ADA4937-2YCPZ-R2 datasheet(25/28 Pages) AD

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Data Sheet

ADA4937-1/ADA4937-2

Rev. F | Page 25 of 28

3.3 V OPERATION

The ADA4937-1/ADA4937-2 provide excellent performance in

3.3 V single-supply applications. Significant power savings can

be realized when the ADA4937-1/ADA4937-2 are used in

combination with a low voltage ADC.

The circuit in Figure 67 is an example of the ADA4937-1 driving

an AD9230, 12-bit, 250 MSPS ADC that is specified to operate with

a single 1.8 V supply. The performance of the ADC is optimized

when it is driven differentially, making the best use of the signal

swing available within the 1.8 V supply. The ADA4937-1/

ADA4937-2 perform the single-ended-to-differential conversion,

common-mode level-shifting, and buffering of the driving signal.

The ADA4937-1/ADA4937-2 are configured with a single 3.3 V

supply and a gain of 2 V/V for a single-ended input to differential

output. The 59 Ω termination resistor, in parallel with the single-

ended input impedance of 306 Ω, provides a 50 Ω termination for

the source. The additional 26 Ω (226 Ω total) at the inverting input

balances the parallel impedance of the 50 Ω source and the

termination resistor that drives the noninverting input. The signal

generator has a symmetric, ground-referenced bipolar output. The

VOCM pin is connected to the CML output of the AD9230, and sets

the output common mode of the ADA4937-1/ADA4937-2 at 1.4 V.

One third of the output common-mode voltage of the amplifier is

fed back to the summing nodes, biasing −IN and +IN at ~0.5 V.

For a common-mode voltage of 1.4 V, each ADA4937-1/

ADA4937-2 output swings between 1.09 V and 1.71 V,

providing a 1.25 V p-p differential output.

A third-order, 125 MHz, low-pass filter between the ADA4937-1/

ADA4937-2 and the AD9230 reduces the noise bandwidth of

the amplifier and isolates the driver outputs from the ADC inputs.

1.8V

DRVDD

AVDD

VIN–

VIN+

AD9230

AGND

CML

D11 TO

30pF

10pF

56nH

33Ω

50Ω

VIN

226Ω

200Ω

VOCM

3.3V

ADA4937-1

59Ω

453Ω

Figure 67. Driving an AD9230, 12-Bit, 250 MSPS ADC

ADA4937-2YCPZ-R2 Datasheet(PDF) 25 Page - Analog Devices

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