ada4937-2 Analog Devices, Inc., ada4937-2 Datasheet - Page 24

• Manufacturer Part Number: ada4937-2
• Description: Ultralow Distortion Differential Adc Driver
• Manufacturer: Analog Devices, Inc.

ADA4937-1/ADA4937-2
The circuit in Figure 63 shows a simplified front-end connection
for an ADA4937 driving an AD9246, 14-bit, 125 MSPS ADC.
The AD9246 achieves its optimum performance when driven
differentially. The ADA4937 performs the single-ended-to-
differential conversion, eliminating the need for a transformer
to drive the ADC.
The ADA4937 is configured with a single 5 V supply and a gain
of ~2 V/V 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 30 Ω (120 Ω total) at the inverting input
balances the parallel ac impedance of the 50 Ω source and the
termination resistor driving the noninverting input.
The signal generator has a symmetric, ground-referenced
bipolar output. The V
unconnected; therefore, the internal pull-ups set the output
common-mode voltage to midsupply. A portion of this is fed
back to the summing nodes, biasing −IN and +IN at 0.55 V. For
a common-mode voltage of 2.5 V, each ADA4937 output swings
between 2.0 V and 3.0 V, providing a 2 V p-p differential output.
The output is ac-coupled to a single-pole, low-pass filter. This
reduces the noise bandwidth of the amplifier and provides some
level of isolation from the switched capacitor inputs of the ADC.
The AD9246 is set for a 2 V p-p full-scale input by connecting
OCM
pin of the ADA4937 is left
50Ω
V
IN
50Ω
10µF
76.8Ω
10µF
76.8Ω
90Ω
90Ω
Figure 63. ADA4937 Driving an AD9246, a 14-Bit, 125 MSPS ADC
ADA4937
+
200Ω
200Ω
5V
Rev. A | Page 24 of 28
10µF
10µF
200Ω
200Ω
33Ω
33Ω
10pF
the SENSE pin to AGND. The inputs of the AD9246 are biased
at 1 V by connecting the CML output, as shown in Figure 63.
The circuit was tested with a −1 dBFS signal at various
frequencies. Figure 62 shows a plot of the second and third
harmonic distortion (HD2/HD3) vs. frequency.
Figure 62. HD2/HD3 for Combination of ADA4937 and AD9246 ADC
–100
–75
–80
–85
–90
–95
VIN–
VIN+
0
G = +2
AGND
AVDD
AD9246
1.8V
20
SENSE
DRVDD
CML
40
FREQUENCY (MHz)
D11 TO
D0
60
HD3
80
HD2
100
120