall Analog
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any cause whatsoever connected to the use of any “Circuit from the Lab”. (Continued on last page)
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CIRCUIT FUNCTION AND BENEFITS
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.
CIRCUIT DESCRIPTION
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.
The AD9246 achieves its optimum performance when driven
differentially. The ADA4937-1 not only provides the single-
ended-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 V
OCM
voltage is
approximately midsupply and is set by an internal divider.
1.8V
DRVDDAVDD
VIN–
VIN+
AD9246
AGND CMLSENSE
10pF
10µF
10µF
10µF
33Ω
33Ω
200Ω
200Ω
50Ω
V
IN
90.9Ω
90.9Ω
76.8Ω
5V
ADA4937
+
200Ω
200Ω
49.9Ω
76.8Ω
10µF
08610-001
VOCM
+IN
V
IN
–IN
GAIN = =2
(V
IN+
) – (V
IN–
)
V
IN
Figure 1. ADA4937-1 Driving the AD9246 14-Bit ADC
(Simplified Schematic: Decoupling and All Connections Not Shown)
