ADA4937-1 Analog Devices, ADA4937-1 Datasheet - Page 23

• Manufacturer Part Number: ADA4937-1
• Description: Ultra-Low Distortion Differential ADC Driver
• Manufacturer: Analog Devices

The circuit in Figure 53 shows a simplified front-end connection
for an ADA4937-1 driving an AD9246, 14-bit, 125 MSPS ADC.
The
differentially. The ADA4937-1 performs the single-ended-to-
differential conversion, eliminating the need for a transformer
to drive the ADC.
The ADA4937-1 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 ADA4937-1 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
AD9246
achieves its optimum performance when driven
is set for a 2 V p-p full-scale input by connecting
OCM
pin of the ADA4937-1 is left
50Ω
V
IN
50Ω
10µF
76.8Ω
10µF
Figure 53. ADA4937-1 Driving an AD9246, a 14-Bit, 125 MSPS ADC
76.8Ω
90Ω
90Ω
ADA4937-1
+
200Ω
200Ω
5V
Rev. 0 | Page 23 of 28
10µF
10µF
200Ω
200Ω
33Ω
33Ω
the SENSE pin to AGND. The inputs of the
at 1 V by connecting the CML output, as shown in Figure 53.
The circuit was tested with a −1 dBFS signal at various
frequencies. Figure 52 shows a plot of the second and third
harmonic distortion (HD2/HD3) vs. frequency.
10pF
Figure 52. HD2/HD3 for Combination of ADA4937-1 and AD9246 ADC
–100
–75
–80
–85
–90
–95
0
VIN–
VIN+
G = +2
AGND
AVDD
AD9246
1.8V
20
SENSE
DRVDD
CML
40
FREQUENCY (MHz)
D11 TO
D0
60
HD3
80
HD2
AD9246
ADA4937-1
100
are biased
120