ADA4937-2YCPZ-R2 datasheet(24/28 Pages) AD

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ADA4937-1/ADA4937-2

Data Sheet

Rev. F | Page 24 of 28

The circuit in Figure 66 shows a simplified front-end connection

for an ADA4937-1 driving an AD9246, 14-bit, 125 MSPS ADC.

The AD9246 achieves optimum performance when driven

differentially. The ADA4937-1/ADA4937-2 perform the single-

ended-to-differential conversion, eliminating the need for a

transformer to drive the ADC.

The ADA4937-1/ADA4937-2 are 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 VOCM pin of the ADA4937-1/ADA4937-2 remains

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/ADA4937-2

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

the SENSE pin to AGND. The inputs of the AD9246 are biased

at 1 V by connecting the CML output, as shown in Figure 66.

The circuit was tested with a −1 dBFS signal at various frequencies.

Figure 65 shows a plot of the second- and third-order harmonic

distortion (HD2/HD3) vs. frequency.

–75

–80

–85

–90

–95

–100

120

100

FREQUENCY (MHz)

HD2

HD3

G = +2

Figure 65. HD2/HD3 for Combination of ADA4937-1/ADA4937-2 and

AD9246 ADC

1.8V

DRVDD

AVDD

VIN–

VIN+

AD9246

AGND

CML

SENSE

D13 TO

10pF

10µF

33Ω

200Ω

50Ω

VIN

90Ω

76.8Ω

ADA4937-1

200Ω

50Ω

76.8Ω

10µF

Figure 66. Driving an AD9246, 14-Bit, 125 MSPS ADC

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