ADA4937-2YCPZ-R2 datasheet(23/28 Pages) AD

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Data Sheet

ADA4937-1/ADA4937-2

Rev. F | Page 23 of 28

HIGH PERFORMANCE ADC DRIVING

The ADA4937-1/ADA4937-2 are ideally suited for broadband IF

applications. The circuit in Figure 64 shows a front-end connection

for an ADA4937-1 driving an AD9445, 14-bit, 105 MSPS ADC.

The AD9445 achieves optimum performance when driven

differentially. The ADA4937-1/ADA4937-2 eliminate the need for

a transformer to drive the ADC and performs a single-ended-

to-differential conversion and buffering of the driving signal.

The ADA4937-1/ADA4937-2 are configured with a single 5 V

supply and unity gain for a single-ended input to differential

output. The 61.9 Ω termination resistor, in parallel with the single-

ended input impedance of 267 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 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 allowing the internal divider to set the

output common-mode voltage at midsupply; one half of the

common-mode voltage is fed back to the summing nodes, biasing

−IN and +IN at 1.25 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 of the amplifier is ac-coupled to the ADC through a

second-order, low-pass filter with a cutoff frequency of 100 MHz.

This reduces the noise bandwidth of the amplifier and isolates

the driver outputs from the ADC inputs.

The AD9445 is configured for a 2 V p-p full-scale input by

connecting the SENSE pin to AGND, as shown in Figure 64.

VIN–

VIN+

47pF

30nH

24.3Ω

50Ω

SIGNAL

GENERATOR

226Ω

200Ω

VOCM

ADA4937-1

61.9Ω

200Ω

BUFFER T/H

ADC

CLOCK/

TIMING

REF

SENSE

AGND

0.1µF

AD9445

3.3V (A)

AVDD1

5V (A)

AVDD2

3.3V (D)

DRVDD

Figure 64. Driving an AD9445, 14-Bit, 105 MSPS ADC

ADA4937-2YCPZ-R2 Datasheet(PDF) 23 Page - Analog Devices

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