ADA4937-1YCPZ-R2 datasheet(22/28 Pages) AD

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ADA4937-1

Rev. 0 | Page 22 of 28

HIGH PERFORMANCE ADC DRIVING

The ADA4937-1 is ideally suited for broadband IF applications.

The circuit in Figure 51 shows a front-end connection for an

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

AD9445 achieves its optimum performance when driven

differentially. The ADA4937-1 eliminates 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 is 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 is left floating,

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 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 51.

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 51. ADA4937-1 Driving an AD9445, 14-Bit, 105 MSPS ADC

ADA4937-1YCPZ-R2 Datasheet(PDF) 22 Page - Analog Devices

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