ADA4937-1YCPZ-R7 datasheet(23/28 Pages) AD

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ADA4937-1

Rev. 0 | Page 23 of 28

The circuit in Figure 53 shows a simplified front-end connection

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

The AD9246 achieves its optimum performance when driven

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 VOCM pin of the ADA4937-1 is left

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 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 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.

–75

–80

–85

–90

–95

–100

120

100

FREQUENCY (MHz)

HD2

HD3

G = +2

Figure 52. HD2/HD3 for Combination of ADA4937-1 and AD9246 ADC

1.8V

DRVDD

AVDD

VIN–

VIN+

AD9246

AGND

CML

SENSE

D11 TO

10pF

10µF

33Ω

200Ω

50Ω

VIN

90Ω

76.8Ω

ADA4937-1

200Ω

50Ω

76.8Ω

10µF

Figure 53. ADA4937-1 Driving an AD9246, a 14-Bit, 125 MSPS ADC

ADA4937-1YCPZ-R7 Datasheet(PDF) 23 Page - Analog Devices

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